Identifying charge-exchange collision products within the ion-energy distribution of electrostatically accelerated plasmas

When used in flowing electrostatically accelerated plasmas, electrostatic energy analyzers, such as Retarding Potential Analyzers (RPA’s) or Cylindrical Mirror Analyzers (CMA’s), occasionally yield data which seem implausible: for a known applied plasma acceleration voltage, electrostatic analyzers may indicate populations of ions having voltages much greater than that available through the electrostatic discharge. The process responsible for this phenomenon is resonant charge-exchange (CEX) collisions between ions of different charge species. Through the transfer of electrons, an ion of charge $n+$ can appear in an electrostatic analyzer with equivalent voltage of n-times the available acceleration potential. This paper discusses the phenomena responsible for the appearance of such high-voltage ions in the measured voltage distribution function and presents evidence for such reactions in the flowing xenon plasma produced by a Hall-effect current accelerator designed for spacecraft propulsion. For a 300 V applied potential on the Hall accelerator electrodes, CEX collisions are shown to produce ions having voltages as high as 800 V when measured using an electrostatic analyzer.