Time-resolved Ion Energy Measurements Using a Retarding Potential Analyzer for Electric Propulsion Applications

To completely characterize the evolving state of a plasma, diagnostic tools which enable measurements of time-resolved behavior are required. In this study a gridded ion source with superimposed oscillations was utilized to verify the functionality of a high-speed retarding potential analyzer (HSRPA), at frequencies equivalent to the low frequency oscillations occurring in Hall effect thrusters (HETs). The verification of this device provides an effective alternative to existing diagnostics for measuring time-resolved ion energies. Due to the inherent simplicity of retarding potential analyzers (RPAs), they have found wide ranging applications across numerous fields.

The work presented demonstrates the capability of a standard RPA in conjunction with high-speed circuitry and data fusion techniques to produce time-resolved IEDFs at higher frequencies and beam potentials than have been previously investigated. Tested frequencies ranged between 20 – 80 kHz with 10 V_pp oscillations at a mean beam potential of 570 V. Additionally, measurements were conducted with several waveforms, functioning as the superimposed oscillation on a Kauffman-style gridded ion source. Utilizing data fusion techniques, we successfully reconstructed the time series data and produced time-resolved IEDFs at all frequency set points and waveforms. This investigation has demonstrated the HSRPA effectiveness at producing time-resolved measurements at conditions similar to those occurring in HETs.