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Particle accelerators so far have relied on electric fields generated by radio waves to accelerate electrons and other particles close to the speed of light. But radio-frequency machines have an upper limit of tens of MeV (millions of electronvolts) per meter of beamline. At higher energies, the RF electric fields can break down and could even generate enough heat to melt accelerator components. So the only way to get more power from an RF accelerator is to build a longer, more expensive beamline.
Experiments indicate that plasma wakefield machines could generate tens of billions of electron volts per meter—as much as 1000 times more acceleration potential per length of accelerator—allowing smaller accelerators of tremendous power. Such a system would use speeding electrons or a laser pulse to create a charge "wake" in a sea of ionized gas, or plasma. Like a surfer on a good wave, particles would ride this plasma wake to greater and greater speeds. But there are technical challenges to overcome before tabletop accelerators and plasma-driven turbo-chargers for larger accelerators can become a reality. Scientists must determine how to accelerate beams of particles—including electrons and their antimatter counterpart, positrons—that are suitable for a future collider.
A plasma accelerator replaces conventional static metallic (copper or niobium) accelerating structures with a structure that is formed dynamically with a tube of plasma. The basic concept of the plasma wakefield accelerator involves the passage of a near-lightspeed electron bunch through a stationary plasma. The plasma can be formed by ionizing a gas with a laser or through field-ionization by the incoming electron bunch itself. This second method allows the production of meter-long, dense plasmas suitable for plasma wakefield acceleration, and greatly simplifies the experimental setup. In single-bunch experiments, the head of the bunch creates the plasma and drives a "wake" of charge. The wake is driven out of the path of the incoming electrons, creating a charge imbalance that pulls it crashing back in, behind the passing electron bunch. This effect produces a strong field that accelerates particles in the back of the bunch. The system effectively operates as a transformer, where the energy from the particles in the head is transferred to those in the back, through the plasma wake. The physics is similar if there are two bunches rather than one; energy from the leading "drive" bunch is transferred to a trailing "witness" bunch.
SLAC National Accelerator Laboratory, Menlo Park, CA
Operated by Stanford University for the U.S. Dept. of Energy