Characterization of the plasma plume in the current overrun regime of cylindrical Hall thrusters

Cylindrical Hall thrusters (HTs)\footnote{Y.~Raitses and N.~J.~Fisch, \emph{Phys.~Plasmas} 8, 2579 (2001)} may be more promising than annular HTs for low-power scaling due to a smaller surface-to-volume ratio. High plasma plume divergence is a main drawback to cylindrical HTs as decreased efficiency and spacecraft integration issues may result. Recent measurements of the plume angle show that overrunning the discharge current above its self-sustained value can significantly decrease plume divergence. In this ``current-overrun'' regime, the half-plume angle of the cylindrical HT was reduced to $55^\circ$. Thrust measurements demonstrate that the current-overrun regime can have an anode efficiency of up to 35--40\% at 100--200~W discharge power levels: an improvement of over 60\%. Measurements of the ion energy distribution function in the plasma plume using a retarding potential analyzer reveal both increased ion current density and ion energy on-axis, indicating that these ions are ionized in a region of higher plasma potential. Also, the average energy of off-axis ions is substantially reduced, resulting in improved performance and lowered risk of damage to spacecraft components.