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

When used in flowing electrostatically accelerated plasmas, electrostatic e nergy analyzers, such as Retarding Potential Analyzers (RPA's) or Cylindric al 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 ava ilable through the electrostatic discharge. The process responsible for thi s phenomenon is resonant charge-exchange (CEX) collisions between ions of d ifferent charge species. Through the transfer of electrons, an ion of charg e n(+) can appear in an electrostatic analyzer with equivalent voltage of n -times the available acceleration potential. This paper discusses the pheno mena responsible for the appearance of such high-voltage ions in the measur ed voltage distribution function and presents evidence for such reactions i n the flowing xenon plasma produced by a Hall-effect current accelerator de signed for spacecraft propulsion. For a 300 V applied potential on the Hall accelerator electrodes, CEX collisions are shown to produce ions having vo ltages as high as 800 V when measured using an electrostatic analyzer. (C) 1999 American Institute of Physics. [S1070-664X(99)02307-1].