Energetic backflow ions caused by microwave neutralizer in gridded ion thruster

The Hayabusa2 spacecraft is equipped with four 10-cm-class microwave discharge ion thrusters (μ10). Onboard quartz crystal microbalance measurements have indicated surface erosion due to ion thruster operation. In this paper, the ion energy distributions (IEDs) of backflow ions were measured at several azimuthal positions around the ion source using retarding potential analyzers in a vacuum chamber. The typical IED had a peak at approximately 20 eV for all azimuthal positions. The IEDs at the high-energy tail (>40 eV), which greatly affects the erosion rate, strongly depend on the azimuthal position relative to the neutralizer position. Furthermore, IEDs were characterized under various operational conditions, including variations in neutralizer operation mode, background pressure, neutralizer gas flow rate, and neutralizer emission current. The results show that high-energy ions appeared only in the presence of a neutralizer plasma column. An increase in background pressure led to an increase in the ion population below 40 eV but a decrease in the ion population above 40 eV. Additionally, increasing the neutralizer gas flow rate suppressed the high-energy ion population, whereas increasing the neutralizer emission current enhanced it. These findings indicate that ions with energies below 40 eV are predominantly generated through charge exchange processes in the ion beam, whereas those above 40 eV are generated due to the neutralizer plasma column.