In this study, performance characteristics of the medium-power (1-3 kW) Hall thruster THT-VI, developed in Osaka Sangyo University, were measured using special gases of carbon dioxide, methane and ammonia as original materials in planets and satellites in the Solar System; that is, propellant can be supplied just on the planets and satellites, although only a heavy gas of xenon is used with higher performance; argon is also used as alternative propellants. Performances with CO2, NH3 and Ar were much lower than those with Xe. The present magnetic field optimized with Xe is not suitable for molecular and light atomic propellants because of poor ionization and acceleration with the Xe-optimized magnetic field. In molecular and light atomic gases, an axially-uniform or widely-stronger magnetic field is preferable for enough ionization and ion acceleration with longer length inside the acceleration channel. As a result, the Hall thruster THT-VI with an improved magnetic field, which was strengthened and flatter in the acceleration channel, was stably operated with CO2 and Ar. For Ar, the thrust after improvement ranged from 20 to 30 mN at input powers of 1-1.8 kW and the specific impulse from 1,000 to 1,700 sec. As for CO2, the thrust was 9.5 to 15 mN at 650-1150 W and the specific impulse 200 to 300 sec, although the performances were still low. Furthermore, pre-ionization by additional discharge in an upstream region of the acceleration channel is expected to be effective for enhancement of ion production and acceleration. The Hall thruster THT-VI with auxiliary electrodes inside the acceleration channel was successfully operated with CO2, resulting in 12.7 mN of thrust and 432 s of specific impulse.

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A relationship between the thickness of the dielectric layer and consumed energy density on a plasma actuator has been experimentally investigated. The plasma actuators with 80 μm and 160 μm dielectric layers have been driven by a compact nanosecond pulsed power supply. Electrically consumed energy for plasma generation has been estimated by applied voltage and discharge current. In the case of the dielectric layer with 80 μm, the discharge current was larger than in the case of the dielectric layer with 160 μm because of the low equivalent impedance of the plasma actuator. At the energy density of 47 μJ/cm, instantaneous power of 17 kW is applied to the plasma actuator with 80 μm dielectric layer. The maximum value of the instantaneous power for the same consumed energy density is superior in the case of the dielectric layer of 80 μm.

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In recent years, a high-power millimeter wave source, a gyrotron capable of oscillating multiple high frequencies has been developed, and the development of a dummy load that supports multi-frequency operation is an issue. We have designed and manufactured a short-pulse dummy load that directly absorbs millimeter waves of different frequencies of 70 GHz and 28 GHz up to 300 kW with cooling water in the transmission line. Measurement results show that the power estimation in the transmission is reliable with high sensitivity at both frequencies. The dependence of the water temperature evolution on the operation conditions has been evaluated through thermal-fluid analysis. The difference in the beam shape does not affect the temperature evolution due to the heat exchange with the waterway, indicating that the same operation can be performed even when operating at different frequencies.

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Hall thrusters have been increasingly used in deep space exploration and commercial communication satellites. This is because they have a simpler structure and higher thrust density than ion engines. For the further development of space exploration, there is a need to reduce the cost and weight of spacecraft. In most Hall thrusters, xenon has been used as the propellant because it has the advantages of high relative density, low ionization cost, and non-toxicity. However, xenon is expensive and requires a heavy tank for a high-pressure filling. As inexpensive and low-pressure-storable propellants, we focused on carbon dioxide (as dry ice). In this study, a 100 W-class Hall thruster was operated using xenon and carbon dioxide as propellants. The thrust was measured using a thrust stand, and the anode efficiency was calculated from the measurement results. The thrust of the carbon dioxide operation was one-fifth that of the xenon operation, and the anode efficiency was one-seventieth that of the xenon operation. The results were discussed in terms of mass utilization efficiency and chemical composition in the plume, suggesting that the loss of mass utilization efficiency and loss due to dissociation are some of the factors that contribute to the reduction of propulsion performance.

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