基本情報
- 所属
- 国立研究開発法人宇宙航空研究開発機構 宇宙科学研究所 准教授
- 学位
- 博士(工学)(2013年3月 東京大学)
- 研究者番号
- 70720697
- ORCID ID
https://orcid.org/0000-0002-6265-1672- J-GLOBAL ID
- 202001010789612597
- researchmap会員ID
- R000013995
経歴
7-
2021年11月 - 現在
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2013年8月 - 2021年10月
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2016年9月 - 2017年10月
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2013年4月 - 2013年7月
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2010年4月 - 2013年3月
学歴
4-
2010年4月 - 2013年3月
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2008年4月 - 2010年3月
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2004年4月 - 2008年3月
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2000年4月 - 2003年3月
委員歴
3-
2023年4月 - 現在
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2022年4月 - 2024年3月
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2019年4月 - 2020年3月
受賞
15-
2024年5月
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2021年4月
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2020年12月
論文
72-
Journal of Applied Physics 135(24) 2024年6月27日 査読有り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.
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AIP Advances 14(6) 2024年6月1日 査読有り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.
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Acta Astronautica 2024年5月 査読有り
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Vacuum 220 112760-112760 2024年2月 査読有り
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Acta Astronautica 212 130-138 2023年11月 査読有り
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Journal of Evolving Space Activities 1 21 2023年6月 査読有り
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Journal of Evolving Space Activities 1 2023年4月 査読有り
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Science (New York, N.Y.) 379(6634) eabn7850 2023年2月24日 査読有りCarbonaceous meteorites are thought to be fragments of C-type (carbonaceous) asteroids. Samples of the C-type asteroid (162173) Ryugu were retrieved by the Hayabusa2 spacecraft. We measured the mineralogy and bulk chemical and isotopic compositions of Ryugu samples. The samples are mainly composed of materials similar to those of carbonaceous chondrite meteorites, particularly the CI (Ivuna-type) group. The samples consist predominantly of minerals formed in aqueous fluid on a parent planetesimal. The primary minerals were altered by fluids at a temperature of 37° ± 10°C, about [Formula: see text] million (statistical) or [Formula: see text] million (systematic) years after the formation of the first solids in the Solar System. After aqueous alteration, the Ryugu samples were likely never heated above ~100°C. The samples have a chemical composition that more closely resembles that of the Sun's photosphere than other natural samples do.
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Science (New York, N.Y.) 379(6634) eabo0431 2023年2月24日 査読有りThe near-Earth carbonaceous asteroid (162173) Ryugu is expected to contain volatile chemical species that could provide information on the origin of Earth's volatiles. Samples of Ryugu were retrieved by the Hayabusa2 spacecraft. We measured noble gas and nitrogen isotopes in Ryugu samples and found that they are dominated by presolar and primordial components, incorporated during Solar System formation. Noble gas concentrations are higher than those in Ivuna-type carbonaceous (CI) chondrite meteorites. Several host phases of isotopically distinct nitrogen have different abundances among the samples. Our measurements support a close relationship between Ryugu and CI chondrites. Noble gases produced by galactic cosmic rays, indicating a ~5 million year exposure, and from implanted solar wind record the recent irradiation history of Ryugu after it migrated to its current orbit.
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Science (New York, N.Y.) 379(6634) eabn9057 2023年2月24日 査読有り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.
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Science (New York, N.Y.) 379(6634) eabn9033 2023年2月24日 査読有り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.
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Nature astronomy 7(2) 170-181 2023年 査読有りWithout a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe3+ to Fe2+ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (-OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss.
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Journal of Electric Propulsion 1(1) 2022年12月 査読有り招待有りAbstract In electron cyclotron resonance (ECR) thrusters, the plasma mode transition is a critical phenomenon because it determines the maximum thrust performance. In ECR ion thrusters, ionization generally occurs in the magnetic confinement region, where electrons are continuously heated by ECR and confined by magnetic mirrors. However, as the flow rate increases, ionization is also observed outside the magnetic confinement region, and this induces the plasma mode transition. In our previous work, two-photon absorption laser-induced fluorescence (TALIF) analysis revealed that the stepwise ionization from the metastable state plays an important role in the ionization process. However, the distribution of the stepwise ionization has not yet been revealed because of the long lifetime of the metastable state. In this study, this distribution was investigated using one experimental and two numerical approaches. First, TALIF was applied to two types of gas injection with clear differences in thrust performance and ground-state neutral density distribution. In the first simulation, the metastable state particle simulation was used to estimate the excitation rate distribution. In the second study, simulations of the electric field of microwaves were used to estimate the contribution of the stepwise ionization to the plasma density. The experimental and numerical results revealed that the stepwise ionization spreads outside the magnetic confinement region because of the diffusion of metastable particles, and this spread induces the plasma mode transition, explaining the difference between the two types of gas injection.
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Science advances 8(46) eabo7239 2022年11月18日 査読有りThe Hayabusa2 spacecraft returned to Earth from the asteroid 162173 Ryugu on 6 December 2020. One day after the recovery, the gas species retained in the sample container were extracted and measured on-site and stored in gas collection bottles. The container gas consists of helium and neon with an extraterrestrial 3He/4He and 20Ne/22Ne ratios, along with some contaminant terrestrial atmospheric gases. A mixture of solar and Earth's atmospheric gas is the best explanation for the container gas composition. Fragmentation of Ryugu grains within the sample container is discussed on the basis of the estimated amount of indigenous He and the size distribution of the recovered Ryugu grains. This is the first successful return of gas species from a near-Earth asteroid.
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NATURE ASTRONOMY 6(10) 1163-+ 2022年10月 査読有りVolatile and organic-rich C-type asteroids may have been one of the main sources of Earth's water. Our best insight into their chemistry is currently provided by carbonaceous chondritic meteorites, but the meteorite record is biased: only the strongest types survive atmospheric entry and are then modified by interaction with the terrestrial environment. Here we present the results of a detailed bulk and microanalytical study of pristine Ryugu particles, brought to Earth by the Hayabusa2 spacecraft. Ryugu particles display a close compositional match with the chemically unfractionated, but aqueously altered, CI (Ivuna-type) chondrites, which are widely used as a proxy for the bulk Solar System composition. The sample shows an intricate spatial relationship between aliphatic-rich organics and phyllosilicates and indicates maximum temperatures of -30 degrees C during aqueous alteration. We find that heavy hydrogen and nitrogen abundances are consistent with an outer Solar System origin. Ryugu particles are the most uncontaminated and unfractionated extraterrestrial materials studied so far, and provide the best available match to the bulk Solar System composition.
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Vacuum 200 110962-110962 2022年6月 査読有りIn gridded ion thrusters, it is generally known that the ion extraction from the grid significantly decreases the neutral density. However, because it is difficult to measure the density inside thruster, it has not been revealed how the local density decreases including the inhomogeneous effects (e.g., gas injection, ionization, and ion recombination). To investigate these inhomogeneous effects, two-photon laser-induced fluorescence (TALIF) and Direct Simulation Monte Carlo (DSMC) applied to a gridded microwave discharge ion thruster. First, the TALIF directly measured the density distribution. Second, the DSMC reproduced the measured density distribution. From these results, the main effect of the ion extraction is the decrease of ion recombination at the grid. This decrease makes the density distribution more sensitive to the ionization distribution and increases the inhomogeneity. Based on this knowledge, we propose improvement against 'simulated experiment without ion extraction' and 'clecoupled numerical simulation' which reproduce the density distribution of ion extraction under the condition without ion extraction by decreasing flow rate. Specifically, we suggest that the grid transparency is expanded and the rest discrepancy is adjusted by the flow rate because the difference of the inhomogeneous effect due to gas injection can be reduced.
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Science 375(6584) 1011-1016 2022年3月4日 査読有りThe Hayabusa2 spacecraft investigated the C-type (carbonaceous) asteroid (162173) Ryugu. The mission performed two landing operations to collect samples of surface and subsurface material, the latter exposed by an artificial impact. We present images of the second touchdown site, finding that ejecta from the impact crater was present at the sample location. Surface pebbles at both landing sites show morphological variations ranging from rugged to smooth, similar to Ryugu’s boulders, and shapes from quasi-spherical to flattened. The samples were returned to Earth on 6 December 2020. We describe the morphology of >5 grams of returned pebbles and sand. Their diverse color, shape, and structure are consistent with the observed materials of Ryugu; we conclude that they are a representative sample of the asteroid.
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Nature Astronomy 6(2) 214-220 2022年2月 査読有り<title>Abstract</title>C-type asteroids1 are considered to be primitive small Solar System bodies enriched in water and organics, providing clues to the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing2–7 and on-asteroid measurements8,9 with Hayabusa2 (ref. 10). However, the ground truth provided by laboratory analysis of returned samples is invaluable to determine the fine properties of asteroids and other planetary bodies. We report preliminary results of analyses on returned samples from Ryugu of the particle size distribution, density and porosity, spectral properties and textural properties, and the results of a search for Ca–Al-rich inclusions (CAIs) and chondrules. The bulk sample mainly consists of rugged and smooth particles of millimetre to submillimetre size, confirming that the physical and chemical properties were not altered during the return from the asteroid. The power index of its size distribution is shallower than that of the surface boulder observed on Ryugu11, indicating differences in the returned Ryugu samples. The average of the estimated bulk densities of Ryugu sample particles is 1,282 ± 231 kg m−3, which is lower than that of meteorites12, suggesting a high microporosity down to the millimetre scale, extending centimetre-scale estimates from thermal measurements5,9. The extremely dark optical to near-infrared reflectance and spectral profile with weak absorptions at 2.7 and 3.4 μm imply a carbonaceous composition with indigenous aqueous alteration, matching the global average of Ryugu3,4 and confirming that the sample is representative of the asteroid. Together with the absence of submillimetre CAIs and chondrules, these features indicate that Ryugu is most similar to CI chondrites but has lower albedo, higher porosity and more fragile characteristics.
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Journal of Applied Physics 131(1) 013301-013301 2022年1月7日 査読有り招待有りAn understanding of the plasma physics inside a microwave discharge cathode is key to extending the lifetime of microwave ion thruster systems. However, probes can only measure the plume region due to their low spatial resolution and electromagnetic disturbance. In this study, we develop a microwave discharge-based cathode with a small optical window in the discharge chamber that provides visual access to the cathode interior. The cathode has the same anode currents as those of a flight model in the diode mode (anode voltage error is within 7%). Laser-induced fluorescence spectroscopy is applied to the cathode. The axial and radial ion velocity distribution functions (IVDFs) in the plume region and the axial IVDFs inside the cathode are measured. The measured functions, which represent the number density of Xe II (P-3(2))6p[3](5/2), are compared to a previously reported number density of Xe II measured by an electrostatic probe in the plume region. The functions exhibit multimodal characteristics. Theoretical models based on the measured current oscillation support these characteristics.
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Review of Scientific Instruments 92(12) 129902-129902 2021年12月1日 査読有りThis article was originally published online on 9 November 2021 with an error in Eqs. (13) and (14). The equations are shown correctly below. All online and printed versions of the article were corrected on 10 November 2021.
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The Review of scientific instruments 92(11) 114501-114501 2021年11月1日 査読有りIn this study, a novel single-piece thin multi-layer tungsten resistive heater was successfully fabricated using additive manufacturing and tested as an electrothermal thruster. The heater has 12 resistive layers, with each layer having a thickness and height of 0.15 and 81 mm, respectively, and can provide high heating efficiency. A single-piece or monolithic heater was manufactured via additive manufacturing technique, which drastically improved its reliability and decreased its manufacturing cost. In the heating and thrust measurement tests that used nitrogen gas as a propellant, the heater reached a gas temperature of ∼2000 K at a 140-A heater current without experiencing any failure. The tungsten-heater resistance linearly increased with an increase in temperature due to the temperature dependence of tungsten's resistivity. The specific impulse and thrust increased with the heater temperature in accordance with the theoretical prediction. Even including a voltage drop due to a contact resistance, the achieved heater efficiency reached 63% at a 100-A heater current even without a thermal insulation around the thruster. The heater efficiency decreased with an increase in the heater temperature due to heat loss to the surroundings. The heat-loss analysis indicated that both thermal conduction and radiation heat losses were crucial for improving the heater performance at a high-temperature operation of over 2000 K.
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Investigation of plasma mode transition and hysteresis in electron cyclotron resonance ion thrustersPlasma Sources Science and Technology 30(9) 095023-095023 2021年9月1日 査読有りPlasma mode-transition and hysteresis have been reported in several electron cyclotron resonance (ECR) plasma sources and have also been observed in ECR ion thrusters. From observation outside the thruster, the optical emission is significantly changed after plasma mode-transition, which indicates the electron heating, excitation and ionization processes are also changed. Therefore, to investigate these processes quantitatively, the ground-state neutral density and spontaneous emission by electron impact excitation are directly measured using two-photon absorption laser-induced fluorescence spectroscopy. This measurement system is applied to two types of thrusters. In one thruster, both mode-transition and hysteresis were observed, while in another, no hysteresis was observed due to the partial prevention of ECR heating. The experimental results indicate that the spontaneous emission sharply increases by the mode-transition and hysteresis. The tendency for this increase was relatively small with the partial prevention of ECR heating; however, the mode-transition was not deleted. Analysis of the excitation and ionization process revealed that the increase of indirect (stepwise) excitation and ionization from metastable particles can contributes to the sharp increase of the spontaneous emission. In addition, quantitative estimation of the collisional and ECR heating indicated that collisional heating cannot be neglected after the mode-transition, which indicates that collisional heating can contribute to the sharp increase due to the increase of electron temperature. The indirect ionization and collisional heating could thus be the main cause of the mode-transition and hysteresis.
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Acta Astronautica 185 179-187 2021年8月 査読有りA cluster of gridded microwave discharge ion thrusters mu 10 contributed to the success of the deep space exploration missions of the Japanese asteroid explorers Hayabusa and Hayabusa2. To meet the demands of the next mission DESTINY+, the maximum ion beam current was increased to 200 mA from the 170 mA used in Hayabusa2 by redesigning the magnetic field and grid configuration. However, this thruster exhibited the plasma hysteresis caused by a high-voltage breakdown. Specifically, the ion beam current (i.e. thrust) has two modes at a given propellant flow rate, namely high- and low-current modes. In nominal operation, the thruster operates in high-current mode. However, once a high-voltage breakdown occurs, the ion beam current transitions to and remains in low-current mode, which significantly decreases thrust efficiency. In this paper, based on an investigation of the physical mechanism that causes the hysteresis, we proposed a recovery method to return the thruster operation to high-current mode. In the investigation, we focused on the plasma mode transition, where plasma is generated inside the waveguide. We experimentally investigated the hysteresis by varying the grid configuration and the waveguide geometries while maintaining the magnetic field. The results show that the neutral density and plasma conditions inside the waveguide are strongly related to the hysteresis, which indicates that the main cause of the hysteresis is plasma generation inside the waveguide.
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VACUUM 190 2021年8月 査読有りThis paper reports measurements of the xenon ground state and excited state densities inside a mu 10 microwave ion thruster. This thruster exhibits a 40% thrust enhancement upon changing from the waveguide to the discharge chamber propellant injection mode. In the present work, the associated mechanism was quantitatively evaluated using two-photon laser induced fluorescence (TALIF) spectroscopy to monitor the thruster waveguide. The 834.7 nm emission from excited state xenon was investigated with a 224.3 nm dye laser to excite the Xe I 5p61 S0 6pMODIFIER LETTER PRIME [3/2]2 state, compared with the emission without the laser. The resulting data confirm that the neutral density exhibits a linear relationship with the propellant flow rate in the cold gas and ionized state, while the ion acceleration decreases the neutral density by the same order of magnitude as the propellant utilization efficiency is changed. As the propellant flow rate increases, the collisions of neutrals that generate excited states occur in the waveguide and, when this process plateaus, the ground state emission suddenly increases. Propellant injection from the discharge chamber is evidently effective at suppressing collisions with electrons in the waveguide that generate excited states and that potentially interfere with microwave propagation.
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Acta Astronautica 181 14-27 2021年4月 査読有り© 2021 IAA This paper presents the design methodology and performance testing of an additively manufactured resistojet operating on hydrogen as propellant. Additive manufacturing allows to produce complex monolithic resistors, resulting in reliable high efficiency thrusters. The concept, to be used in combination with advanced cryogenic storage technologies, is proposed for short time and high specific impulse orbit transfers. The simplified two-dimensional thermal design approach adopted is discussed, and its application to the engineering of the resistor is shown for both Inconel 718 and tungsten. The paper reports the performance testing of the proof-of-concept version of the thruster, manufactured in Inconel 718. Experiments on hydrogen show near ideal performance, demonstrating a peak thermal efficiency of 96%. The thruster proves the validity of the design methodology proposed, and the feasibility of the approach to develop monolithic additively manufactured hydrogen resistojets as main propulsion units.
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JOURNAL OF PROPULSION AND POWER 37(2) 223-230 2021年3月 査読有りThis Paper explores keeper ignition and discharge characteristic of a hollow cathode center mounted on a Hall thruster to discuss the optimum startup sequence. A strong magnetic field along the hollow cathode in the center-mount configuration is considered to play an important role in the ignition and discharge; however, the characteristics in this configuration has never been reported. The effect of cathode mass flow rate on the ignition and discharge with 3 A keeper ignition is examined both with and without magnetic field. The influence of cathode heater power and flow from the anode is also investigated. The lower flow rate causes strong ionization instability, while the higher flow rate seems to induce ion acoustic turbulence. Applied magnetic field improves the ionization instability due to the decrease in electron temperature through plasma production. These results indicate ignition sequences over 1.5mg/s flow rate with magnetic field appear promising. On the other hand, the confined electrons around the magnetic field line prevent the initiation of the discharge. The ignition and instability are improved with high heat-up power because more thermionic emission is obtained. The anode flow provides slight improvement to the characteristics. The Fourier transform also demonstrates the presence of ionization instability and ion acoustic turbulence. The behavior of peak frequencies of ionization instability can be explained based on the predator-prey theory.
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TRANSACTIONS OF THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES 64(3) 189-192 2021年 査読有り
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Nature Astronomy 5(3) 246-250 2021年 査読有り© 2021, The Author(s), under exclusive licence to Springer Nature Limited. Analyses of meteorites and theoretical models indicate that some carbonaceous near-Earth asteroids may have been thermally altered due to radiative heating during close approaches to the Sun1–3. However, the lack of direct measurements on the subsurface doesn’t allow us to distinguish thermal alteration due to radiative heating from parent-body processes. In April 2019, the Hayabusa2 mission successfully completed an artificial impact experiment on the carbonaceous near-Earth asteroid (162173) Ryugu4,5, which provided an opportunity to investigate exposed subsurface material and test potential effects of radiative heating. Here we report observations of Ryugu’s subsurface material by the Near-Infrared Spectrometer (NIRS3) on the Hayabusa2 spacecraft. Reflectance spectra of excavated material exhibit a hydroxyl (OH) absorption feature that is slightly stronger and peak-shifted compared with that observed for the surface, indicating that space weathering and/or radiative heating have caused subtle spectral changes in the uppermost surface. The strength and shape of the OH feature suggests that the subsurface material experienced heating above 300 °C, similar to the surface. In contrast, thermophysical modelling indicates that radiative heating cannot increase the temperature above 200 °C at the estimated excavation depth of 1 m, even at the smallest heliocentric distance possible for Ryugu. This supports the hypothesis that primary thermal alteration occurred on Ryugu’s parent body.
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Nature Astronomy 5(1) 39-45 2021年1月 査読有り© 2020, The Author(s), under exclusive licence to Springer Nature Limited. The asteroid (162173) Ryugu and other rubble-pile asteroids are likely re-accumulated fragments of much larger parent bodies that were disrupted by impacts. However, the collisional and orbital pathways from the original parent bodies to subkilometre rubble-pile asteroids are not yet well understood1–3. Here we use Hayabusa2 observations to show that some of the bright boulders on the dark, carbonaceous (C-type) asteroid Ryugu4 are remnants of an impactor with a different composition as well as an anomalous portion of its parent body. The bright boulders on Ryugu can be classified into two spectral groups: most are featureless and similar to Ryugu’s average spectrum4,5, while others show distinct compositional signatures consistent with ordinary chondrites—a class of meteorites that originate from anhydrous silicate-rich asteroids6. The observed anhydrous silicate-like material is likely the result of collisional mixing between Ryugu’s parent body and one or multiple anhydrous silicate-rich asteroid(s) before and during Ryugu’s formation. In addition, the bright boulders with featureless spectra and less ultraviolet upturn are consistent with thermal metamorphism of carbonaceous meteorites7,8. They might sample different thermal-metamorphosed regions, which the returned sample will allow us to verify. Hence, the bright boulders on Ryugu provide new insights into the collisional evolution and accumulation of subkilometre rubble-pile asteroids.
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Astrodynamics 4(4) 349-375 2020年12月 査読有り© 2020, Tsinghua University Press. The Japanese interplanetary probe Hayabusa2 was launched on December 3, 2014 and the probe arrived at the vicinity of asteroid 162173 Ryugu on June 27, 2018. During its 1.4 years of asteroid proximity phase, the probe successfully accomplished numbers of record-breaking achievements including two touchdowns and one artificial cratering experiment, which are highly expected to have secured surface and subsurface samples from the asteroid inside its sample container for the first time in history. The Hayabusa2 spacecraft was designed not to orbit but to hover above the asteroid along the sub-Earth line. This orbital and geometrical configuration allows the spacecraft to utilize its high-gain antennas for telecommunication with the ground station on Earth while pointing its scientific observation and navigation sensors at the asteroid. This paper focuses on the regular station-keeping operation of Hayabusa2, which is called “home position” (HP)-keeping operation. First, together with the spacecraft design, an operation scheme called HP navigation (HPNAV), which includes a daily trajectory control and scientific observations as regular activities, is introduced. Following the description on the guidance, navigation, and control design as well as the framework of optical and radiometric navigation, the results of the HP-keeping operation including trajectory estimation and delta-V planning during the entire asteroid proximity phase are summarized and evaluated as a first report. Consequently, this paper states that the HP-keeping operation in the framework of HPNAV had succeeded without critical incidents, and the number of trajectory control delta-V was planned efficiently throughout the period.
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ACTA ASTRONAUTICA 176 413-423 2020年11月 査読有りA hollow cathode is an efficient electron source in the self-heating mode utilizing the discharge power. However, in sub-ampere currents, it needs keeper power to maintain the thermionic electron discharge, which could decrease the thrust efficiency. To address this problem, we propose using a microwave cathode, which is based on the flight model of a microwave ion thruster neutralizer cathode, as an alternative to a hollow cathode. First, we redesigned the magnetic field of a microwave cathode discharge chamber and tested it in the diode mode configuration. The electron emission current is doubled compared to the original performance. Next, we coupled the improved microwave cathode with a 200-W class Hall thruster and compared the characteristics and performance with a hollow cathode. We confirmed that the magnetic field polarity affects the ignition characteristics. We measured the thrust by an inverted thrust stand, the ion energy distribution functions by a retarding potential analyzer, and the beam profiles by an ion collector. The thrust and thrust efficiency are equivalent for both types of cathode. The specific impulse is 10% higher in the case of the microwave cathode. Since the potential difference between the microwave cathode and ground rapidly increased at currents above 600 mA, this could be taken to be the trade-off point against the hollow cathode.
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ACTA ASTRONAUTICA 176 77-88 2020年11月 査読有りThe authors redesigned the discharge chamber for the mu 10 microwave discharge ion thruster to improve its thrust performance and succeeded in enhancing the maximum beam current and thrust efficiency. However, it was found that the ion current ratio extracted from the discharge chamber with the redesigned configuration was lower than that obtained with the original configuration. To investigate the relationship between ion extraction and the magnetic field geometry, the ion loss current distribution in these two types of discharge chamber were measured by electrostatic probes. Using planar probes with a guard ring, the ion current that flowed into the wall was measured without disturbing the ion beam current. The results show that ionization occurs mainly near the upstream magnet. In addition, the ion flux on the sidewall in the redesigned discharge chamber is about 1.5-2 times larger than that in the original discharge chamber. This suggests that the distance between the edge of the plasmaproduction region and the chamber wall with consideration of the Larmor radius of ions is an important parameter in discharge chamber design. In addition, although the ion beam current showed a tendency to saturate at high microwave power, the ion loss to each part in the discharge chamber increased in proportion to input microwave power. The decrease in the extracted ion ratio in the redesigned discharge chamber is considered to be caused by a decrease in the electrostatic ion transparency of the screen grid. Therefore, in a well-tuned microwave discharge ion thruster, it is difficult to improve the thrust efficiency by increasing the discharge power. A design that suppresses the wall loss of ions is thus important.
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ACTA ASTRONAUTICA 174 367-376 2020年9月 査読有りIn space operation of the microwave discharge ion thruster mu 10 on the asteroid explorers Hayabusa and Hayabusa2, the propellant utilization efficiency deteriorated much more than in the ground endurance test. In this study, a fault tree analysis and experimental simulations of space operation were performed, focusing on the grid-derived internal carbon contamination. It was found that the performance deterioration due to the wave-guide contamination matched that in Hayabusa2 qualitatively and quantitatively. Based on the experimental verification, the future performance is experimentally predicted.
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JOURNAL OF PROPULSION AND POWER 36(4) 586-592 2020年7月 査読有りA low-cost and reliable propellant flow controller is necessary to offer competitive electric propulsion systems. This paper first reviews xenon flow control methods for Hall thrusters and ion engines. Through a trade study, a simple and low-cost xenon flow controller applying a heated capillary is proposed. The test article is fabricated based on a developed theory and tested with xenon. The estimation based on laminar flow shows slightly different characteristics from those of the experiment, although the Reynolds number indicates a laminar condition. The turbulent flow assumption is in good agreement with the experiment at a high mass flow rate while the flow characteristics are between laminar and turbulent at a low mass flow rate. That implies that the surface roughness and tube curvature induce the laminar-turbulent transition even in the low mass flow rate case. To obtain a wider flow rate range, maintaining laminar flow in the capillary is desirable because of the high sensitivity of the friction coefficient with temperature in the laminar flow. Component improvements achievable through miniaturization and reduction of the inner wall surface roughness and capillary curvature are considered to be necessary to prevent the turbulent transition.
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Science 368(6491) 654-659 2020年5月8日 査読有り© 2020 American Association for the Advancement of Science. All rights reserved. The near-Earth asteroid (162173) Ryugu is thought to be a primitive carbonaceous object that contains hydrated minerals and organic molecules. We report sample collection from Ryugu’s surface by the Hayabusa2 spacecraft on 21 February 2019. Touchdown images and global observations of surface colors are used to investigate the stratigraphy of the surface around the sample location and across Ryugu. Latitudinal color variations suggest the reddening of exposed surface material by solar heating and/or space weathering. Immediately after touchdown, Hayabusa2’s thrusters disturbed dark, fine grains that originate from the redder materials. The stratigraphic relationship between identified craters and the redder material indicates that surface reddening occurred over a short period of time. We suggest that Ryugu previously experienced an orbital excursion near the Sun.
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Science 368(6486) 2020年4月 査読有り© 2020 American Association for the Advancement of Science. All rights reserved. The Hayabusa2 spacecraft investigated the small asteroid Ryugu, which has a rubble-pile structure. We describe an impact experiment on Ryugu using Hayabusa2's Small Carry-on Impactor. The impact produced an artificial crater with a diameter >10 meters, which has a semicircular shape, an elevated rim, and a central pit. Images of the impact and resulting ejecta were recorded by the Deployable CAMera 3 for >8 minutes, showing the growth of an ejecta curtain (the outer edge of the ejecta) and deposition of ejecta onto the surface. The ejecta curtain was asymmetric and heterogeneous and it never fully detached from the surface. The crater formed in the gravity-dominated regime; in other words, crater growth was limited by gravity not surface strength. We discuss implications for Ryugu's surface age.
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NATURE 579(7800) 518-522 2020年3月 査読有りCarbonaceous (C-type) asteroids(1) are relics of the early Solar System that have preserved primitive materials since their formation approximately 4.6 billion years ago. They are probably analogues of carbonaceous chondrites(2,3) and are essential for understanding planetary formation processes. However, their physical properties remain poorly known because carbonaceous chondrite meteoroids tend not to survive entry to Earth's atmosphere. Here we report on global one-rotation thermographic images of the C-type asteroid 162173 Ryugu, taken by the thermal infrared imager (TIR)(4) onboard the spacecraft Hayabusa2(5), indicating that the asteroid's boulders and their surroundings have similar temperatures, with a derived thermal inertia of about 300 J m(-2) s(-0.5) K-1 (300 tiu). Contrary to predictions that the surface consists of regolith and dense boulders, this low thermal inertia suggests that the boulders are more porous than typical carbonaceous chondrites(6) and that their surroundings are covered with porous fragments more than 10 centimetres in diameter. Close-up thermal images confirm the presence of such porous fragments and the flat diurnal temperature profiles suggest a strong surface roughness effect(7,8). We also observed in the close-up thermal images boulders that are colder during the day, with thermal inertia exceeding 600 tiu, corresponding to dense boulders similar to typical carbonaceous chondrites(6). These results constrain the formation history of Ryugu: the asteroid must be a rubble pile formed from impact fragments of a parent body with microporosity(9) of approximately 30 to 50 per cent that experienced a low degree of consolidation. The dense boulders might have originated from the consolidated innermost region or they may have an exogenic origin. This high-porosity asteroid may link cosmic fluffy dust to dense celestial bodies(10).Thermal imaging data obtained from the spacecraft Hayabusa2 reveal that the carbonaceous asteroid 162173 Ryugu is an object of unusually high porosity.
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TRANSACTIONS OF THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES, AEROSPACE TECHNOLOGY JAPAN 18(3) 57-63 2020年 査読有り<p>The authors investigate the discharge chamber of the microwave ion thruster μ10 by using kinetic particle simulation. First, to investigate the plasma phenomena qualitatively, we conduct a particle-in-cell (PIC) simulation model. The simulation results indicate that the distribution of ion density is ring-shaped. To verify the simulation result with the experimental result, the simulation result is compared with the optical emission distribution. In low propellant flow rates, the distribution of ion density agrees with the optical emission distribution. However, in high propellant flow rates, the optical emission distribution is different from simulation results in the waveguide due to the excited neutral particles. In the thruster, the performance strongly depends on the location of injecting the propellant. Hence, to develop the plasma simulation for quantitative comparison with the experiment, the distribution of the neutral density is evaluated by using direct Monte Carlo simulation (DSMC). The results show the neutral density in the waveguide increases corresponding to the ratio of waveguide injection, which indicates that the density is one of the most important parameters for quantitative evaluation with the experiment.</p>
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ACTA ASTRONAUTICA 166 69-77 2020年1月 査読有りJapan's second asteroid explorer Hayabusa2 was successfully launched on Dec 3, 2014, to return a sample from asteroid 162173 Ryugu by 2020. Four xenon ion thrusters based on electron cyclotron resonance discharge propelled the spacecraft for 547 h during its first year in space. Hayabusa2 completed an Earth gravity assist on Dec 3, 2015, followed by 798 and 2593 h of ion thruster operation, called the first and second transfer phases of delta-v, respectively. The third transfer phase of delta-v was conducted from Jan 10, 2018, to Jun 6, 2018, in which the final 2475-h ion thruster operation was executed before the rendezvous with Ryugu. The cumulative operating times for the four ion thrusters are 6,450, 11, 5,193, and 6418 h. This paper summarizes the 6515-h powered flight by the ion engine system, which produced 1015 m/s delta-v, in terms of thruster performance change, roll torques generated by various combinations of ion thrusters, and spacecraft surface erosion history measured by two quartz crystal microbalances located near the thrusters. In parallel with the space flight operation, an engineering model of the microwave discharge neutralizer has been under long-duration testing on the ground since 2012. It has accumulated 55,170 h of diode-mode operation as of Mar 15, 2019.
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TRANSACTIONS OF THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES 63(6) 281-283 2020年 査読有りTo investigate the neutral xenon density distribution of electric thrusters such as ion and Hall thrusters, two-photon absorption laser-induced fluorescence (TALIF) spectroscopy was applied to a microwave cathode. First, the background pressure of the vacuum chamber was measured by TALIF. In the present measurements, the ground state was excited by a 224.29 nm laser, and 834.68 nm fluorescence was detected. The first measurement confirmed that the fluorescence intensity linearly increases with respect to the ground state number density. Based on this result, the density of neutral ground-state xenon was measured at the exit of the nozzle of the microwave cathode. The variation in the density with the microwave power was successfully measured at xenon flow rates of 0.029 and 0.098 mg/s. The measured densities varied from 2.3 x 10(19) to 8.4 x 10(19) m(3) with a maximum error of +/- 20% due to the plasma fluorescence.
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宇宙太陽発電 5 101-106 2020年 査読有り<p> 電気推進を用いた軌道間輸送機の質量と輸送コストについて調査した. まず電気推進システムの理論式において, 質量とコストそれぞれを最小化する推進機比推力を検討し, 後者の方が低い比推力になることを示した. 次に, JAXAで開発中の技術試験衛星9号機をもとに, 静止軌道までの軌道間輸送機の質量とコストモデルを構築し, 感度解析を行った. 結果として, これまであまり議論されてこなかった, 電力系・推進系の比質量[kg/kW]の影響が大きいことがわかった. また, 現状達成可能な比質量範囲では多くの場合, 低軌道ではなく静止トランスファ軌道を出発軌道として静止化を行った方が, 質量・コストメリットが得られやすいことがわかった.</p>
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ACTA ASTRONAUTICA 165 25-31 2019年12月 査読有りThe microwave cathode was developed as a neutralizer for the microwave ion thrusters of the Japanese asteroid explorers Hayabusa and Hayabusa2. Since it emits hundreds of mA of electron current, ion currents collect at the wall of the cathode, which causes fatal destruction due to sputtering. In an effort to reduce the sputtering voltage, this study investigates the effect of the strength of the magnetic field at the nozzle on the anode voltage. Firstly, a magnetic field is applied at the nozzle by a coli. Using the coil, decreasing the magnetic field intensity increases the electron density at the exit of the nozzle. It is presumed that the applied magnetic field facilitates the detachment of magnetic lines by the electrons inside the microwave cathode, resulting in a reduction of the anode voltage. By weakening the nozzle magnetic field, trapped electrons are reduced and the transportability to the outside is improved. Secondly, to realize the same magnetic field intensity achieved in the first experiment without any additional power consumption, the author proposes the use of a magnetic shield. The magnetic shield reduces the anode voltage from 37 V to 32 V at 180 mA, the nominal current of the flight model. Since the sputtering rate exponentially increases with the anode voltage, reducing the anode voltage through these techniques is effective in increasing the lifetime of the cathode.
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REVIEW OF SCIENTIFIC INSTRUMENTS 90(10) 2019年10月 査読有りMagnetoplasmadynamic (MPD) thrusters are operated in a quasisteady state with about 1.0 ms pulse created by a pulse forming network (PFN). However, there is still no precedent to verify the operation time quantitatively. The nonsteady region of the pulse can lead to an error of the thrust performance against that of steady state operation. In addition, the propellant gas outside the discharge chamber can be consumed since the exhaust velocity exceeds the estimated velocity. This paper shows the first step in quantitative evaluation of the quasisteadiness of an MPD thruster operation. First, we developed a new power supply that outputs a flat-topped and less nonsteady region pulse with a variable pulse width. Compared with that of a PFN, the nonsteady region "t(r) + t(f) " decreased from 0.532 to 0.110 ms. By implementing the circuit shorter and adjusting the gate resistance, the surge voltage in the experiment was suppressed to 309 V, which is less than 2% error of that in the PSIM simulation, 305 V. Second, we operated an MPD thruster using the new power supply for discharge and the external magnetic field. As a result, we obtained operation time characteristics by sweeping the operation time from 0.3 to 5.0 ms. The current waveforms are in a range of 620 +/- 70 A. We confirmed the consistency of the thrust, 0.32 +/- 0.03 N. From the correlation between the input energy and the impulse, it is possible to discuss quasisteadiness of the MPD thruster operation using the determination coefficient and the offset. Published under license by AIP Publishing.
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VACUUM 168 2019年10月 査読有りThis paper reports the first study to measure xenon neutral ground state particle density of microwave cathode by two-photon laser induced fluorescence spectroscopy (TALIF). Xenon is commonly used as a propellant in electric propulsion like Hall thrusters, ion thrusters, and their cathodes. For electric propulsion, information about neutral particles is important such as the ionization degree and the charged exchange collisions (CEX). The measurement target is XeI 5p(61)S(0)6p[3/2](2), which absorbs at a wavelength of 224.29 nm and emits fluorescence of 834.7 nm. The measurement system was demonstrated for three cases: cold gas, without electron extraction, with electron extraction. From three cases, the measurement system can detect a neutral ground state particle density of 10(19)m(-3) order without and with a plasma. In a cold gas, the neutral ground state particle density is (8.4 +/- 0.4) x 10(19)m(-3 )at 0.098 mg/s. Without electron extraction, the neutral ground density decreases by ionization and excitation With electron extraction, the density varied from 0.6 to 2.3 times compared to without electron extraction depending on anode voltage.
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ACTA ASTRONAUTICA 161 382-388 2019年8月 査読有りElectrostatic plasma thrusters such as ion engines and Hall thrusters commonly use xenon as a propellant and several measurement techniques for xenon ions and metastable neutrals have been applied to evaluate the characteristics of the thrusters. Although density measurements of ground state neutral xenon can provide crucial information on the ionization characteristics and help explain charge exchange phenomenon, much less research is available due to its technical difficulty. Two-photon absorption laser induced fluorescence is promising because it allows access to ground state xenon atoms by using around 220-260 nm wavelength lasers which have become more readily available lately. In this study, observation of the fluorescence following two-photon excitation from a room temperature (cold) xenon gas cell is conducted with 249 and 252 nm wavelength excitation at xenon pressures of 0.1 and 10 Torr. The fluorescence signals are obtained against a wide range of laser intensities, and the resulting fluorescence response comprises of a few regimes - weak-excitation, saturation, and an intermediate regime. The natural lifetime and quenching rate are evaluated by analyzing the fluorescence decay, and the result is consistent with published literature. Finally, actual application to ground tests of Hall thrusters is discussed based on the experimental results, especially with respect to their fluorescence responses.
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Physics of Plasmas 26(7) 2019年7月1日 査読有り© 2019 Author(s). This paper reports the numerical investigation of plasma properties for the microwave discharge ion thruster μ10. The model consists of a particle in cell simulation and a Monte Carlo collision simulation. The results indicate that the plasma density and the electron temperature in the confined region are larger than those in other regions and are qualitatively consistent with probe measurements. Moreover, we traced the trajectories of charged particles to investigate the plasma generation and transport. The electron trajectories indicate that electrons are strongly confined by the mirror magnetic field and the sheath, which indicates that the confinement depends on the electron energy. As a result, the electron energy distribution function is a combination of two Maxwellian distributions. Although the hot electrons account for 3.4% of all electrons, they account for 50.1% of the ionization and can generate plasma with an excitation loss of 1/3 of that of cold electrons. The ion trajectories indicate that they are affected by the magnetic field. To investigate the effect of the magnetic field on the transport, we evaluate the ion and electron current percentage toward the wall and compare with the wall surface percentage. The ion and electron current ratios differ because of diffusion with respect to the magnetic field. The ion current percentage is larger than the surface area percentage in the grid, which indicates that ions are transported to the grid more efficiently due to the magnetic field. Therefore, the effect on ions by the magnetic field is one of the most important criteria for microwave discharge ion thrusters.
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JOURNAL OF PROPULSION AND POWER 35(3) 565-571 2019年5月 査読有りThe neutralizers in ion thruster systems do not generate thrust force. Hence, the power consumption of a neutralizer limits the thrust efficiency of the ion thruster system. Therefore, an ion thruster system is proposed that uses a negative ion source that generates thrust force as well as neutralizes the positive ion beam. In this study, in order to verify the feasibility of this ion thruster system by ground experiments, a negative fullerene ion source was developed and three experiments were conducted. First, in order to demonstrate that the satellite can accelerate ions without becoming charged, the positive and negative ion beams were accelerated in the floating condition using a feedback circuit. Second, in order to verify whether the formation of the virtual anode could be suppressed, the space potential and the beam profile in the downstream region of the ion beam were measured. Third, in order to demonstrate that the thruster system generates thrust force, the thrust with a beam target was measured. From the aforementioned results, it was concluded that the experimental verification of the feasibility of the ion thruster with combined positive and negative ions without a neutralizer was successful.
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Acta Astronautica 157 425-434 2019年4月 査読有り© 2019 IAA To improve the performance of the 10-cm-class microwave discharge ion thruster μ10 for use in future deep space exploration missions planned by the Japan Aerospace Exploration Agency (JAXA), a new discharge chamber was designed, and its performance was tested. The maximum beam current in the new discharge chamber geometry was 16% higher than that in the original geometry, which was used in the Hayabusa 2 space explorer, under the same discharge power. To investigate the reason for this performance improvement, the multi-charged ion ratio in the plume, the beam current density profiles, and the ion current in the discharge chamber were measured by probes. It was found that the multi-charged ion efficiency and the beam divergence efficiency in the redesigned configuration were not significantly different from those in the Hayabusa 2 configuration. This shows that the increase in the ion beam current enhances the thrust. In addition, it was confirmed that the total ion current inside the new discharge chamber is higher than that in the Hayabusa 2 configuration. The ion extraction efficiency, however, was lower than that in the Hayabusa 2 configuration. This suggests that the increase in the total ion current per unit of incident microwave power is the cause of the performance improvement. In the redesigned configuration, the thrust is 12.0 mN, the specific impulse is 3122 s, the discharge loss is 162 W/A, and the propulsion efficiency is 39.6% at the peak performance point.
MISC
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令和4年度宇宙輸送シンポジウム: 講演集録 = Proceedings of Space Transportation Symposium FY2022 2023年1月令和4年度宇宙輸送シンポジウム（2023年1月12日-13日. 宇宙航空研究開発機構宇宙科学研究所（JAXA)(ISAS)) , 相模原市, 神奈川県 Space Transportation Symposium FY2022 (January 12-13, 2023. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan In the 3D-printed resistojet that the authors are working on, it is necessary to provide an electrical contact point at a high temperature part considering the limitations in 3D printing. So far, we have made some prototypes of flat contact type, tapered contact type, and screw type. In addition, as a new attempt, we also describe design guidelines for banana plug type that contact with radial surface pressure. 資料番号: SA6000184068 STEP-2022-016
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日本航空宇宙学会誌 = Aeronautical and space sciences Japan 70(10) 214-217 2022年10月
書籍等出版物
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Elsevier 2022年 (ISBN: 9780323997317)
担当経験のある科目(授業)
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2022年12月 - 2023年1月推進エネルギー変換工学 (東京大学大学院 新領域創成科学研究科)
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2021年9月 - 2022年2月エネルギー推進工学特論 (東京大学大学院 新領域創成科学研究科)
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2021年9月 - 2021年11月エネルギー推進工学特論 (東京大学)
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2013年9月 - 2016年3月地球と宇宙 (駒沢女子大学)
所属学協会
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2011年7月 - 現在
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2009年9月 - 現在
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2008年4月 - 現在
共同研究・競争的資金等の研究課題
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日本学術振興会 科学研究費助成事業 2023年4月 - 2027年3月
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日本学術振興会 科学研究費助成事業 2023年4月 - 2027年3月
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日本学術振興会 科学研究費助成事業 若手研究(A) 2017年4月 - 2021年3月
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日本学術振興会 科学研究費助成事業 若手研究(B) 2014年4月 - 2017年3月
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日本学術振興会 科学研究費助成事業 基盤研究(A) 2010年4月 - 2015年3月
学術貢献活動
1メディア報道
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J-WAVE JK Radio Tokyo United https://www.j-wave.co.jp/original/tokyounited/archives/the-hidden-story/2021/02/19-111253.html 2021年2月 テレビ・ラジオ番組
