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THEORY DEPARTMENT
R&R Seminars
Plasma Webinars
Stellarator Series
EP Seminars
Frontiers Colloquia
Heliophysics Seminars
OLTP Seminar Series
Asia Pacific -USA "Highlights in Low Temperature Plasma Physics"
CSG Seminars
The Frontiers of Plasma Physics colloquium is jointly sponsored by PPPL and the Journal of Plasma Physics.
A full list of upcoming colloquia can be found on the JPP Frontiers of Plasma Physics Colloquium Upcoming Talks webpage.
Chris Hamilton, IAS, Princeton, USA
#s1618, Thursday, 23 Feb 2023, 11:00am
Marta Fajardo, Instituto Superior Técnico, Portugal
PIC simulations of the magnetorotational instability (MRI) in stratified, collisionless accretion disks Google Scholar (abstract)
Mario Riquelme, University of Chile, Chile
Plasma accretion onto astrophysical compact objects, such as black holes and neutron stars, is often considered to occur in the collisionless regime. This implies that kinetic plasma processes may play a crucial role in the physics of accretion in these systems, giving rise to important phenomena like non-thermal particle acceleration, temperature anisotropies and different temperatures between ions and electrons. Kinetic simulations, beyond MHD modeling, are thus needed in order to acquire a fully self-consistent picture of the processes that participate in disk accretion and to predict their observational signatures. In this talk, we will first present our initial results of 2D and 3D fully kinetic, particle-in-cell (PIC) plasma simulations of the collisionless magnetorotational instability (MRI), an MHD-scale instability that is crucial to produce outward transport of angular momentum in accretion disks. Our simulations are local and stratified, which means that we use the shearing box approximation and self-consistently include the vertical stratification of the disk. We find that the MRI saturation and the transport of angular momentum in our stratified simulations are more efficient than in the case where disk stratification is not included. Particle acceleration in our runs is efficient and mainly driven by magnetic reconnection, and is also more significant when the simulations include stratification. In the final part of the presentation, we will also focus on the non-linear evolution of (microscopic) temperature anisotropy instabilities, which play a crucial role in the MRI evolution by providing an effective collisionality to the plasma. We will show that these instabilities can also contribute to non-thermal particle acceleration in collisionless accretion disks. Our simulation results mainly concentrate on the sub-relativistic plasma regime, relevant for accretion onto black hole systems like Sgr A* and M87.
On the origin of magnetic fluctuations in low magnetic Prandtl number plasmas Google Scholar
Maarit-Korpi-Lagg, Aalto University, Finland , abstract
[#s1616, 02 Feb 2023]
Magnetic fields on small scales are ubiquitous in the universe. For example, the fluctuating magnetic fields in star-forming regions of galaxies are more than twice the strength of the magnetic fields coherent over large scales. On the solar surface, magnetic fields are mostly concentrated in medium and small-scale structures, while the proportion comprising the mean field strength is even lower than in galaxies. The generation mechanisms of the fluctuating magnetic fields are not fully understood. One possibility is the so-called small-scale dynamo (SSD), the other is tangling of the large-scale field structures through turbulence acting on them. In the interstellar medium of galaxies, the resistivity η is much lower than the viscosity ν, such that magnetic instabilities are easier to excite relative to the turbulence. SSD in such high magnetic Prandtl number (Pm= ν / η) conditions has therefore been predicted to be easily excited. In the Sun and cool stars, Pm is much lower, namely in the range of 10-6 - 10-3. Both theoretically and especially numerically, SSD is more difficult to excite at such very low magnetic Prandtl numbers. Indeed, some recent numerical studies had indicated that the threshold for SSD excitation should systematically increase with decreasing Pm, concluding that SSD would be impossible in the Sun and cool stars. Continuing increases in CPU resources, for now, have permitted us to perform even higher-resolution simulations employing the lowest Pm values to date to mimic solar conditions. Contrary to earlier findings, the SSD turns out not only to be possible for Pm down to 0.0031, but even to become increasingly easy to excite for Pm below ≃ 0.05. We relate this behaviour to the known hydrodynamic phenomenon, referred to as the bottleneck effect. Extrapolating our results to solar values of Pm indicates that an SSD would be possible under such conditions. We have recently developed a GPU-accelerated version of our solver, capable of bringing us to the solar parameter regime in terms of Pm in the near future, rendering the extrapolations into real findings in the solar regime.
Electron scale kinetic instabilities in magnetized plasmas via radiation reaction and laser ionization , Google Scholar
Luis Oliveira e Silva, Instituto Superior Técnico, Universidade de Lisboa, Portugal , abstract
[#s1613, 26 Jan 2023]
Anisotropic electron distribution functions in a strong magnetic field can be unstable to the electron cyclotron/synchrotron maser instability (ECMI), leading to the generation of coherent radiation. We explore laboratory and astrophysical scenarios where this instability can be present and/or generated due to collective processes and/or by carefully tuning the experimental conditions. We show that radiation reaction, in the classical and quantum regimes, depicts a general property that can lead to ring-like electron populations that are unstable to the ECME. A theoretical model is developed and compared with QED PIC simulations [1]. Laboratory scenarios where ring-like distributions can be generated via radiation reaction are also explored and demonstrated. We generalize previous works on anisotropic distributions driven by laser-ionized gases to demonstrate that similar distribution functions and the ensuing instability are naturally driven via a careful combination of laser parameters (intensity and polarization), plasma density, and external magnetic fields [2]. Fully self-consistent simulations demonstrate the key signatures of these processes, with an excellent agreement on the instability's features and the emitted radiation properties. Our results open two closely connected new directions, showing that radiation reaction naturally leads to strongly anisotropic ring-like distribution functions, prone to collective plasma instabilities and that these distributions and conditions can be mimicked in the laboratory allowing for the detailed exploration of maser radiation with the state-of-the-art laser technology. [1] P. Bilbao, L. O.Silva, arXiv:2212.12271 [2] T. Silva, P. Bilbao, L. O. Silva, in preparation (2023)
New insights on plasma turbulence in the boundary region of tokamaks , Google Scholar
Paolo Ricci, EPFL, Switzerland University, Switzerland , abstract
One of the greatest uncertainties in the success of the fusion program is related to the turbulent dynamics of the fusion fuel in the boundary region. The plasma behavior in the boundary governs the overall confinement properties of the device, regulates the impurity dynamics and the level of fusion ashes, and determines the heat load to the vessel walls – a showstopper for the whole fusion program if material requirements cannot be met. With the goal of improving our understanding of the boundary dynamics, the GBS code was developed during the past years. By solving the drift-reduced Braginskii equations self-consistently with the kinetic neutral atom dynamics, GBS simulates the plasma turbulence in the boundary of tokamaks as it results from external sources, recycling, turbulent transport and flows, and plasma losses at the vessel. GBS simulations have allowed us to advance the basic understanding of boundary turbulence. This incudes progress in estimating the SOL width, a crucial quantity to determine the heat flux on the vessel walls, and the identification of a turbulent regime characterized by a catastrophically large turbulent transport, which has been associated with the crossing of the density limit. We will present an overview of our simulation and theoretical results, as well as their comparison with experimental measurements from several tokamaks worldwide. Predictions for ITER and other future reactors will be discussed.
6-dimensional hybrid-Vlasov modelling of the near-Earth space: First Vlasiator results , Google Scholar
Minna Palmroth, University of Helsinki, Finland , abstract
Numerical simulations are key in modern space physics, as they can be used as 1) context to data, 2) predict future behaviour of the system, 3) understand the system using unforeseen boundary conditions, and increasingly also in 4) discovering new phenomena that are hard to be observed using point-wise satellite measurements. Especially, the discovery of new phenomena pertains to global systems, where phenomena of interest may be initiated far away from the point of observations. The most typical method of simulating the global solar wind - magnetosphere - ionosphere system is based on magnetohydrodynamics (MHD), which is however not representing the actual plasma behaviour in locations where kinetic physics becomes important. Such regions are e.g., the foreshock - magnetosheath interaction, reconnection, and the inner magnetosphere. Vlasiator is the world's first and so far the only global simulation based on the hybrid-Vlasov approach that simulates the ion distributions accurately without noise. Earlier Vlasiator results have shown without a doubt that ion-kinetic effects cannot be neglected from the large scales, as small-scale phenomena affect large scales and vice versa. This scale coupling leads to phenomena that are not predicted using local simulations without proper boundary conditions, or with spacecraft measurements lacking the global context. Here, we present the world's first global 6-dimensional ion-kinetic global magnetospheric simulation run, accurate both locally and globally. The simulation box extends from the dayside to the nightside, and includes global dynamics and both dayside and nightside reconnection regions. We show the newest runs showing a magnetotail eruption, and discusses the physics leading to this phenomenon that has puzzled space scientists for decades.
Magnetospheric waves and bursts from magnetars , Video
Andrei Beloborodov , Columbia University, USA
[#s1577, 15 Dec 2022]
Fundamental Scaling of Adiabatic Compression of Field Reversed Configuration Thermonuclear Fusion Plasmas , Video
David Kirtley, Helion Energy, USA
Fully kinetic simulations of plasma accretion in a three-dimensional shearing box , Google Scholar , Video
Fabio Bacchini, Katholieke Universiteit, Leuven, Belgium , abstract
One of the main uncertainties in the physics of plasma accretion onto compact objects (black holes and neutron stars) is represented by the dynamics of microscopic processes at kinetic scales. Although often expected to be collisionless, accretion flows around black holes are customarily studied with magnetohydrodynamic (MHD) models; these models however do not include out-of-equilibrium physics and cannot describe nonthermal particle acceleration and radiation, particle scattering and diffusion, and the transport of angular momentum through astrophysical accretion disks. To study these processes self-consistently, fully kinetic simulations are necessary; but the prohibitive costs typically associated to such numerical experiments have so far inhibited significant progress in this direction. In this talk, we will present the first mesoscale (i.e. attaining global MHD-like behavior) simulations of plasma accretion carried out with a fully kinetic approach in three dimensions. Our work is based on the shearing-box paradigm, which allows the first-principles study of a localized sector of an accretion disk at affordable costs. Our 3D kinetic simulations are large enough to reach convergence (with respect to physical parameters and box size), allowing us to analyze the detail of particle acceleration and angular-momentum transport in the turbulent plasma dynamics developing in a typical collisionless accretion flow.
No seminar today due to Thanksgiving Holiday
[#s1574, 24 Nov 2022]
Accelerating the rate of discovery: Toward high-repetition-rate laser experiments for High-Energy Density (HED) Science and Inertial Fusion Energy (IFE) , Google Scholar , Website , Video
Tammy Ma, Lawrence Livermore National Laboratory, USA , abstract
As high-intensity short-pulse lasers that can operate at high-repetition-rate (HRR) (>10 Hz) come online around the world, the high-energy-density (HED) science they enable will experience a radical paradigm shift. The >10^3 increase in shot rate over today's shot-per-hour drivers translates into dramatically faster data acquisition and more experiments, and thus the potential to significantly accelerate the advancement of HED science. However, to fully realize the potential benefits of HRR facilities requires a fundamental shift in how they are operated, and in fact, how the experiments done on them are designed and executed. Current energetic driver facilities depend on the ability to manually tune the lasers, the targets, the diagnostics settings, and more, between single shots or sets of shots through a manual feedback loop of data collection, data analysis, and optimization largely driven by experience and intuition. At 10 Hz, this paradigm is no longer sustainable as more complex data is collected more quickly than is possible to analyze manually. Simultaneously, on-the-fly optimization of experiments will become ever more crucial as higher repetition rates will lead to more deliberate inter-shot variations and the improved operational range to allow exploration over larger regions of phase space. Consequently, it is likely that the next generation of laser facilities will be limited not by their hardware but by our ability to use it effectively. We will present the vision and ongoing work to realize a HRR framework for rapidly delivered optimal experiments coupled to cognitive simulation to provide new insights in HED science, and lay the groundwork for Inertial Fusion Energy (IFE) rep rate operations.
The root cause of 2,1 tearing instabilities in DIII-D H-mode plasmas , Google Scholar
László Bardóczi, General Atomics, USA , abstract
Tearing modes open magnetic islands on rational surfaces of tokamaks, which ruin the confinement and can lead to plasma termination. The Neoclassical Tearing Mode is the leading single root cause of disruptions of high performance tokamak experiments. In order to develop and project stable operating solutions to future machines, a first principles based and experimentally validated model is necessary. I will present the analysis of an ensemble of over 16,000 DIII-D H-mode discharges exploring the root cause of 2,1 tearing onsets and its parameter dependency. The onset time distribution is exponential and the instability onset rate is constant in intermediate and high edge safety factor plasma scenarios, in accordance with Poisson-point processes as for example the radioactive decay. This supports that the instability has a threshold for growth and it is triggered by random events, in line with the Rutherford-theory of neoclassical tearing modes (NTMs). In scenarios characterized by low edge safety factor, such as the ITER baseline scenario, the onset rate increases over the course of the βN flattop, indicating that this group of plasmas evolve toward more unstable conditions. The onset statistics are well replicated by Poisson point process models, in contrast to models of classical stability index evolution. At a low edge safety factor, the increasing rate parameter is correlated with the drop of differential rotation between the q=1 and q=2 surfaces to near zero. Explanation is offered by the reduction of stabilizing polarization currents acting on sawtooth driven 2,1 seed islands when the differential rotation is approaching zero. The rate parameter and onset relative frequency analyses show strong correlation with βN , in agreement with NTMs. The magnetic perturbation amplitude of these islands grows linearly in time, as expected from NTMs, and it is unaffected by the current profile relaxation, suggesting that classical effects are weak. Recent experiments testing the effect of the current profile shape at q=2 on the 2,1 tearing instability found no causality between the island onset and the shape of the current profile. Overall, the agreement of various aspects of the data with the Rutherford theory supports that the 2,1 islands are NTMs triggered by random events in DIII-D at all edge safety factor values. This database analysis also suggests that the plasma stability can be improved by reducing the elongation, triangularity, density, impurity density, increasing the internal inductance and by running the plasmas in upper single null configuration.
Evolution of current end vorticity sheets in collisionless plasma turbulence , Video , Google Scholar
Daniela Grasso, Lawrence Livermore National Laboratory, USA , abstract
The evolution of current and vorticity sheets in collisionless plasmas, where magnetic reconnection within turbulence may take place driven by the electron inertia, is studied. We start from a linear analysis of magnetic vs. fluid instability, that might affect these layers, to understand the complex situation that generates in a turbulent plasma. Here, due to the presence of strong velocity shears, the typical plasmoids formation, observed to influence the energy cascade in the resistive magnetohydrodynamic context, has to coexist with the Kelvin-Helmholtz instability. We find that the current density layers may undergo the plasmoid or the Kelvin-Helmholtz instability depending on the local values of the magnetic and velocity fields. The competition among these instabilities affects not only the evolution of the current sheets, that may generate plasmoid chains or Kelvin-Helmholtz-driven vortices, but also the energy cascade, that is different for the magnetic and kinetic spectra.
ST40: Advancing the Physics Basis of Spherical Tokamak Reactors , Video , Google Scholar , Website
Michele Romanelli, Tokamak Energy Ltd, UK
[#s1545, 27 Oct 2022]
Collective modes in QED plasma , Website
Mikhail Medvedev, University of Kansas, USA , abstract
Ultra-magnetized plasmas where the magnetic field strength exceeds the Schwinger (critical) field become of great scientific interest, thanks to the astrophysical observations of magnetar emission and current advances in laser-plasma experiments. These advances demand better understanding of how quantum electrodynamics (QED) effects that are present in ultra-strong-field environments affect plasma dynamics. Interestingly, magnetars -- neutron stars with magnetic fields of ~1e15 Gauss or greater -- do exist and QED effects on their magnetospheric plasma cannot be ignored. In particular, Maxwell's equations become nonlinear in the strong-QED regime. This effect has not been systematically considered in theoretical studies. Here we discuss how "textbook" linear plasma modes are modified in an arbitrarily strong magnetic field. These results can be important for understanding of a magnetospheric pair plasma of a magnetar and for future laser-plasma experiments.
Advances in the understanding of ultrarelativistic beam-plasma instabilities , Video
Laurent Gremillet, CEA-DAM-DIF, France, abstract
Relativistic beam-plasma instabilities are thought to arise in many astrophysical systems. One of their major effects is to dissipate into heat, suprathermal particles and radiation the kinetic energy of fast outflows from powerful objects. This occurs via self-induced, kinetic-scale electromagnetic fields that scatter and decelerate particles to the point of forming collisionless shocks waves or triggering bright synchrotron-type emissions. In initially unmagnetized, relativistic beam-plasma systems, two main instability classes are known to prevail: the essentially electrostatic, oblique two-stream modes (OTSI) and the essentially magnetic, transverse current filamentation (CFI) modes. These can also operate, usually detrimentally, in laboratory schemes involving the interpenetration of relativistic electron streams and plasmas, as in plasma-wakefield accelerators or ultraintense laser-solid interactions. My talk will cover some recent theoretical and particle-in-cell simulation results on relativistic beam-plasma instabilities. Most of this work is connected with the E 305 project underway at the SLAC/FACET-II accelerator, which aims to probe the instabilities excited by a 10 GeV electron beam through gas or solid materials. First, I will show that the OTSI dynamics in the case of a finite-size beam impinging onto a collisionless, semi-infinite plasma deviates from the standard picture of a uniform, infinite system. I will present a model for the spatiotemporal effects caused by a sharp beam front and discuss the competition between the OTSI and the self-focusing of a finite-width beam, an issue of prime relevance for experiments. Then, I will consider the case of solid-density target and demonstrate that the hierarchy between the OTSI and CFI depends on the target collisionality and the beam density. The simulation results will be interpreted in light of a general kinetic linear theory of the OTSI and CFI modes in the presence of collisional background electrons. Finally, I will report our latest analysis of the saturation mechanisms of the CFI in asymmetric plasma flows, under conditions representative of relativistic collisionless shocks.
Relativistic Magnetospheres: Current sheets, Reconnection, Particle Acceleration , Website , Video
Benoit Cerutti, University of Grenoble, France, abstract
[#s1540, 29 Sep 2022]
Spinning neutron stars and black holes are the central engines of some of the most extreme astrophysical phenomena such as gamma-ray bursts, pulsars, X-ray binaries, binary mergers, or active galactic nuclei. The activity of these compact objects is often associated with the creation and the launching of a relativistic magnetized plasma within their magnetospheres. Similarly to their non-relativistic analog in the Solar System, these magnetospheres are highly dynamical. Large-scale current sheets form and reconnect, leading to efficient particle acceleration, pair production and non-thermal radiation. However, the interplay between general relativity, quantum electrodynamics and plasma physics makes this problem not easily tractable. In this talk, I will review current efforts to model pulsar and black hole magnetospheres from first principles by means of global particle-in-cell simulations. Results will be discussed in the context of gamma-ray pulsars, and recent horizon-scale observations of the weakly accreting supermassive black holes M87* and SgrA*.
A private path to laser-fusion energy, Focused Energy, a new startup in the fusion community , Video , Website
Markus Roth, Focused Energy Inc., Germany , abstract
As clean and safe energy is needed more than ever new developments have led to the rise of startup companies around the globe taking advantage of the science developed of the years and combining the results of the past with the technology of the 21st century to make fusion energy a reality. Focused Energy is a US/German startup supported by the TU Darmstadt and deeply embedded in the international science community. We are focusing on the concept of direct-drive laser-based inertial confinement fusion and fast ignition. In our approach, a small pellet containing a milligram of DT is directly irradiated by intense laser light and compressed to roughly 1000 times solid density. At the moment of maximum density, a burst of energetic, laser-driven ion beams is focused into a small part of the compressed fuel to rapidly rise the temperature above ignition temperature and start a bootstrap fusion reaction, which results in a supersonic burn wave consuming the fuel. More than two decades of research have led to this path, which has recently been quoted the most promising approach in inertial fusion energy by international leaders in the field. We have started assembling the best scientist in the field and will move on a fast track to get fusion demonstrated in a decade time frame. Thus, we build a first facility, T-STAR and UT in Austin, while we are working on laser and target development in Darmstadt. Focused Energy finally plans to develop a demonstration facility within this decade to demonstrate ignition, burn and gain sufficient for attractive energy production based on the unique combination of high-energy and high-power lasers.
Quantum computing for plasma physics: An overview of recent progress , Video , Google Scholar , Website
Yuan Shi, LLNL, USA , abstract
Quantum computing (QC) promises to bring game changing capabilities. However, the potential for quantum speedup of plasma problems remains unclear. This colloquium surveys recent progress on applying QC to plasma related problems. To connect plasma physics with quantum hardware, both top-down and bottom-up approaches have yielded interesting results. The top-down approach starts from future ideal computers and reconceptualizes plasma problems to fit into the QC framework. The bottom-up approach starts from near-term noisy devices and builds up toy problems towards more realistic applications. The research field is still in its early stage, and wide gaps remain to be bridged before QC will become useful for plasma physics.
Plasma physics of Fast Radio Bursts , Video , Google Scholar , Website
Maxim Lyutikov, Purdue University, USA , abstract
Fast Radio Bursts are millisecond long events of coherent radio emission coming from half way across the Universe. The inferred plasma and radiation conditions are extreme: radio waves carry macroscopic amount of energy, the laser non-linearity parameter $a_0$ can be as large as millions, the guiding magnetic field may reach critical quantum values. I will highlight plasma challenges of producing high brightness coherent emission in astrophysical surrounding, new regimes of plasma-laser interaction, and discuss how the concept of Free Electron Lasers may help us to understand these mysterious events.
Plasma wave topology and topological plasma waves , Google Scholar
Hong-Qin, PPPL, USA , abstract
The hairy ball theorem is well-known to plasma physicists because it implies that the surface of the magnetic bottle for confining fusion plasmas should not look like a 2D sphere. Chern's celebrated method of 1944 to prove the hairy ball (Gauss-Bonnet-Poincare) theorem can be used to prove that the cyclotron wave must vanish somewhere on a 2D sphere enclosing the Weyl point of Langmuir-cyclotron resonance in the parameter space. A physical consequence of this fact is that there must exist a topological surface excitation called Topological Langmuir-Cyclotron Wave (TLCW) in magnetized plasmas, which can propagate along complex phase transition interfaces in a unidirectional manner and without scattering. Due to this topologically protected robustness, the TLCW could be explored as an effective mechanism to drive current and accelerate particles in plasmas. The topological methods that we recently proposed for plasma waves bear similarities to those used in condensed matter physics in the past four decades. But there are significant differences. In condensed matters, periodic lattices lead to nontrivial topology in momentum space, but we show that in continuous media such as plasmas, nontrivial topology only exists in phase space because of the contractibility of momentum space. Using the algebraic topological concepts and tools that we recently developed, such as the boundary isomorphism theorem, and an Atiyah-Patodi-Singer type of index theorem formulated by Faure, the existence of TLCW as a spectral flow across the band gap is rigorously proven. We also show that the TLCW can be faithfully represented by a tilted Dirac cone. The entire spectrum of a generic tilted Dirac cone in phase space, including its spectral flow, is found analytically. [This research was supported by the U.S. Department of Energy (DE-AC02-09CH11466).] References: 1) Topological Langmuir-cyclotron wave. 2) The dispersion and propagation of topological Langmuir-cyclotron waves in cold magnetized plasmas. 3) Topological phases and bulk-edge correspondence of magnetized cold plasmas.
Investigating radiatively driven, magnetized plasmas with a university scale pulsed-power generator , Video , Website
Jack Halliday, Imperial College London, UK , abstract
[#s1522, 25 Aug 2022]
We describe results from a novel experimental platform that is able to access physics relevant to topics including indirect-drive magnetized inertial confinement fusion, laser energy deposition, various topics in atomic physics, and laboratory astrophysics (for example, the penetration of B-fields into high energy density plasmas). This platform uses the x-rays from a wire array Z-pinch to irradiate a silicon target, producing an outflow of ablated plasma. The ablated plasma expands into ambient, dynamically significant B-fields (∼5 T), which are supported by the current flowing through the Z-pinch. The outflows have a well-defined (quasi-1D) morphology, enabling the study of fundamental processes typically only available in more complex, integrated schemes. Experiments were fielded on the MAGPIE pulsed-power generator (1.4 MA, 240 ns rise time). On this machine, a wire array Z-pinch produces an x-ray pulse carrying a total energy of ∼15 kJ over ∼30 ns. This equates to an average brightness temperature of around 10 eV on-target.
Exploring Novel Properties of Strongly Magnetized Plasmas , Google Scholar , Website
Scott Baalrud, University of Michigan, USA, abstract
An often-underappreciated aspect of plasma theory is that it assumes weak magnetization, applying only when the electron gyroradius is much larger than the Debye length. This allows one to ignore the magnetic field at the scale of Coulomb interactions. Here, we use first-principles molecular dynamics simulations and new theoretical methods to explore the properties of strongly magnetized plasmas. A few novel behaviors have been uncovered. One is that the friction force on a test ion moving through a strongly magnetized plasma shifts to obtain components that act perpendicular to its velocity. These components cause qualitative changes to the average trajectory of the ion, such as changing its gyroradius and gyrofrequency in non-intuitive ways. They also translate to qualitative changes in macroscopic material properties of the plasma, such as the electrical conductivity, viscosity, and energy relaxation rates. Although strongly magnetized plasmas are not the norm, they do arise in several contexts, including non-neutral plasmas, antimatter traps, high magnetic field approaches to fusion energy, and in dense astrophysical objects such as magnetars. These results suggest that unexpected behaviors arise in these systems, and it motivates potential applications that make use of these novel properties.
Turbulence and transport research beyond the burning plasma era , Google Scholar , Website
Anne White, MIT, USA, abstract
The prospect of near-term fusion electricity opens new doors for university-based plasma physics research. Even after the grand societal challenge of putting fusion on the grid is achieved, research addressing grand intellectual challenges in plasma transport will remain vibrant. University groups will engage with sponsors and collaborators including not only governments and national labs around the world, but also private companies and utilities. In this talk I present side-by-side examples of recent research results on turbulence and transport measurements, as well as predictive simulation and modeling, carried out by researchers at MIT in support of both the nascent fusion industry and the established fission industry. I will share my perspective, as an academic department head, on the future of fusion research in universities as we move through and beyond the era of burning plasmas.
Highlights of T and DT results from JET-ILW experiments , Google Scholar
Jon Hillesheim, UK Atomic Energy Authority, UK - Chaired by: Bill Dorland, Editor, JPP , abstract
JET has recently completed the first DT campaign in a tokamak since 1997, and for the first time ever with ITER-like W/Be divertor and first wall materials. Scientific results have been achieved in all goals of the campaign including demonstration of high (average Pfus>10 MW ) sustained fusion power for 5 s, demonstration of an integrated radiative scenario, demonstration of clear alpha particle effects, exploration of isotope and mixed plasma species effects on energy and particle transport, addressing plasma-wall interaction in DT plasmas, and demonstration of RF schemes relevant to ITER DT operation. Highlights from the DT campaign, and from the preparatory isotope campaigns, will be presented.
Compressed Current Sheets in the Magnetotail: Importance of the Ambipolar Electric Field video , Google Scholar
Ami Dubois, Naval Research Laboratory, USA , abstract
[#s1510, 28 Jul 2022]
Micro-scale features are now being resolved by NASA's Magnetospheric Multi-Scale (MMS) mission, which means for the first time, we are able to investigate thin and non-ideal current sheets (i.e. current sheets that cannot be explained by the Harris equilibrium model) in detail and assess their role in magnetic reconnection. We use MMS satellite data to analyze kinetic-scale structures and dynamics associated with compressed current sheets. Our analysis shows that a transverse ambipolar electric field is localized to the region of lower hybrid fluctuations and the pressure gradient in this region is comparatively small, leading to the interpretation that compression of the current sheet and the resulting velocity shear is the underlying fluctuation driving mechanism. Our kinetic equilibrium model shows that as a large scale Harris current sheet is compressed, an ambipolar electric field forms and produces velocity shear near the magnetic null, indicating that velocity shear-driven waves can arise in the center of compressed current sheets. The presence and location of shear-driven waves at the center of current sheets is notable for a couple of reasons. First, because laboratory experiments and PIC simulations have shown that shear-driven lower hybrid fluctuations are capable of producing significant anomalous cross-field transport and resistivity, which can trigger magnetic reconnection. Second, using MMS wave data we can calculate the anomalous resistivity directly and show that the resistivity is significant, particularly at the magnetic null. Finally, we show that the electron distribution function is non-gyrotropic, which theoretical arguments suggest is an indicator of the possibility for magnetic reconnection to occur. Our kinetic equilibrium shows that such non-gyrotropic distribution functions can be generated by a quasi-static electric field and does not necessarily arise from wave induced effects.
Particle acceleration in turbulent plasmas: colourizing the Fermi picture video , Website
Martin Lemoine, Institut d'Astrophysique de Paris, France
Validation of low-Z impurity transport theory using boron perturbation experiments in ASDEX Upgrade video , Google Scholar
Rachael McDermott, IPP, Garching, Germany , abstract
Impurities are unavoidable in fusion plasmas and potentially problematic as they result in fuel dilution and radiative energy losses. Therefore, it is important to have accurate predictions of impurity behavior in future fusion devices, which requires a validated theoretical description of impurity transport. With this goal, an experimental technique was developed at ASDEX Upgrade (AUG) to separately identify the diffusive and convective components of the boron particle flux [1-2]. Using this technique, a database of B transport coefficients covering a wide range of plasma parameters has been assembled and can now be used to validate theoretical predictions of low-Z impurity transport [2]. This database shows that the normalized ion temperature gradient (R/LTi) is the strongest organizing parameter for both the B diffusion and convection and strong R/LTi (>6) is a necessary ingredient to obtain hollow B density profiles in AUG. This database also shows that large changes in the applied neutral beam injection (NBI) have a relatively small impact on impurity transport compared to similar changes in electron cyclotron resonance heating (ECRH). Even low levels of ECRH power dramatically increase both the diffusive and convective fluxes and lead to peaking of the impurity density profile. Comparisons to a combination of neoclassical and quasi-linear gyrokinetic simulations show good agreement in the measured and predicted diffusion coefficients. The outward convection measured in NBI dominated plasmas, however, is not well captured by the simulations, despite the inclusion of fast ions [3]. In contrast, the convection is reasonably well reproduced for plasmas with flat or peaked boron density profiles. This dataset provides an excellent experimental validation of the non-monotonic, predicted, convective-particle-flux created by the combination of pure-pinch, thermodiffusion, and roto-diffusion. In addition, this dataset demonstrates a non-monotonic dependence of the experimental particle diffusivity to ion heat conductivity (D/χi) in qualitative agreement with theoretical predictions. [1] C. Bruhn et al Plasma. Phys. Control. Fusion 60 (2018) 085011 [2] Corrigendum Bruhn C. et al. Plasma Phys. Control Fusion 62 (2020) 049501 [2] R. M. McDermott et al. 2022 Nucl. Fusion 62 (2022) 026006 [3] P. Manas et al Nucl. Fusion (60) 2020 056005
Plasma Physics of Pulsar Magnetospheres , Website
Alice Harding, NASA Goddard Space Flight Center, USA , abstract
Pulsars turn out to be much more complex than a seemingly simple rotating dipole field. The relativistic plasma required for current closure in their magnetospheres requires two signs of charge, so production of electron-positron pairs is required. A great deal of progress has been made over the last 15-20 years in understanding the structure of fields and currents in the pulsar magnetosphere. However, the source of the plasma and how the microphysics of its production is self-consistently coupled with the global magnetosphere are still not resolved. While we have determined that the main site of particle acceleration and high-energy radiation is in the current sheet outside the light cylinder, the details of the mechanisms involved are also not resolved. I will review the theoretical progress to date, from force-free MHD global models to particle-in-cell simulations. I will also review the recent ideas for generating the pulsar emission from radio to Very-High-Energy wavelengths that is ultimately needed to connect with observations.
Collisionless Shockwaves in Magnetized High-Energy-Density Laboratory Plasmas , Video , Google Scholar , Website
Derek Schaeffer, Princeton University, USA , abstract
[#s1506, 30 Jun 2022]
As a fundamental process for converting kinetic to thermal energy, collisionless shocks are ubiquitous throughout the heliosphere and astrophysical systems, from Earth's magnetosphere to supernova remnants. While these shocks have been studied for decades by spacecraft, telescopes, and numerical simulations, there remain key open questions in the fundamental physics of collisionless shocks, such as how do shocks accelerate particles to extremely high energies? Or, how is energy partitioned between particles across a shock? In this talk, I will discuss results from high-energy-density experiments and simulations on the formation of supercritical collisionless shocks created through the interaction of a supersonic laser-driven magnetic piston and magnetized ambient plasma. Through proton and refractive imaging, we observe for the first time a magnetized collisionless shock, comparable to some of the strongest shocks in the heliosphere. By probing particle velocity distributions with Thomson scattering, we directly measure the coupling interactions between the piston and ambient plasmas that are critical steps in the formation of magnetized collisionless shocks. Particle-in-cell simulations constrained by experimental data further detail the shock formation process and predict key signatures that are observed in experiments. I will also discuss how the development of this experimental platform can complement, and in some cases overcome, the limitations of similar measurements undertaken by spacecraft missions and can allow novel investigations of energy partitioning and particle acceleration in shocks.
Plasma Physics of Pulsar Magnetospheres video
Piero Martin, University of Padova, Italy , abstract
This talk illustrates the physics basis which supports the main engineering choices in the Divertor Test Tokamak facility (DTT) under construction in Frascati, Italy. DTT is a superconducting tokamak with 6 T on-axis maximum toroidal magnetic field, carrying plasma current up to 5.5 MA in pulses with total length up to 100 s. The D-shaped device has a major radius R=2.19 m, minor radius a=0.70 m, with an average triangularity 0.3. The auxiliary heating power coupled to the plasma at maximum performance is 45 MW, which allows matching the PSEP/R values with those of ITER and DEMO, where PSEP is the power flowing through the last closed magnetic surface. The primary mission of DTT is the study of the plasma exhaust and of tokamak divertor performance in conditions relevant to ITER and DEMO and in regimes where plasma core and edge behaviors are integrated. In addition to that DTT will provide a facility for high performance tokamak physics and to address core confinement and stability issues in a variety of plasma configurations, including negative triangularity scenarios and the management of transient events like disruptions and ELMs.
Turbulence and thermodynamics in expanding, collisionless, magnetised plasma , Google Scholar
Archie Bott, Princeton University, USA , abstract
The magnetised plasma composing many different astrophysical systems of interest – from the solar wind to the intracluster medium of galaxy clusters – is often weakly collisional or collisionless, with the Larmor radii of the constituent particles being many orders of magnitude below their Coulomb mean free paths. This feature results in a complex interplay between a plasma's macrophysical evolution (e.g., due to expansion, compression, or large-scale shear) and its microphysical response (e.g., triggering of kinetic instabilities). In this talk, we will elucidate this phenomenon aided by the results of several hybrid-kinetic expanding-box simulations. We will show how the nonlinear dynamics of strong Alfvénic turbulence in a collisionless plasma efficiently adapts to changes in fundamental wave physics that are induced by the effect of macroscopic expansion on microscopic particle motions. This adaptation holds irrespective of a qualitative transformation to the plasma's thermodynamics caused by pressure-anisotropy-driven kinetic instabilities. We will also demonstrate that different rates of expansion can lead to two qualitatively distinct thermodynamic states: in one state, Alfvén waves are supported; in the other, they are suppressed. These states will be characterised in detail, including the firehose-induced effective collisionality. Our results may help to disentangle the signatures of kinetic instabilities and strong Alfvénic turbulence in key observables in the near-Earth solar wind, such as magnetic power spectra and ion velocity distribution functions.
Prospects for real-time, first-principles transport simulations and stellarator optimization including turbulence , Video , Google Scholar
Bill Dorland,University of Maryland, Princeton Plasma Physics, USA , abstract
The open-source Trinity code solves for the time-dependent radial profiles of density, temperature, etc, using turbulent fluxes obtained from any radially local gyrokinetic turbulence code, neoclassical fluxes obtained from any drift kinetic solver, external sources, and edge boundary conditions supplied by the user. While originally developed for tokamak applications, the multiscale approach of Trinity is easily generalized for stellarator applications, as long as the equilibrium is assumed to consist of nested flux surfaces without islands. We present results using the original Trinity code together as well as a new Python version that will enable broader, easier uptake by the community. In 2018, we embedded Trinity into an optimization framework and demonstrated the ability to optimize tokamak shaping to maximize fusion power using first-principles estimates for turbulence-induced fluxes. Here, we will present our approach to embedding these gyrokinetic tools into the SIMSOPT framework. GX is an open-source, radially-local, GPU-native, gyrokinetic turbulence code that uses pseudo-spectral methods and native CUDA libraries to calculate turbulence-induced fluxes and critical gradients. At high resolution, GX is simply yet another GK code, but it can be run successfully at low resolution, in lieu of uncontrolled approximations and reduced models. With these tools, we demonstrate the ability to solve for the time-dependent evolution of core fusion reactor profiles in approximately real time, without resorting to reduced models. We also demonstrate the ability to find a shape, size, etc, that maximizes fusion performance by minimizing turbulence-induced losses "inside the optimization loop" for families of tokamak and stellarator reactor concepts, using equilibrium information calculated by VMEC and/or a near-axis expansion approximation, and we present machine-learned, sub-grid techniques that could further accelerate these calculations. I will show linear and nonlinear benchmarks against standard codes from the community, for both tokamak and stellarator configurations. Finally, we introduce the concept of specific computational intensity and use it to demonstrate how one can decide when to retire a given reduced model and rely instead on a higher-fidelity approach. There are many, many reduced models available for modeling fusion plasmas and without some kind of easy-to-use, objective method to distinguish the appropriateness of one approach from another, modelers and designers are often left to work out how to proceed more or less randomly. This leads to severe combinatoric complexity in design and interpretation efforts, which we hope to help bring under control.
The road to pedestal tailoring at ASDEX Upgrade , Website , Video
Elizabeth Wolfrum, IPP Garching, Germany , abstract
In the narrow edge region of a tokamak transport can be reduced by suppression of turbulence. The core plasma confinement is then elevated and consequently, the region with reduced turbulence is called 'pedestal'. This work gives an overview of recent investigations at ASDEX Upgrade that show our current understanding of the transport mechanisms in the pedestal and how transport and stability in this narrow region can be influenced. For electron heat transport a constant temperature gradient length hints towards a local small-scale turbulent transport mechanism. The ion heat transport is close to neoclassical values, however in some cases this only holds in the central part of the pedestal with deviations at the pedestal top and foot. The shape and position of the edge density profile are key to both stability and transport and remains the parameter which can be most varied in the pedestal. In our search for a scenario without large edge localised modes, ballooning modes can be driven unstable at the pedestal foot. Careful balance of the drive and stabilising terms allows the pedestal to be tailored such that the global peeling-ballooning stability limit is not breached. Another globally stable regime is achieved with strong nitrogen seeding, leading to the formation of an X-point radiator. These two ELM-free regimes are important research topics for the extrapolation to larger devices.
Cross-scale interactions between ion and electron-scale turbulence in magnetized plasmas , Video , Google Scholar
Shinya Maeyama, Nagoya University, Japan , abstract
[#s1501, 26 May 2022]
Recent gyrokinetic simulations have revealed the existence of cross-scale interactions between disparete turbulence at ion and electron gyroradius scales. I would like to start my talk by reviewing recent studies of multi-scale gyrokinetic simulations and discussing problems to be solved in future. For addressing one of these issues, we examine the extrapolation of cross-scale interactions toward high electron temperature burning plasmas, and demonstrate a possibility of reduction of turbulent transport by cross-scale interactions. In the latter part of this talk, I also would like to discuss the methodology for extracting and modeling cross-scale interactions between disparate-scale turbulence. To this end, we have developed a statistical analysis technique based on Mori-Zwanzig projection operator method, which decomposes time evolution of variable of interests into correlated/uncorrelated terms with regard to the explanatory variables. We discuss validity/applicability of the method to multi-scale turbulence problem based on the results of application example to simple plasma turbulence problem.
Overview of plasma transport processes relevant to Inertial Confinement Fusion at the National Ignition Facility , Google Scholar
Mark Sherlock, Lawrence Livermore National Laboratory, USA , abstract
A predictive simulation capability is a long term goal of the Inertial Confinement Fusion research program at the Nation Ignition Facility laser. Achieving this goal requires us to understand a number of plasma transport processes in detail in order to assess their overall impact on achieving a sufficiently efficient and symmetric energy transfer from the laser "drive" to the fusion fuel. This talk will give an overview of the processes currently being explored and the associated computational and theoretical techniques. Topics include: modeling electron thermal transport in the kinetic regime with Vlasov-Fokker-Planck codes; generation and transport of magnetic field by lasers; generation of ion turbulence by strong heat flow; ion kinetic effects inside the fuel capsule; transport instabilities involving magnetic field including the thermomagnetic, collisional Weibel, electrothermal and magnetothermal instabilities; the effect of laser speckles on transport; and the theory of laser absorption in non-thermal plasmas.
Quantum-inspired methods for solving the Vlasov-Poisson equation ,video
Erika Ye, MIT, USA , abstract
Kinetic simulations of collisionless (or near-collisionless) plasmas using the Vlasov equation are often infeasible due to high resolution requirements and the exponential scaling of computational cost with respect to dimension. Recently, it has been proposed that matrix product state (MPS) methods, a quantum-inspired but classical algorithm, can be used to approximately solve partial differential equations with exponential speed up, provided that the solution can be compressed and efficiently represented as an MPS within some tolerable error threshold. In this work, we explore the practicality of MPS methods for solving the Vlasov-Poisson equations in 1D1V, and find that important features of linear and nonlinear dynamics, such as damping rates and saturation energies, can still be captured while compressing the solution by at least a factor of 8. Furthermore, by comparing the performance of different mappings of the distribution functions onto the MPS, we generate some intuition of the MPS representation and its behavior, which will be useful for extending these methods to higher dimensional problems.
Michael Barnes, University of Oxford, UK , Google Scholar
[#s1452, 28 Apr 2022]
Vladimir Zhdankin, Flatiron Institute, USA , Google Scholar
Particle acceleration in collisionless shocks: connecting micro and macro scales
Anatoly Spitkovsky, Princeton University, USA , Google Scholar , abstract
Sudden deceleration of supersonic flows results in shock waves, which in the conditions of low density plasmas are mediated by collisionless processes. Such colliisionless shocks in astrophysical environments are thought to be responsible for the generation of nonthermal particles that span many decades in energy. These particles produce synchrotron radiation from astrophysical sources, such as supernova remnants and relativistic jets, or are observed directly as energetic cosmic rays. The main acceleration mechanism for these particles is known as "diffusive shock acceleration" and involves particle scattering and diffusion around a shock wave. In the nonlinear stage, shock acceleration couples together the internal structure of the shock with magnetic turbulence generated by accelerated particles, and presents a fascinating self-propagating nonlinear system with multiscale feedbacks. With the development of ab-initio numerical simulations of collisionless shocks, many details of the shock acceleration mechanism can now be studied directly. In this talk I will review the progress in kinetic (PIC) simulations of shock structure and particle acceleration in various regimes, and focus on processes that lead to electron acceleration in non-relativistic shocks, including field amplification, electron heating, and nonlinear regulation of shock injection. The lessons learned from microscopic PIC simulations suggest pathways to larger simulations that use augmented MHD techniques to study shock acceleration on the scales of astrophysical objects. I will discuss such MHD-PIC approaches and applications of current results to morphologies and spectra of nonthermal emission from supernova remnants and galaxy clusters.
Laser-matter interactions at ultra-high intensity: how do we simulate them and what can experiments tell us?
Tom Blackburn, University of Gothenburg, Sweden , Google Scholar , abstract
As the intensity frontier pushes past 1023 W/cm-2, experiments with high-intensity lasers interacting with matter, whether plasma or relativistic particle beams, enter a new regime. Here the dynamics arise from the interplay between relativistic plasma physics and strong-field, nonperturbative, quantum electrodynamics (QED). Understanding these processes is essential for developing our knowledge of extreme astrophysical environments, such as pulsars, magnetars and black-hole magnetospheres. In this talk I will present an overview of the progress that has been made in investigating the strong-field regime, from the simulation models we use, to the experiments that are possible with today's high-power lasers.
Improvement of confinement in tokamaks by a weakening the poloidal magnetic field at the boundary, invariants, and attractors ,video
Vladimir Yankov, Ergophos LLC, USA , Google Scholar , abstract
[#s1464, 31 Mar 2022]
Density profiles of tokamaks are enigmatically peaked and can be described as a turbulent attractor defined by a conservation law, namely, the plasma is frozen in the poloidal magnetic field. The profiles aka Turbulent EquiPartition are accurately described by a simple formula nv=const where v is the specific poloidal volume. The formula predicts that density and temperature at the border will decrease if the v is increased. This can be done in many ways and was observed experimentally before any theory emerged. The first way observed was current rampdown and the latest way was negative triangularity. Since almost all results in tokamaks were obtained experimentally, the theory will be presented briefly as well as several new ways to improve confinement. The theory will include the origin of many plasma and tokamak invariants from the Poincare invariant.
Primordial magnetic fields ,video
Axel Brandenburg, NORDITA, Sweden , WebSite , Google Scholar
Electron holes in collisionless plasmas: how long do these common nonlinear structures last? ,video
Ian Hutchinson, MIT, USA , abstract
Electron phase-space holes are now widely observed in space plasmas. They consist of a solitary positive potential peak with depleted electron population on trapped orbits that sustains the potential; and so they are intrinsically kinetic: governed by the Vlasov equation. Important new details about their speed and structure are now emerging from multi-satellite measurements. This talk will introduce the principles, observations, and simulations of electron holes; explain the ways that they behave like composite objects possessing lumped momentum, negative mass, and kinematic properties; and show how these concepts determine how and when they break up by instabilities. Instability probably determines the lifetime of a hole when collisions are negligible.
Exploring edge turbulence in the low and improved confinement regimes at the ASDEX Upgrade tokamak ,video
Rachel Bielajew, MIT , WebSite
, abstract
Future tokamak fusion reactors will need to operate in a regime of high energy confinement while particle confinement remains low enough that impurities can be exhausted. The low confinement operating regime L-mode has no edge transport barrier and lacks high energy confinement. The high confinement operating regime H-mode has been a target for high confinement operation, however its steep pedestal gradients lead to the edge instability Edge Localized Modes (ELMs). ELMs exhaust impurities and allow for steady-state high confinement operation, but they also release substantial energy which can damage material surfaces. The "improved" confinement regime I-mode is a promising operational scenario for future fusion reactors because it features an edge energy transport barrier without a particle transport barrier and it is naturally ELM-free. The mechanism that leads to this separation of transport channels in I-mode is an open question. The nature of the edge and pedestal turbulence in I-mode plasmas, and its role in determining transport, is still under investigation. In this work we explore edge fluctuations in the L-mode and I-mode edge at the ASDEX Upgrade tokamak through detailed study with turbulence diagnostics. In conjunction, linear gyrokinetic studies probe the nature of the turbulence from the outer core to the pedestal top. We find that the pedestal Weakly Coherent Mode (WCM) remains similar in nature in L-mode and I-mode and that ion-scale fluctuations in the outer core and pedestal top also undergo little change between L-mode and I-mode. The electron scale is a potential candidate for the suppression of heat flux in I-mode, separated from possible particle flux mechanisms. Cross-scale coupling is seen to be important in the I-mode outer core and pedestal.
Tokamak turbulence stabilization by electromagnetic effects and fast ions ,video
Paola Mantica, IFP-CNR, Milan , Google Scholar
A decade ago the experimental discovery on JET that ion temperature profile stiffness (due to the strength of turbulence reaction to changes in ion temperature normalized gradient) is reduced by increased Neutral Beam and/or Ion Cylotron Resonant Heating power triggered an intense work to understand and reproduce/expand these results. The JET results were explained by means of gyrokinetic simulations as due to non-linear electromagnetic stabilization of ion turbulence associated with pressure gradients (including thermal and suprathermal components). In both JET and ASDEX-Upgrade evidence has been found that these mechanisms are at the basis of improved ion confinement and ion temperature peaking in high power Hybrid scenarios. On DIII-D a similar stabilizing effect was found. An additional mechanism linked to a purely fast ion driven resonant linear electrostatic stabilization has been found in JET high ICRH power discharges and very recently used in ASDEX-Upgrade to design pulses with improved ion temperature peaking. Significant progress has been achieved in the theoretical understanding of these stabilizing effects and work is still in progress to better understand the extrapolability to ITER conditions, especially in presence of highly energetic α particles. This talk will present an overview of the experimental and theoretical work on this topic and will discuss its impact on our predictive capabilities of tokamak scenarios.
Fast and furious: reconnection and turbulence in magnetically-dominated astrophysical plasmas ,video
Lorenzo Sironi, Columbia University, USA , Google Scholar , Website
In the most powerful astrophysical sources, reconnection and turbulence operate in the "relativistic" regime, where the magnetic field energy exceeds even the rest mass energy of the plasma. Here, reconnection and turbulence can lead to fast dissipation rates and efficient particle acceleration, thus being prime candidates for powering the observed fast and bright flares of high-energy non-thermal emission. With fully-kinetic particle-in-cell (PIC) simulations and analytical theory, we investigate the physics of relativistic reconnection and turbulence, and demonstrate that they can be the "engines" behind: (1) high-energy flares in blazar jets; and (2) the hard-state spectra of black hole X-ray binaries and Active Galactic Nuclei.
New horizons for stellarator optimization via fast 3D MHD equilibrium and stability calculations with islands and chaos ,video
Joaqium Loizu, EPFL, Switzerland , Google Scholar , Website
Turbulence and transport in the Large Plasma Device: shear suppression, nonlinear instability and electromagnetic turbulence video
Troy Carter, UCLA, USA , Google Scholar , Website
Anomalous Electron Diffusion in Magnetic Islands and Stochastic Magnetic Fields , Video
Eva Kostadinova, Auburn University, USA , Google Scholar , Website
Magnetic islands and regions of stochastic magnetic fields originate from the dynamical processes of magnetic reconnection and turbulence in plasma. These structures are ubiquitous in both laboratory settings (e.g., tokamaks and stellarators) and space environment (e.g., solar wind plasma and Earth's magnetosphere). An interesting feature of magnetic islands and stochastic regions in plasmas is their connection to plasma particle acceleration, often resulting in anomalous diffusion. An important question is what universal principles relate the properties of energetic particles as a function of the underlying magnetic field topology in both lab and space. The answer to this question requires the development of universal transport models. This talk will introduce a Fractional Laplacian Spectral (FLS) approach to anomalous diffusion in plasmas with magnetic islands and stochastic magnetic fields. The FLS is a novel technique which computes the probability for particle transport as a function of nonlocal interactions and stochasticity in the examined field. The inputs for the model are informed from DIII-D experiments where energetic electrons (exhibiting anomalous diffusion) were observed in the presence of resonant magnetic perturbation (RMP) of the magnetic field and from simulations of the corresponding B-field topology. The perturbation on the B-field results in two characteristic structures: magnetic islands (leading to nonlocal transport) and stochastic regions (leading to chaotic transport). We show how the interplay between typical island scale and the magnitude of stochasticity determine the resulting electron diffusion.
Dusty plasma experiments: strong coupling, shocks, and testing theories of statistical physics,, Video
John Goree, University of Iowa, USA , Google Scholar , Website
Dusty plasmas contain small solid particles, which gain large electric charges. Typically they are micron size polymer spheres. Unlike the electron and ion components, the dust particle component tends to behave like a strongly coupled plasma, with Coulomb collisions dominating to such a degree that particles arrange themselves like atoms in a liquid or a solid. Due to their large size, the dust particles can be imaged individually in video recordings. This video imaging allows experimenters to track individual particles, which is an enormously powerful diagnostic that is unavailable in traditional plasma physics experiments, where electrons and ions cannot be imaged individually. In this talk I present, as two example research topics: shocks and tests of theories of statistical physics. While the shock topic is a traditional one for plasma physics, the topic of testing statistical physics takes the discipline of plasma physics in a new direction, by exploiting particle tracking.
Relativistic plasma physics and high field phenomena using intense lasers video
Karl Krushelnick, University of Michigan Ann Arbor, USA , Google Scholar
The past two decades have witnessed the development of revolutionary light sources having the unprecedented ability to probe new physical regimes and control matter with atomic scale precision. The ongoing development of multi-Petawatt lasers around the world will allow exploration of fundamental yet unanswered questions regarding non-linear Quantum Electrodynamics in relativistic plasmas, including non-perturbative quantum radiation reaction and electron-positron pair production mechanisms. Further experiments enabled by such lasers will include pump-probe experiments using femtosecond x-rays as a probe of material dynamics on ultra-short timescales, the production of GeV ion beams, the generation of instabilities in electron-positron jets, the exploration of vacuum polarization effects, relativistic shocks and the production of "exotic" particles such as pions and muons. I will review recent advances in this field and also describe the new NSF funded ZEUS facility under construction at the Center for Ultrafast Optical Science (CUOS) at the University of Michigan. ZEUS will be a dual-beamline 3 PetaWatt laser system that will provide unique capabilities for research. This will be a new high power laser user facility for US scientists as well as for the wider international research community, and will have an open and transparent external review panel for facility access and 30 weeks per year dedicated to external user experiments. After completion in 2023, the ZEUS laser system will be the highest-power laser system in the US.
Maximal Energy Release and the Rules of Rearrangement video
Elijah Kolmes, Princeton University, USA , Google Scholar
Throughout plasma physics, we are often interested in processes through which kinetic energy is transferred out of a distribution of particles. Examples of these processes include wave-particle interactions (for instance, the amplification of a wave) as well as the growth of turbulent internal modes. Some particle distributions are more prone to these energy transfers than others. Of interest is the maximal possible energy that can be liberated from a distribution function by wave-particle interactions with constraints on the nature of the interaction. These constraints might be that the waves can only rearrange the 6D phase space, or that they must conserve adiabatic invariants, or that instead they can only act to diffuse particles. This talk will trace the development of these ideas, starting in the 1960s. Among the developments that we will cover is the recent and surprising result that, with enough fine-tuning, the energy recoverable from diffusive processes can reach the energy recoverable from entropy-conserving processes.
Laser plasma accelerators: First results from the HIGGINS 2x100 TW laser
Victor Malka, Weizmann Institute of Science, Israel , Google Scholar
Laser Plasma Accelerators (LPA) are changing the scientific and societal landscape. Opening new hopes for high energy physics, offering alternative to synchrotron light sources with the recent demonstration with LPA's based Free Electron Radiation, and delivering particle and radiation beams for medical and security applications, they are among the most innovative tools of modern sciences. The laser plasma accelerators are a perfect illustration of what cross-domain fertilization with a zest of imagination can produce. In this talk I'll explain the main involved concepts, and why these wonderful machines rely on our ability to control finely the electrons motion with intense laser pulses. I'll show how the electrons collective manipulation permits to produce giant electric fields of value in the TV/m exceeding by 3 orders of magnitude or more the ones used in current machines. These collective motions when controlled permits also to modify and to shape the longitudinal and radial components of the plasma fields for either accelerating efficiently electrons or for producing energetic photons by wiggling electron during their acceleration. This control is crucial for electrons injection that is essential for delivering stable ultra-short and ultra-bright energetic particle or radiation beams. To illustrate the beauty of laser plasma accelerators I will show some concepts we recently demonstrated that allow these controls for beams improvements. Finally, I will show the commissioning of the HIGGINS dual laser system of the Weizmann Institute of Science, together with a set of first experimental results showing new insights of the relativistic plasma fields and a new approach for producing plasma refractive optics for relativistic beam manipulation.
The helicity barrier: how low-frequency turbulence triggers high-frequency heating of the solar wind ,video
Jonathan Squire, University of Otago, New Zealand , Google Scholar , Webpage
Weakly magnetized, relativistic collisionless shocks have been studied extensively over the past couple of decades using electron-ion and pair plasma compositions, whereas the broader landscape of electron-ion-positron mixtures has been left unexplored. The more general case is of astrophysical relevance for the early afterglow phase of gamma-ray bursts (GRBs), where the prompt radiation loads the external medium ahead of the shock with electron-positron pairs. In this talk, I will address the microphysics of external, pair-loaded GRB shocks using a set of first-principles kinetic simulations. I will show that even a small number of electron-positron pairs per ion significantly changes the shock structure. In particular, I will demonstrate that a pair-loaded shock is mediated by the Larmor gyration of ions in the compressed mean magnetic field even when this field is extremely weak, and therefore, pair-loaded shocks accelerate ions only in the limit of vanishing external magnetization. Electrons, on the other hand, can form distinctively non-thermal distributions even when the ions are essentially thermal. Although the shock structure significantly changes with respect to the plasma composition, the energy fraction carried by the pairs downstream of the shock is nearly independent of the pair-loading factor. Finally, I will comment on the implications of the results for the early afterglow emission of GRBs.
Daniel Groselj, Colombia University, USA , Google Scholar
Ilya Dodin, Princeton Plasma Physics Laboratory USA , Webpage
Modern geometrical optics is a powerful framework that allows modeling wave processes more efficiently than just via solving wave equations like any other PDEs. I will overview some of the recent applications of this theory to reduced linear modeling of radiofrequency waves, including mode conversion, cutoffs, and caustics, which is usually assumed to require the full-wave approach. I will also show how modern wave theory helps fundamentally improve quasilinear theory and understand inhomogeneous turbulence. The presentation is mainly targeted at curious theorists, but the applications addressed are also of practical importance for fusion science and beyond.
The magnetised plasma sheath and its role in the boundary of magnetic fusion devices video
Alessandro Geraldini, EPFL, Switzerland , Google Scholar , Webpage
Sheaths form wherever a plasma is in contact with a solid target. They are characterised by a spatial variation of the electrostatic potential and density over very small length scales normal to the surface. Within a few Debye lengths of the target, the electric field is so strong that the plasma is non-neutral. Debye sheaths exist in order to repel electrons, which are lighter and more mobile than ions. When a magnetic field is present in the plasma at an oblique angle with the target, the sheath develops a two-scale structure, with a part of the electrostatic potential variation occurring in a larger quasineutral magnetic presheath (or Chodura sheath) a few ion gyro radii from the target. When simulating turbulence in the open field line region (Scrape-Off Layer) of a fusion device, it is numerically prohibitive to resolve the small timescales and length scales of the magnetised sheath, which comprises the magnetic presheath and Debye sheath. Instead, an iterative method can be used to directly obtain numerical solutions for the electrostatic potential in the magnetised sheath in steady state. This is demonstrated using a fully kinetic model exploiting the grazing magnetic field angle (typical of fusion devices) to approximate the ion and electron trajectories. Numerical solutions include the ion distribution function reaching the target, important for sputtering predictions. Analytical calculations show that the kinetic Chodura condition must be satisfied by the ion distribution function reaching the magnetised sheath from the rest of the plasma. The implications of this for boundary conditions to gyrokinetic codes of the open field line region are discussed.
Scott Hsu, ARPA-E, USA , Google Scholar , Webpage
Since 2015 with the launch of the ALPHA program [1], the Advanced Research Projects Agency-Energy (ARPA-E) of the U.S. Department of Energy has supported over 50 R&D projects relating to fusion energy. ARPA-E's fusion R&D portfolio is focused on high-risk, high-reward translational/applied R&D to enable timely commercially viable fusion energy, while incentivizing collaborations between privately and publicly funded fusion teams. This colloquium will be organized into two parts: (1) brief overview of the mission/approach of ARPA-E as an R&D funding agency, and how fusion-energy R&D is motivated/pursued within the overall context of the agency, and (2) overview of ARPA-E's active fusion programs (BETHE, GAMOW, and Fusion Diagnostics), including its technology-to-market (T2M) approach, and technical research highlights from selected fusion projects.
Research on Complex/Dusty Plasmas in the Lab and in Space video
Hubertus Thomas, DLR — German Aerospace Centre, Germany , Google Scholar
Complex/dusty plasmas are plasmas containing small solid particles, which get charged by the collection of plasma electrons and ions. Due to their high charge in laboratory plasmas they start to interact strongly and can form liquid and solid structures, the latter is called plasma crystal. This can be seen as a classical condensed matter system where the main component – the solid particles – can be visualized and tracked dynamically. This allows investigations of fundamental processes in liquids and solids and their transitions. Solid particles of sizes of around a micrometer in diameter start to react strongly on gravity and levitating forces are mandatory. The sheath electric field of a rf-discharge allows the trapping of microparticles in the sheath and can be used to form2-dimensional (horizontal) or compressed 3-dimensional (with a small extend in the vertical direction) complex plasma systems. To study large 3-dimensionalcomplex plasmas in the bulk of a discharge microgravity experiments are necessary. PK-4 is the third plasma crystal facility on the International Space Station ISS continuing the successful research under microgravity conditions started in 2001 already. In this presentation I will give an overview on complex plasma research and will show recent results like active matter and electrorheological plasmas from ground based and ISS-based laboratories. This work was supported in part by DLR (BMWi), ESA, Roscosmos and NASA/NSF.
Exploring Stellar Nucleosynthesis and Basic Nuclear Science using High Energy Density plasmas at OMEGA and the NIF
Maria Gatu Johnson, MIT, USA , Website
Abstract: Thermonuclear reaction rates and nuclear processes have been explored traditionally by means of accelerator experiments, which are difficult to execute at conditions relevant to Stellar Nucleosynthesis. High-Energy-Density (HED) plasmas closely mimic astrophysical environments and are an excellent complement to accelerator experiments. This talk will focus on HED experiments to study the T+T reaction at the OMEGA laser facility, and the mirror 3He+3He reaction at OMEGA and at the National Ignition Facility (NIF). We present neutron spectra from the T(t,2n)α(TT) reaction measured in HED experiments at ion temperatures from 4 to 18 keV, corresponding to center-of-mass energies (Ec.m.) from 16 to 50 keV. A clear difference in the shape of the TT-neutron spectrum is observed between the two Ec.m., providing the first conclusive evidence of a variant TT-neutron spectrum in this Ec.m. range. Preliminary data from a recent discovery science experiment at the NIF exploring the solar 3He+3He reaction at Ec.m. from 60-120 keV will also be discussed. In addition, the talk will cover the potential of this new field of research, ongoing efforts, and future directions for studying nuclear astrophysics-relevant nuclear processes at OMEGA and the NIF. This work was supported in part by the U.S. DOE, the MIT/NNSA CoE, LLE and LLNL.
The role of turbulence in determining the density profile in magnetic confinement devices
Saskia Mordijck, College of William & Mary, USA , Google Scholar , Web Page
Abstract: The fusion gain in a tokamak is directly linked to the density of the plasma. However, due to the high temperatures necessary for fusion, it is impossible to fuel the core of the plasma directly and directly influence the core density. Without any direct fueling in the core of a tokamak, the plasma density is fully controlled by transport perpendicular to the confining magnetic field surfaces. In this talk, I will show how cross-field transport of electrons is dominated by turbulence in the plasma core by comparing experiments with existing models. These models capture how various types of turbulence influence transport and thus the density profile. While the density profile in the core is fully determined by turbulent transport, at the plasma edge, the picture is more complicated. At the edge of the tokamak, turbulent transport effects intermingle directly with fueling through ionization of the surrounding gas. To better understand the impacts of turbulence on the particle flux, we perform a series of experiments on LAPD varying the neutral density and electron density gradient. While some trends follow linear predictions of resistive drift wave turbulence, other phenomena cannot be explained using linear predictions..
Extreme Plasma Astrophysics: a Shining New Frontier
Dimitri Uzdensky, University of Colorado Boulder, USA , Google Scholar ,Video
While traditional plasma physics deals with plasmas made up of a fixed number of electrons and ions that are nonrelativistic and nonradiative, there exist in the Universe important plasma environments with physical conditions so extreme that additional "exotic physics" (from a plasma physicist's point of view) processes come into play: special and general relativity, strong coupling between plasma particles and photons, and, in most extreme cases, QED effects like pair production and annihilation. These processes alter the plasma dynamics near compact relativistic astrophysical objects — neutron stars and black holes — arguably, the most enigmatic and fascinating objects in the Universe. Understanding how collective plasma processes (waves, instabilities, shocks, magnetic reconnection, turbulence, etc.) operate under these exotic conditions calls for the development of a new, richer physical framework, which forms the scope of Extreme Plasma Astrophysics. I will review the rapid progress that is being made now in exploring this exciting new frontier, stimulated by the exploding astrophysical motivation and enabled by concerted, vigorous theoretical efforts and recent computational breakthroughs such as the advent of novel relativistic kinetic plasma codes incorporating radiation reaction and pair creation. I will illustrate this progress with recent studies of radiative relativistic turbulence and magnetic reconnection with pair creation in the context of accreting black-hole coronae and neutron-star magnetospheres. I will end by outlining the future directions of the burgeoning field of Extreme Plasma Astrophysics.
Fast Magnetic Reconnection
Allen Boozer, Columbia University, USA , Google Scholar , Web Page ,Video
When a magnetic field undergoes a near-ideal evolution that involves all three spatial coordinates, mathematics and Maxwell's equations give a characteristic time scale for the initiation of reconnection. This time is given by the ideal evolution multiplied by a factor that depends only logarithmically on the strength of the non-ideal effects. The critical mathematical concept is chaos, which means the streamlines of the ideal flow of the magnetic field lines can separate exponentially in time. The mathematics of vector representations in three dimensions together with Faraday's law define the ideal flow velocity of magnetic field lines as well as an electromotive-like constant on each line which gives the non-ideality. Maxwell's equations imply chaotic flows are energetically impossible in a two-dimensional evolution, which makes conventional two-dimensional reconnection theory an extremely specialized subject. The magnitude of the current density in a three-dimensional reconnection depends only logarithmically on the strength of the non-ideal effects instead of being inversely proportional as in two dimensions. The current density lies in numerous thin but wide ribbons along the magnetic field lines. The concepts that underlie three-dimensional reconnection theory are unfamiliar to the plasma physics community. The talk will both explain these concepts and give simple examples of their application. To ensure those who would like have time to assess unfamiliar concepts, the slides that will be used are attached here.
Modeling Complex Interactions in a Complex Plasma
Lorin Swint Matthews, Baylor University, USA , Google Scholar , Web Page
A complex, or dusty, plasma consists of sub-micron to micron-sized grains immersed in a plasma environment. Micron-sized dust grains have been successfully employed as non-perturbative probes to measure variations in plasma conditions on small spatial scales, such as those found in plasma sheaths. Within a sheath, ions are accelerated from the bulk plasma towards the charged boundary. Ions flowing past a dust grain form a positively charged spatial region downstream of the grain, called the ion wake. The ion wake in turn modifies the interaction potential between charged grains and can contribute to the stability of the dust structures which are formed in a given plasma environment. Thus, although dust grains can be used as non-invasive probes on "small scales", on even "smaller scales" the perturbations to the plasma flow are necessary to establish a stable dust configuration. Here we present a multi-scale N-body model of the dust-plasma interactions. Results are compared with ground-based lab experiments as well as microgravity experiments onboard the International Space Station to determine quantities such as the charge on individual grains, the electric field within the region, and the nature of the ion wakefield.
Reconnection-controlled decay of magnetohydrodynamic turbulence and the role of invariants
David Hosking, University of Oxford, UK , Google Scholar , Web Page
In this talk, I will describe a new theoretical picture of magnetically dominated, decaying turbulence in the absence of a mean magnetic field. I shall argue that the energy-decay rate of such a system is governed by the reconnection of magnetic structures, and not necessarily by ideal dynamics, as has previously been assumed. I will explain how a prediction for the decay law of magnetic energy can be obtained by assuming reconnection-mediated dynamics that respects the conservation of certain integral invariants, which represent topological constraints satisfied by the reconnecting magnetic field. As is well known, the magnetic helicity is such an invariant for initially helical field configurations, but does not constrain non-helical decay, where the volume-averaged magnetic-helicity density vanishes. For such a decay, I shall propose a new integral invariant, analogous to the Loitsyansky and Saffman invariants of hydrodynamic turbulence, that expresses the conservation of the random [scaling as volume^(1/2)] magnetic helicity contained in any sufficiently large volume. The existence of this `Saffman helicity invariant' leads to a natural explanation of the inverse-transfer phenomenon reported by previous numerical studies. Finally, I shall describe an application of these results to the decay of primordial magnetic fields in the early Universe.
Magnetospheric Multiscale Observations of Collisionless Plasma Turbulence in Earth's Magnetosheath: Turbulent Electric Fields & Turbulence-Driven Magnetic Reconnection
Julia Stawarz, Imperial College London, UK , Google Scholar , Web Page
Plasmas throughout the Universe undergo complex, highly nonlinear turbulent dynamics, which transfer energy from large to small-scale fluctuations and in the process generate a multitude of small-scale structures, such as current sheets. However, many space plasmas are nearly collisionless, making the question of how the turbulent fluctuations are dissipated a particularly challenging question. NASA's Magnetospheric Mutiscale (MMS) mission is a formation of four Earth-orbiting satellites providing the high-resolution plasma measurements and inter-spacecraft separations necessary to examine plasma dynamics at scales approaching those of the electrons. In this presentation, I will discuss two recent studies that make use of the unique measurements from MMS in Earth's magnetosheath to examine the small sub-proton scale dynamics of turbulent plasmas in greater detail than previously possible. In the first study, the behaviour of the turbulent electric field is examined by directly measuring the contributions from the terms in generalised Ohm's law from fluid to electron-scales. In the second study, MMS observations are used to systematically identify magnetic reconnection events at the thin current sheets that are generated by the turbulent fluctuations. The large-scale properties of the turbulent fluctuations, in particular the correlation length, are found to influence the nature of the reconnection dynamics potentially leading to, so called electron-only reconnection, in which there is not enough space for ion to fully couple to the newly reconnected magnetic fields. Both of these studies provide insight into the nonlinear couplings, and potentially the dissipative dynamics, in collisionless plasmas.
Some basic principles of inertial confinement fusion and some recent "burning plasma" results*
Omar Hurricane, Lawrence Livermore National Laboratory, USA ,Google Scholar
Abstract: Inertial confinement fusion (ICF) has existed as a field of study since the 1970s, but the field was born out of the Cold War. In the decades since the 1970s, pioneering research developing the principles and technologies of ICF culminated in the creation of several major facilities that exist today. While the technology of ICF facilities themselves is fascinating, this talk concentrates upon a handful of basic physics principles of "indirect-drive" (x-ray driven) targets fielded on the National Ignition Facility (NIF) in Northern California and upon some key results from the last decade of research, including some recent experiments that appear to have broached the burning plasma regime [1,2,3].
[1] A.B. Zylstra, O.A. Hurricane, D.A. Callahan, et al., in preparation (2021) [2] J.S. Ross, J.E. Ralph, A.B. Zylstra, et al., in preparation (2021) [3] A.L. Kritcher, C.V. Young, H.F. Robey, et al., in preparation (2021) *Work performed under the auspices of the U. S. Department of Energy by LLNL under contract DE-AC52-07NA27344
Progress and plans for Princeton Field-Reversed-Configuration Research
Samuel Cohen, PPPL, USA ,Webpage
Abstract: The Princeton Field Reversed Configuration-2 (PFRC-2) is a research device for studying innovative physics methods to enable small clean fusion reactors. PFRC novel physics regimes are characterized by J┴B and kinetic conditions. Based on the limited availability of one fuel component, 3He, such reactors would be limited to use in niche applications, such as for spacecraft propulsion or emergency terrestrial power generation. First experiments, motivated by single-particle simulations of plasma heating by rotating magnetic fields of odd-parity symmetry (RMFo), produced electron temperatures in excess of 100 eV. The present research program addresses three topics: ion heating by RMFo; confinement; and stability. To achieve bulk ion energies in excess of 100 eV, ARPA-E-supported upgrades are being made to machine hardware, modeling capabilities, and diagnostics. Two new diagnostics have been installed, two-photon laser-induced fluorescence (TALIF, PU-MAE) and Thomson scattering (TS, ORNL). The TALIF diagnostic has measured the H° density in quasi-state-state and puffed gas discharges, allowing evaluation of particle confinement time and energy loss by CX. TS is now being put into operation. Additional planned increased capabilities include reflectometry (UC-Davis), DFSS for internal fields (ORNL), an ion energy analyzer (PPPL), and a PIC simulation code (U Rochester). Benefits of and the requirements for scrape-off-layer modification are described.
Effects of distribution structure on predictions of plasma behavior in marginally unstable plasma
Emily Lichko, University of Arizona, USA Google Scholar ,Webpage
Abstract: Due to low collisionality in space and astrophysical plasmas, distributions of ions and electrons observed by spacecraft exist in a state far from thermodynamic equilibrium. The non-Maxwellian features in these distribution functions can trigger microinstabilities, which likely play a role in some of the largest open questions in solar physics, including coronal heating, heating of the bulk solar wind, and accounting for high-frequency waves observed alongside the Alfvenic turbulent cascade. While there is a tremendous amount of information in the structure of these distribution functions, they are typically only represented by a fit of one or two Maxwellian or bi-Maxwellian distributions. In this work, we examine how the fidelity of the model to the observed distribution function affects our predictions for the stability of the plasma, and how much of the information in the distribution function is needed to accurately predict the behavior of the plasma. To do this, we use marginally stable one-dimensional, electrostatic simulations of the electron two-stream instability. For these simulations, there is significantly better agreement between the behavior of the plasma and the predictions of linear theory when a higher-fidelity representation of the distribution function is used. Preliminary work on the extension of these electrostatic results to the electromagnetic regime and the comparison of the predictions of linear wave activity with measurements of waves from Parker Solar Probe will be presented as well.
Muni Zhou, Massachusetts Institute of Technology, USA Google Scholar, News
William Daughton, Los Alamos National Laboratory, USA Google Scholar
Benedikt Geiger, University of Wisconsin, Madison, USA Webpage
Solar Wind Turbulence: in-situ observations from magneto-fluid to kinetic plasma scales
Olga Alexandrova, LESIA, Observatoire de Paris, France Google Scholar
Abstract: Solar wind turbulence was mostly studied at MHD scales: there, magnetic fluctuations follow the Kolmogorov spectrum. The fluctuations are mostly incompressible and they have non-Gaussian statistics (intermittency), due to the presence of coherent structures in the form of current sheets, as it is widely accepted. Kinetic range of scales is less known and the subject of debates. We study the transition from Kolmogorov inertial range to small kinetic scales with a number of space missions. It becomes evident that if at ion scales (100-1000 km) turbulent spectra are variable, at smaller scales they follow a general shape. Thanks to Cluster/STAFF, the most sensitive instrument to measure magnetic fluctuations by today, we could resolve electron scales (1 km, at 1 AU) and smaller (up to 300 m) and show that the end of the electromagnetic turbulent cascade happens at electron Larmor radius scale, i.e., we could establish the dissipation scale in collisionless plasma. Furthermore, we show that intermittency is not only related to current sheets, but also to cylindrical magnetic vortices, which are present within the inertial range as well as in the kinetic range. This result is in conflict with the classical picture of turbulence at kinetic scales, consisting of a mixture of kinetic Alfven waves. The dissipation of these waves via Landau damping may explain the turbulent dissipation. How does this picture change if turbulence is not only a mixture of waves but also filled with coherent structures such as magnetic vortices? These vortices seem to be an important ingredient in other instances, such as astrophysical shocks: for example, they are observed downstream of Earth's and Saturn's bow-shocks. With the new data of Parker Solar Probe and Solar Orbiter we hope to study these vortices closer to the Sun to better understand their origin, stability and interaction with charged particles.
Tim Horbury, Imperial College London, UK ,Webpage
Christopher Reynolds, University of Cambridge, UK Webpage
Abstract: The baryonic component of galaxy clusters is dominated by the intracluster medium (ICM), a hot and tenuous plasma atmosphere in an approximate state of hydrostatic equilibrium within the gravitational potential of the dark matter halo. The ICM is an important actor in many astrophysical processes within the cluster - the ram pressure of the ICM can strip cold gas out of orbiting galaxies, and radiative cooling can lead to significant galaxy building in the ICM core in a manner that is well-known to be regulated by feedback from the central supermassive black hole. However, all of these phenomena are influenced by transport processes within the weakly-collisional and high-beta ICM which are still poorly understood. In this talk I focus on the physics and astrophysical role of thermal conduction in the ICM. I summarize recent developments in understanding the role of whistler modes in the regulation of thermal heat transport and proceed to discuss some astrophysical implications of this new transport model. I end by discussing the future observational landscape of these ICM plasma studies.
Reduced turbulence in optimised maximum-J stellarators video
Josefine Proll, Eindhoven University of Technology, Holland Google Scholar
Abstract: Turbulence is one of the main obstacles to a working fusion reactor. Especially in stellarators, the large space of available magnetic field shapes allows for optimisation towards low levels of turbulence. A useful nonlinear measure of turbulence is that of available energy. Here I will show that the available energy calculated for trapped-electron-mode turbulence in different magnetic configurations can predict trends in the (simulated) heat flux of trapped-electron mode (TEM) turbulence in these configurations and could thus serve as a valuable proxy in future optimisation routines. Both, the available energy and the nonlinear simulations, support a previous linear prediction: that the class of optimised maximum-J stellarators, amongst them Wendelstein 7-X, particularly benefits from reduced turbulence. Previously, we had analytically shown that in these devices, the electron-driven TEM is absent. Here I will show that the stabilising property of the electrons also extends to ion-temperature gradient (ITG) modes and can thus explain the levels of low turbulence in the record-shots of Wendelstein 7-X at finite density gradient. Finally, I will present evidence that in the absence of TEMs, the universal instability can emerge and actually dominate the turbulence in optimised stellarators
Expanding frontiers for dusty plasmas: magnetic fields to microgravity
Edward Thomas, Auburn University, USA Google Scholar
Abstract: The presence of charged, solid, particulate matter in plasmas, i.e., "dust", is ubiquitous. From stellar nurseries to planetary rings and from fusion experiments to plasma processing reactors, "dusty" plasmas are found in a wide variety of naturally occurring and human-made plasma systems. Therefore, understanding the physics of dusty plasmas can provide insights into a broad range of astrophysical and technological problems. This presentation will focus on how the small charge-to-mass (q/m) ratio of the charged microparticles gives rise to many of the unique spatio-temporal properties of dusty plasmas. Moreover, this small charge-to-mass ratio strongly influences how magnetic field and microgravity studies of dusty plasmas are performed, leading to new investigations of previously unexplored regimes of plasma parameters. This presentation will discuss results from our studies of dusty plasmas in high magnetic fields (B ≥ 1 T) using the Magnetized Dusty Plasma Experiment (MDPX) device at Auburn University and in microgravity experiments using the Plasmakristall-4 (PK-4) laboratory on the International Space Station. At the end, the presentation will discuss the prospects for the future of dusty plasma research., abstract
Zonally dominated dynamics and the transition to strong turbulence in cold-ion Z-pinch plasma
Plamen Ivanov, University of Oxford, UK Google Scholar
Abstract: Following the discovery of the Dimits shift (Dimits et al. 2000), the role of zonal flows (ZFs) for the transition to turbulence in tokamak plasmas has been an area of intense research. We attempt to shed some light on this problem by studying the transition to turbulence in a simplified cold-ion fluid model for ion-scale turbulence in Z-pinch magnetic geometry. Our equations are obtained in a highly collisional, cold-ion, asymptotic limit of the ion gyrokinetic equation and capture the two well-known ion-temperature-gradient (ITG) instabilities driven by either magnetic curvature or parallel compression. We find that this model has a well-defined Dimits (low-transport, ZF-dominated) state characterised by a staircase-like arrangement of ZFs and zonal temperature that suppresses turbulence. Viscous decay of the ZFs leads to occasional turbulent bursts that reconstitute the staircase by providing a negative zonal turbulent viscosity. In 2D, at sufficiently large equilibrium temperature gradients, the zonal turbulent viscosity switches sign, hence the turbulent bursts no longer reinforce the zonal staircase and the Dimits state is destroyed. In 3D, the Dimits state is much more resilient and can always be sustained provided sufficient parallel extent of the system. This is because the large-scale curvature-driven perturbations go unstable to small-scale "parasitic" 3D slab-ITG modes that give rise to a negative zonal turbulent viscosity and provide an effective thermal diffusion for the large-scale modes. If we restrict the parallel extent of the system, the Dimits state is destroyed, and a strongly turbulent, high-transport state is established. In this state, energy is injected into large-scale perturbations by the curvature-ITG instability, then transferred into the parasitic small-scale modes, and finally dissipated by the finite collisionality. Moreover, we find that sufficient parallel resolution is critical for the 3D Dimits state and failure to resolve the small parallel scales of the parasitic modes results in a non-physical transition to strong turbulence. This analysis is based on analytical calculations and numerical simulations of the cold-ion fluid model.
Vortex dynamics in non-neutral electron plasmas subject to externally imposed ExB flows
Noah Hurst, University of Wisconsin, USA Webpage
Abstract: A series of experiments is described in which magnetized non-neutral electron plasmas are subjected to strong applied electric fields in the plane perpendicular to the magnetic field. The resulting ExB drift dynamics are isomorphic to those of a two-dimensional ideal fluid described by the Euler equations. In this correspondence, the electron density is analogous to the fluid vorticity, and so the plasmas mimic the behavior of fluid vortices. The transverse electric fields act as externally imposed ExB strain flows which can deform and destroy the vortices. Details of the experimental procedure are given, as well as an overview of the experiments that have been carried out so far using this technique. Recent work is then discussed in greater detail, including studies of adiabatic behavior of elliptical electron vortices subject to slowly growing strain flows, and studies of spatial Landau damping of vortex oscillations due to a fluid-wave resonance near the vortex edge. The results are compared with a low-dimensional theoretical model of elliptical vortices, and with particle-in-cell simulations. Finally, the relationship of these results to other similar systems in geophysics, astrophysics, and plasma physics is discussed.
Turbulence in high-energy-density experiments: inference and generation
Seth Davidovits, Lawrence Livermore National Laboratory (LLNL), USA
Abstract: High-energy-density (HED) experiments pursuing fusion or X-ray generation can become turbulent. Facilities for HED experiments are also utilized for generating plasma turbulence for study, often with astrophysical applications in mind. The first part of this talk discusses the inference of turbulent flow in experiments without spatial (diagnostic) resolution of the flows; a need for such inference often arises in fusion or X-ray generation experiments, where the plasma is rapidly compressed to small size. Here I highlight examples from Z-pinch experiments optimized for X-ray production, and also briefly discuss recent work showing that turbulence in such two-dimensional compressions may exhibit stronger growth rates with decreasing volume than three-dimensional compressions. The second part of the talk discusses the turbulence generation principles underlying a new experimental design being developed for future laboratory studies of astrophysically-relevant turbulence.
X-ray view of the Coma galaxy cluster with SRG/eROSITA
Eugene Churazov, Max Planck Institute for Astrophysics, Garching, Germany and Space Research Institute, Moscow, Russia Google Scholar, abstract
Coma (Abell 1656) is a massive nearby galaxy cluster famous for being the first object where the presence of Dark Matter was noted by Fritz Zwicky back in 1933. In radio band, it became the first cluster where a "radio halo" and a "radio relic" were detected. In X-rays, which are emitted by hot plasma filling the cluster gravitational well, it is one of the three brightest clusters in the sky. Coma is also a spectacular case of cluster merger with a smaller galaxy group. All this makes Coma a testbed for studies of the phenomena ranging from collisionless dynamics of merging clusters to hydrodynamics, particle acceleration, and weakly collisional intracluster plasma on small scales. In X-rays, the only "trouble" is the large angular size (a few degrees) of the Coma cluster, which is difficult to map with telescopes having a small field of view. This difficulty was recently overcome with the SRG/eROSITA observations yielding a spectacular X-ray map of the entire cluster. Preliminary results of the analysis of these data will be discussed.
Direct laser acceleration (DLA) of leptons in plasma channels in radiation-dominated regime
Marija Vranic, Istituto Superior Tecnico, Lisbon, Portugal Google Scholar, abstract
DLA occurs in partially void plasma channels as a consequence of the simultaneous interaction of particles with the laser field and the plasma background. The particles perform betatron oscillations in the large-scale electric and magnetic field generated by displacing plasma electrons. In addition, they oscillate in the rapidly alternating laser field. By gaining momentum in the direction of laser propagation, the particles perceive a lower laser frequency, and the two types of oscillations can become resonant. The DLA electrons to ~500 MeV were obatianed in experiments using near-critical plasma densities and ps optical lasers. The principal advantage of DLA is that it generates relativistic electron beams with > 100 nC of charge. Using the next generation of lasers (~10 PW power), one could expect energies > 10 GeV, maintaining the high-charge content. In this regime, the interaction becomes dominated by the radiation losses, which counter-intuitively become favourable for acceleration. With a few modifications, DLA can be used for positron acceleration as well. I will address the underlying physics, the analytical model of the acceleration and the scaling laws predicting the asymptotic energy of the accelerated particles. The presented results are supported by particle-in-cell simulations.
Collisional transport in large aspect ratio stellarators
Feliz I. Parra, University of Oxford, UK Google Scholar, abstract
Collisional transport at the small collision frequencies characteristic of fusion reactors can be enormous in stellarators. In order to reduce this transport and the associated energy loss, the position and strength of the external magnets that produce the magnetic field in stellarators must be optimized. In this talk, I will revisit collisional transport in the particularly interesting limit of large aspect ratio stellarators. I will derive a new formulation to calculate collisional transport in the small collision frequency regime relevant to stellarator reactors. This new formulation has been implemented in KNOSOS (KiNetic Orbit-averaging SOlver for Stellarators), a very efficient, fast code that calculates collisional transport in a variety of regimes and can hence be used in stellarator optimization exercises. I will show both numerical and analytical results obtained using the new model that illustrate the nature of stellarator collisional transport at small collision frequencies.
Understanding the complex interaction between supra-thermal particles and turbulence in magnetic confinement devices
Alessandro di Siena,University of Texas, Austin, USA Google Scholar, abstract
The performance of magnetic confinement devices is strongly limited by turbulent transport inducing particle and energy losses and reducing plasma confinement. Among the different experimental actuators of turbulence, supra-thermal particles – generated via external heating schemes – are typically considered one of the most efficient in suppressing ion-temperaturegradient (ITG) driven turbulence in the core of fusion devices. In this talk, I will present some of the most recent insights into understanding the underlying physical mechanisms responsible for this turbulence regulation from first principle gyrokinetic simulations, theory and experiments. Finally, I will discuss the possible implications of this turbulence stabilization via energetic particles to existing and future tokamak and optimized stellarator devices.
The runaway electron landscape of cooling plasmas
Tünde Fülöp, Chalmers University of Technology, Sweden Google Scholar, abstract
The phenomena of runaway acceleration in plasmas has general importance in many fields of physics, for example it is a candidate mechanism for lightning initiation in thunderstorms and electron acceleration in solar flares. In fusion plasmas, understanding of runaways has a great practical importance, as the severity of runaway avalanches increases strongly with plasma current. Therefore, generation of runaways is expected to be a serious issue in ITER and other high-current reactor-scale fusion devices. We will discuss the characteristics and consequences of runaway generation, as well as possible mitigation strategies in fusion devices.
HelioSwarm: Leveraging Multi-point, Multi-scale Observations to Uncover the Nature of Turbulence in Space Plasmas
Kristopher Klein, University of Arizona Webpage, abstract
There are many fundamental questions about the temporal and spatial structure of turbulence in space plasmas. Answering these questions is complicated by the multi-scale nature of the turbulent transfer of mass, momentum, and energy, with characteristic scales spanning many orders of magnitude. The solar wind is an ideal environment in which to measure turbulence, but multi-point observations with spacecraft separations spanning these scales are needed to simultaneously characterize structure and cross-scale couplings. Recently selected for phase A study for NASA's Heliophysics MidEx Announcement of Opportunity, the HelioSwarm Observatory proposes to transform our understanding of the physics of turbulence in space and astrophysical plasmas by deploying nine spacecraft to measure the local plasma and magnetic field conditions at many points, with separations between the spacecraft spanning MHD and ion scales. HelioSwarm resolves the transfer and dissipation of turbulent energy in weakly-collisional magnetized plasmas with a novel configuration of spacecraft in the solar wind. These simultaneous multi-point, multi-scale measurements of space plasmas allow us to reach closure on questions of how energy is distributed in typical solar wind conditions, as well as in extreme conditions relevant to astrophysical plasmas.
Magnetic reconnection andplasmoid formation in black hole accretion flows
Bart Ripperda, Flatiron Institute and Princeton University Webpage, abstract
Plasmoids, or hotspots, forming due to magnetic reconnection in current sheets, are conjecturedto power frequent X-ray and near-infrared flares from Sgr A*, the black hole inthe center of our Galaxy. It is unclear how, where, and when current sheetsform in black-hole accretion flows. We show extreme resolution 3Dgeneral-relativistic resistive magnetohydrodynamics and 2D general-relativisticparticle-in-cell simulations to model reconnection and plasmoid formation inblack hole magnetospheres. Plasmoids can form in thin current sheets In theinner 15 Schwarzschild radii from the event horizon, after which they canmerge, grow to macroscopic hot spots of the order of a few Schwarzschild radiiand escape the gravitational pull of the black hole. Large plasmoids areenergized to relativistic temperatures via magnetic reconnection near the eventhorizon and they significantly heat the jet, contributing to itslimb-brightening. We find that only hot plasmoids forming in magneticallydominated plasmas can potentially explain the energetics of Sgr A* flares. Theflare period is determined by the reconnection rate, which we find to beconsistent with studies of reconnection in isolated Harris-type current sheets.
Studies of plasma confinement in a gas-dynamic trap
Alexander Ivanov, Budker Institute of Nuclear Physics, Novosibirsk, Russia
The gas dynamic trap (GDT) was invented by Vladimir Mirnov and Dmitrii Ryutov in Novosibirsk in the late 1970s. It is basically a version of a magnetic mirror which is characterized by a long mirror-to-mirror distance exceeding the effective mean free path of ion scattering into a loss cone, a large mirror ratio (R ~ 100) and axial symmetry. Under these conditions the plasma confined in a GDT is isotropic and Maxwellian. The plasma loss rate out of the end mirrors is governed by a set of simple gas-dynamic equations, hence the device's name. Plasma magnetohydrodynamic stability in GDT can be achieved through a favorable averaged pressure-weighted curvature of the magnetic field lines, as was initially proposed, or, alternatively through a sheared plasma rotation at periphery induced by electrically biased electrodes at the end wall. A high flux volumetric neutron source based on a GDT is proposed, which benefits from the high β achievable in magnetic mirrors. Axial symmetry also makes the GDT neutron source more maintainable and reliable, and technically simpler. This review discusses the results of a conceptual design of the GDT-based neutron source which can be used for fusion materials development and as a driver of fission–fusion hybrids. The main physics issues related to plasma confinement and heating in a GDT are addressed by the experiments at the GDT device in Novosibirsk. The review concludes by updating the experimental results obtained, a discussion about the limiting factors in the current experiments and a brief description of the design of a future experimental device for more comprehensive modeling of the GDT-based neutron source. Conceivable approaches to improvement of plasma confinement in a GDT are also considered which would allow to consider the concept application in a fusion reactor.
Waves, Turbulence, and Transport in Weakly Collisional, High-Beta Plasmas
Matt Kunz, Princeton University
Many space and astrophysical plasmas are so hot and dilute that they cannot be rigorously described as fluids. These include the solar wind, low-luminosity black-hole accretion flows, and the intracluster medium of galaxy clusters. All of these plasmas are magnetized and weakly collisional, with plasma beta parameters of order unity or even much larger ("high-beta"). In this regime, deviations from local thermodynamic equilibrium ("pressure anisotropies") and the kinetic instabilities they excite can dramatically change the material properties of such plasmas and thereby influence the macroscopic evolution of their host systems. This talk outlines an ongoing programme of kinetic calculations aimed at elucidating from first principles the physics of waves, turbulence, and transport under these conditions. Three key results will be featured. (1) Shear-Alfvén waves "interrupt" themselves at sufficiently large amplitudes by adiabatically driving a field-biased pressure anisotropy that both nullifies the restoring tension force and excites a sea of ion-Larmor-scale instabilities. (2) Ion-acoustic waves in a collisionless, high-beta plasma similarly excite Larmor-scale instabilities, which ultimately aid the waves' propagation by rendering the plasma more fluid-like and, therefore, incapable of Landau damping. (3) Pressure anisotropy generated either by turbulent fluctuations or by global expansion (as in the solar wind) qualitatively change the properties of magnetized turbulence, affecting plasma heating and the so-called "critical balance". Contact with observations of the near-Earth solar wind and the intracluster medium of galaxy clusters will be made.
Plasma Modelling in Support of the STEP Fusion Reactor Programme, video
Howard Wilson, University of York, UK
STEP – the Spherical Tokamak for Electricity Production – aims to deliver a prototype power plant by 2040 that will deliver net electricity at the 100MW level. This is a challenging timescale, that will require disruptive changes to how we design and regulate. The plasma scenario presents some of the biggest challenges, and this talk will discuss some of them. A spherical tokamak plasma has some advantages over one at conventional aspect ratio, allowing access to high elongation and beta (ratio of thermal plasma pressure to magnetic field pressure), in a compact geometry. Therefore, while much of the physics in the two designs is similar, there are also key differences. For example, the compact nature means there is little space for a solenoid, so non-inductive current drive is essential; the low magnetic field and high density require novel radio frequency methods for this current drive; the high beta affects the micro-instabilities that drive plasma turbulence and influence confinement; magnetohydrodynamic instabilities must be controlled to limit disruptions while achieving high fusion power and bootstrap current fraction; novel systems are required to manage the exhaust power loads, especially for the inner divertor leg. This talk will explore progress and challenges in modelling these key physics issues in support of STEP.
Progress and Challenges in TAE's Quest Towards an FRC-Based Fusion Reactor
Artem Smirnov, TAE Technologies, Inc
TAE Technologies, Inc. (TAE) is a privately funded company pursuing a novel approach to magnetic confinement fusion, which relies on Field-Reversed Configuration (FRC) plasmas composed of mostly energetic and stable particles. This advanced FRC-based system simplifies the reactor design and could offer a path forward to clean, safe, and economical aneutronic p-B11 fusion. To validate the science behind the FRC-based approach to fusion, an active experimental program is underway at TAE's state-of-the-art plasma research facility in Orange County, California. The core of the facility is the world's largest FRC device named Norman. In Norman, tangential injection of variable energy neutral beams (15 – 40 keV hydrogen, up to 20 MW total), coupled with plasma edge biasing, active plasma control, and advanced surface conditioning, led to production of steady-state, hot FRC plasmas dominated by fast ion pressure. High-performance, advanced beam-driven FRCs were produced,1-4 characterized by (1) macroscopic stability, (2) steady-state plasma sustainment, and (3) dramatically reduced transport rates (more than an order of magnitude improvement over conventional FRCs). Collectively, these accomplishments represent a strong argument validating the FRC-based approach to fusion power. This talk will provide a comprehensive overview of the TAE experimental program.
Astrophysical collisionless shock formation and nonthermal electron acceleration in laboratory experiments
Hye-Sook Park, Lawrence Livermore National Laboratory, USA
Collisionless shocks are ubiquitous in astrophysical environments such as in supernova remnants, jets in active galactic nuclei and gamma ray bursts and are known to be responsible for cosmic ray acceleration. While the theory of diffusive, or Fermi, shock acceleration (DSA) is well-established, the plasma microphysics responsible for the generation of the shocks, the nature of their resulting magnetic turbulence residue, and the injection of particles into DSA is not yet well understood. With the advent of high-power lasers, laboratory experiments with high-Mach number, collisionless plasma flows can provide critical information to help understand the mechanisms of shock formation by plasma instabilities and self-generated magnetic fields. A series of experiments were conducted on Omega and the National Ignition Facility to observe: the filamentary Weibel instability that seeds microscale magnetic fields [1, 2]; collisionless shock formation (seen by an abrupt ~4x increase in density and ~6x increase in temperature); and electron acceleration distributions that deviated from the thermal distributions [3]. Experimental results along with theoretical interpretations aided by particle-in-cell simulations will be discussed. [1] H.-S. Park et al., High Energy Density Phys. 8, 38 (2012); [2] C. Huntington et al., Nat. Phys. 11, 173 (2015); [3] F. Fiuza et al., Nat. Phys. 16, 916 (2020).
Thermonuclear Fusion in an Equilibrium Z Pinch
Uri Shumlak, University of Washington, USA and Zap Energy, Inc.
The equilibrium Z pinch is a novel approach to magnetic confinement fusion because it does not rely on external magnetic field coils. Equilibrium conditions are reached through the use of sheared plasma flows, which enhance stability and provide a path to thermonuclear fusion. Simple geometry and strong scaling of fusion gain with pinch current form the cornerstones of this compact fusion device. The sheared-flow-stabilized Z pinch has been developed through integrated computational and experimental investigations at the University of Washington in collaboration with Lawrence Livermore National Laboratory. Experimental results demonstrate plasma stabilization, sustained thermonuclear fusion, and agreement with theoretical and computational predictions. Building on these advances, Zap Energy Inc. is developing a low-cost fusion reactor core based on the equilibrium Z pinch.
Plasma Physics Challenges on the Road to a Tokamak DEMOnstration Fusion Power Plant
Hartmut Zohm, Max Planck Institute for Plasma Physics, Germany
In the EU Roadmap to Fusion Electricity, DEMO is the step between ITER and a commercial power plant. It is supposed to generate net electricity and have a self-sufficient fuel cycle. The pre-conceptual studies carried out for a tokamak-based DEMO show that the plasma scenario cannot be simply transferred from the ITER Q=10 scenario. We will discuss the physics issues encountered and possible solutions how to overcome them. The focus of the talk will be on the plasma physics aspects, not on reactor integration, and therefore meant to be exciting for plasma physicists from all fields.
Random flows and rotation in galactic coronae
Anvar Shukurov, Newcastle University, UK
After a review and summary of the observational evidence for random flows and rotation at large altitudes (1-10 kpc) above the discs of spiral galaxies, I discuss the physical parameters of the off-planar gas and the energy sources of the random motions. I argue that the effects of the turbulent viscosity on large-scale gas flows are significant and propose an exploratory model of the viscous coupling of the rotating galactic disc and the corona.
Understanding the Building Blocks of the Solar Wind and How They Fit Together: Heat Flux, Radiation, and Alfven-Wave Turbulence
Benjamin Chandran, University of New Hampshire, USA
A growing body of evidence suggests that the solar wind is powered to a large extent by an Alfven-wave energy flux that is generated by convective motions on the surface of the Sun. The solar wind and solar corona are also affected by the flux of heat, including conductive losses into the radiative lower solar atmosphere. Numerical simulations that account for the above physics are increasingly able to reproduce remote observations of the corona and solar wind. On the other hand, we still lack an analytic theory that provides formulas for key quantities such as the solar mass-loss rate. Analytic treatments are needed for several reasons. They deepen our understanding by distilling complex processes into their most essential elements, they show how different quantities scale with one another, and they encapsulate our understanding into a portable form that can be applied to other systems and used by anyone. In this talk, I will present a recently developed analytic theory of coronal heating and solar-wind acceleration that provides analytic formulas and intuitive explanations for the solar mass-loss rate, the solar-wind speed far from the Sun, the coronal temperature, the heat flux from the corona into the lower solar atmosphere, and the plasma density at the base of the corona.
Drift kinetic theory of alpha particle transport by tokamak perturbations
Elizabeth Tolman, Institute for Advanced Study, USA
Upcoming deuterium-tritium tokamak experiments are expected to have large energetic alpha particle populations. These experiments can be used to study the interaction between these alpha particles and perturbations to the tokamak's electric and magnetic fields. In this talk, I will first describe why this behavior is important and interesting. Then, I will discuss a new drift kinetic theory to calculate the alpha heat flux resulting from a wide range of perturbation frequencies and periodicities. This theory suggests that the alpha heat flux caused by toroidal field ripple, one type of perturbation, is small. Applied to the toroidal Alfvén eigenmode (TAE), another type of perturbation, the theory suggests a significant alpha heat flux that scales with the square of the TAE amplitude. The TAE amplitude calculated from one saturation condition suggests that TAEs in SPARC, one upcoming deuterium-tritium experiment, will not cause significant alpha transport via the mechanisms in this theory. However, saturation above the level suggested by the simple condition, but within numerical and experimental experience, could cause significant transport.
Exploring the physics of turbulent collisionless shocks in conditions of laboratory experiments
Anna Grassi, Sorbonne University, Paris, France
Collisionless shocks are ubiquitous in astrophysical plasmas and play an important role in magnetic field generation/amplification and particle acceleration. While diffusive shock acceleration (DSA) is well established, the details of particle injection into DSA remain a long-standing puzzle, particularly for electrons. High-energy-density (HED) plasma experiments and kinetic plasma simulations offer a promising route to identify the dominant processes at play. Very recently experiments performed at the National Ignition Facility have observed for the first time the formation of high-Mach number collisionless shocks mediated by electromagnetic instabilities and nonthermal electron acceleration. I will discuss the physics behind shock formation and particle acceleration in these laboratory systems and how they can be connected to astrophysical models. Using large-scale, multi-dimensional particle-in-cell (PIC) simulations, we find that the inhomogeneous profiles of laser-ablated plasmas lead to shock formation that can be up to 10 times faster than previous models predicted. The shock front can also develop strong corrugations at the ion gyroradius scale, which can be controlled by changing the electron temperature of the flow. Finally, we show that electrons can be effectively accelerated to nonthermal energies and injected into DSA via a Fermi-like mechanism occurring within the finite, turbulent shock transition. These findings can help guide the development and interpretation for current experimental programs and open exciting prospects for studying the microphysics of turbulent collisionless shocks in the laboratory.
When will we be able to predict plasma confinement in fusion devices?
Frank Jenko, IPP Garching, Germany
It is a key goal of magnetic confinement fusion (MCF) research to develop and build devices that allow us to create a plasma at sufficiently high pressure and energy confinement time, so that Lawson's criterion for a burning plasma can be met. There was breathtaking progress along these lines between the 1970s and 1990s, largely based on a "trial-and-error" approach. With the preparation of ITER operation and attempts to design first versions of future fusion power plants, it became clear, however, that a more targeted "predict-first" approach is needed at this point to save significant amounts of time and resources in the further development of fusion energy. Fortunately, the power of High Performance Computing keeps growing at a remarkable speed, with exascale systems around the corner. These platforms open up new possibilities to solve the complex nonlinear equations underlying many observed phenomena in MCF plasmas, and to move from an interpretative to a truely predictive approach. In this context, computing, data analysis, and machine learning are increasingly intertwined to provide reliable predictions. So how and when will we be able to predict plasma confinement in fusion devices? This question will be at the heart of this presentation.
Some open questions in the plasma physics of cosmic rays
Ellen Zweibel, University of Wisconsin, Madison, USA
It's now widely recognized that cosmic rays have considerable influence on the dynamics and energy balance of thermal gas in and around galaxies. While it's their collisional interactions that render cosmic rays most directly visible, in many respects their collisionless interactions, mediated by kinetic scale plasma waves and instabilities, are more significant. These microscale interactions are now captured by fluid models and used in large scale simulations of galaxy formation and evolution, where they are revealing surprising behavior. After briefly reviewing the basics, I will discuss some open questions in the microphysics of cosmic ray - thermal gas coupling, and how the large scales react back on the small ones.
For a full list of past colloquia, see the JPP Frontiers of Plasma Physics Colloquium webpage.
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Research & Review Seminars
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