Performance comparison between standard and magnetically shielded 200 W Hall thrusters with BN-SiO2 and graphite channel walls

Magnetic shielding is a specific magnetic topology that can increase the lifespan of Hall thrusters by an order of magnitude. Magnetically shielded thrusters have similar performances as standard unshielded thrusters in the 5-15 kW power range. A comparison of the performances of two 200 W shielded (ISCT200-MS) and unshielded (ISCT200-US) Hall thrusters is presented here. The effects of replacing the usual BN-SiO2 walls with graphite are investigated on both thrusters. The ISCT200-US thruster has a peak anode efficiency of 39% and a specific impulse of 1400 s at 250 W with the ceramic discharge channel. However with graphite wall, the discharge current increases by 25% which reduces the efficiency down to 31% (1360 s specific impulse). The ISCT200-MS performances are significantly lower than the ISCT200-US at similar operating points. It only reaches 24% anode efficiency and 1020 s specific impulse at 250 W. The switch to graphite has little effects on its performances below 300 V. The lower performances are due to a low propellant utilization. We propose that this low propellant utilization is caused by lower ionization near the walls which as a large impact in small thrusters due to their higher surface to volume ratio.