Ryudo Tsukizaki

Backflowing ions generated by electric thrusters are known to cause sputtering and surface degradation on spacecraft, which may ultimately shorten mission lifetimes. In this study, velocity measurements for metastable xenon ions were conducted near the cathode plume of the microwave discharge ion thruster μ10 using the laser-induced fluorescence technique. In addition, a spatial distribution of plasma potential was measured by an emissive probe. The ion velocity distribution functions showed different features depending on the measurement position. The mean velocity of the drifting population at each measurement position was visualized as a velocity field. A comparison of the velocity field with the plasma potential distribution or ion number density distribution revealed a diverging center that coincided with the cathode plume, suggesting that a certain number of backflow ions originated from the cathode plume region. In particular, high-energy ions were observed between the cathode plume and the spacecraft surface. Asymmetric features were found in both the plasma potential distribution and the ion number density distribution, which implies that the effect of the magnetic field is not negligible in this region.

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 (IL) electrospray thrusters offer a promising space propulsion solution for small spacecraft; however, their performance is limited by variability in ion emission current. Understanding these variations is critical for the reliability of the thrusters. This study focuses on passively fed porous emitters, which are commonly used for their effective IL transport via capillary action without active flow control. Nonetheless, such configurations often suffer from poor repeatability in ion emission current versus applied voltage ( I – V ) characteristics, known as I – V variability. We systematically investigated the trends and contributing factors to I – V variability by identifying the emitter filling state—linked to the surface pressure on the porous emitter—as a key driver. We also examined the impact of emitter geometry and found that increased emitter tip curvature reduces the threshold electric field, thereby affecting the I – V characteristics. These findings provide valuable insights into the mechanisms driving emission variability and offer guidance for the design optimisation of passively fed IL electrospray thrusters.

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.

The Hayabusa2 spacecraft collected samples from the surface of the carbonaceous near-Earth asteroid (162173) Ryugu and brought them to Earth. The samples were expected to contain organic molecules, which record processes that occurred in the early Solar System. We analyzed organic molecules extracted from the Ryugu surface samples. We identified a variety of molecules containing the atoms CHNOS, formed by methylation, hydration, hydroxylation, and sulfurization reactions. Amino acids, aliphatic amines, carboxylic acids, polycyclic aromatic hydrocarbons, and nitrogen-heterocyclic compounds were detected, which had properties consistent with an abiotic origin. These compounds likely arose from an aqueous reaction on Ryugu’s parent body and are similar to the organics in Ivuna-type meteorites. These molecules can survive on the surfaces of asteroids and be transported throughout the Solar System.

Samples of the carbonaceous asteroid (162173) Ryugu were collected and brought to Earth by the Hayabusa2 spacecraft. We investigated the macromolecular organic matter in Ryugu samples and found that it contains aromatic and aliphatic carbon, ketone, and carboxyl functional groups. The spectroscopic features of the organic matter are consistent with those in chemically primitive carbonaceous chondrite meteorites that experienced parent-body aqueous alteration (reactions with liquid water). The morphology of the organic carbon includes nanoglobules and diffuse carbon associated with phyllosilicate and carbonate minerals. Deuterium and/or nitrogen-15 enrichments indicate that the organic matter formed in a cold molecular cloud or the presolar nebula. The diversity of the organic matter indicates variable levels of aqueous alteration on Ryugu’s parent body.