Ion Acoustic Turbulence and Ion Energy Measurements in the Plume of the HERMeS Thruster Hollow Cathode

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Hollow cathodes serve as the electron source in ion and Hall thrusters. One of the life limiting factors of the propulsion system is cathode failure due to erosion from high energy ion bombardment. Despite the successful application of hollow cathodes on commercial and deep-space missions, the fundamental physical processes of erosion are not fully understood, particularly the source of the high energy ions. A recent experimental study of the near-plume in a high current hollow cathode confirmed the existence of ion acoustic turbulence (IAT), a phenomenon that was only previously suspected and modeled in numerical simulations. Theoretical analyses and turbulence measurements in the plume established that ion acceleration due to turbulence could explain the existence of high energy ions. This paper is a continuation of this work, focusing on detecting and quantifying instabilities in the cathode near-plume under conditions relevant for the thruster being developed for the proposed Asteroid Robotic Redirect Mission (ARRM). Ion acoustic turbulence and ion energy measurements were taken at the nominal discharge currents and cathode flow rates expected for ARRM, as well as off-nominal conditions to determine where it might be susceptible to erosion by high energy ions. Dual ion saturation probes were used to measure fluctuations in ion saturation current and assess the wave dispersion relation. A retarding potential energy analyzer was used to measure ion energy distribution functions for ions with velocities perpendicular to the plume flow. The effect of cathode orifice size on the onset of IAT has also been studied. Peaks in wave amplitude were found at extremely low flow rates and high discharge currents, where instabilities other than IAT dominate. In addition, the onset of IAT was found to occur at discharge currents as low as 35 A. Measurements also confirm that at high discharge currents above 30 A, a larger orifice size reduces the magnitude of turbulence. However, large orifices are more susceptible to low frequency instabilities at flow rates below 10 sccm.