Aerospace Leads Effort to Create Standard on RF Breakdown
Advances in solar cell and power system technology have been a double-edged sword for space systems. On the one hand, the increase in available power has enhanced the capabilities of satellites using radio frequency (RF) transmissions; on the other hand, the high voltages that form in the presence of these RF fields can give rise to a destructive phenomenon known as RF breakdown, which can melt, short, or degrade sensitive components.
As explained by Preston Partridge of the Antenna Systems Department and Aimee Hubble of the Propulsion Systems Department, RF breakdown refers to two related phenomena: multipactor breakdown and ionization breakdown. “Multipactor is a resonant electron breakdown that requires a vacuum (or near vacuum) inside the RF component, whereas ionization refers to a plasma breakdown created from gas or other vaporized material internal to the component,” Partridge said.
Multipactor breakdown is a function of the frequency, geometry, and intensity of an electric field. When free electrons within an electrode gap are accelerated in the presence of an alternating RF field, the subsequent collisions can release more electrons in a cascade reaction, which can form a plasma. “The components most often affected are those that see high-power RF,” said Partridge. “This includes high-power amplifiers and all the components downstream — isolators and circulators used to protect amplifier outputs, switches and combining/dividing networks used to route the signals, filters and multiplexers used for managing the frequencies of operation, and antennas used to radiate or receive the RF signals.”
To avoid permanent damage, space system designers and operators face difficult choices. “One solution is to completely fill the device with a dielectric (insulating material), but this often comes with performance tradeoffs, and either cannot be done or results in reduced capability,” Partridge said. “Another solution is to reduce the power level used on the spacecraft — again, this results in reduced capability.”
As one of the few organizations in the country with lab facilities dedicated to studying RF breakdown, Aerospace has a unique understanding of both the phenomenon itself and its engineering impacts. In 2014, Aerospace convened a special panel at the Mission Assurance Improvement Workshop to address the issue. This meeting brought together industry and government to generate consensus for a comprehensive standard that incorporates best practices. “The MAIW process involves gathering representatives and subject matter experts from across the industry and meeting several times over the year to produce a final document,” Partridge said. “The committee members were drawn from suppliers, prime contractors, FFRDCs, and academic institutions.”
The standard delineates a variety of new tests that can identify problems early, preventing expensive failures late in production or on orbit. “The best way to mitigate RF breakdown is to address the risk in the design phase. If a component is found to be susceptible to breakdown late in the development phase, it can be difficult to mitigate,” Partridge said. The standard also establishes new bounding test cases that enable programs to confidently remove excessive margin from their designs, which can result in large cost savings. The standard also accounts for new numerical analysis techniques that can, in certain cases, eliminate the need for expensive qualification and acceptance testing.
AIAA Adopts Standard
The American Institute of Aeronautics and Astronautics (AIAA) recently adopted “Standard/Handbook for Multipactor Breakdown Prevention in Spacecraft Components” as ANSI/AIAA S-142-2016. The document covers worst-case conditions, margin requirements, and verification of those requirements using state-of-the-art methodologies. It also recommends methods, with examples, to ensure proper requirement verification for all satellite components susceptible to RF breakdown. “The standard is intended to provide a baseline of minimum requirements for component verification,” said Partridge. “It is meant to take into account known system-level uncertainties and the application of modern test and analysis techniques when considering margin and verification requirements.” A second standard, which focuses on ionization breakdown, is also under development.
Programs are already seeing improvements. For example, JPL’s Surface Water and Ocean Topography (SWOT) program recently caught a design issue in one of its prototypes, enabling the problem to be corrected before building the actual flight hardware. “For programs adopting the standard in the beginning, the risk of RF breakdown may be addressed with minimal impact to cost and schedule,” Partridge said.