Energetics of Propellant Options for High-Power Hall Thrusters

Krypton, cadmium, iodine, cesium, mercury, and bismuth are compared with xenon in several areas of performance, including thrust, specific impulse, probability of ionization, maximum theoretical efficiency, and sputter yield. The lighter propellants such as krypton and cadmium are favorable for high-4P, low-thrust applications, whereas heavier propellants such as mercury and bismuth are preferable for low-4p, high-thrust missions. Calculations of the ionizing collision rate show that cesium had the highest ionization probability (and lowest ionization energy), krypton had the lowest probability (and highest ionization energy), with the other propellant falling between cesium and krypton. Sputter erosion calculations show that for a carbon surface, heavier atoms will sputter less at low ion energies (less than 2000 eV) than light atoms,, and will sputter much less on a kilograms-per-kilogram basis.