月崎 竜童

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.

Ionic liquid electrospray thrusters represent an alternative propulsion method for spacecraft to conventional plasma propulsion because they do not require plasma generation, which significantly increases the thrust efficiency. The porous emitter thruster has the advantages of simple propellant feeding and multi-site emissions, which miniaturize the thruster size and increase thrust. However, the multi-scale nature, that is, nano- to micrometer-sized menisci on the millimeter-size porous needle tip, makes modeling multi-site emissions difficult, and direct observation is also challenging. This paper proposes a simple model for multi-site emissions, which assumes that the ionic conductivity or ion transport in the porous media determines the ion-emission current. The conductivity was evaluated by comparing the experimental and numerical data based on the model. The results suggest that the ionic conductivity of the porous emitter is suppressed by the ion–pore wall friction stress. Additionally, the model indicates that the emission area expansion on the porous emitter creates the unique curve shape of the current vs voltage characteristics for multi-site emissions.

An understanding of the degradation mechanism of a microwave discharge cathode is the key to extending the lifetime of microwave ion thruster systems. This study investigates the effect of nozzle contamination by sputtered Ag-polytetrafluoroethylene (PTFE) on microwave discharge cathode performance. The current–voltage characteristics were measured for nominal and contaminated (by PTFE spray with 0.2 µm thick or tape with 0.15 mm thick) cathodes. The contamination thickness and area on the nozzle were varied to investigate the characteristic differences. It was confirmed that the anode voltage increased by 20 V or more in the case of the contaminated cathode. The anode voltage was measured for the sputter-contaminated cathode to evaluate the effect of contamination under more realistic conditions. After 630 h of sputter-contamination operation, it is estimated that sputtered particles were deposited to a thickness of 77 µm at most, and the anode voltage increased by 8 V. The results show that the downstream surface of the nozzle is critical for maintaining cathode performance. The insulating coating formed by the sputtered PTFE may interfere with ion absorption and degrade electron emission capability. A theoretical model based on the extended Brophy model supports these results. This study provides important information for the use of PTFE-based materials around ion thrusters.