Cu nanoparticles were synthesized by a cavitation bubble plasma with a pair of Cu rod electrodes in deionized water. The cavitation bubble plasma was generated by a bipolar high voltage pulse with a voltage of 10 kV, a pulse width 1.0 μs and a repetition rate 200 kHz in deionized water in which many cavitation bubbles produced by a high-speed rotor were feeding. The shape and size of Cu nanoparticles were observed by TEM. It was found that the particle shape was spherical. The median diameter and span of the Cu nanoparticles increased from 4.0 to 9.4 nm and from 2.9 to 6.1 nm with increasing processing time from 1 to 20 min, respectively. The observed increase in the particle size of Cu nanoparticles should be ascribed to Ostwald ripening. These results suggested that the particle size of Cu nanoparticles could be controlled by the processing time.

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Laser-supported detonation (LSD), a type of supersonic flow, is regarded as a very important phenomenon in Laser propulsion systems, because it can generate the required high pressures and temperatures. In this study, the unsteady performances are investigated by simple one-dimensional geometry and variable laser intensity using a computational fluid dynamics (CFD) analysis with the thermochemical non-equilibrium model for inert gas plasma. Though the periodic irradiative laser intensity is too exaggerated below the level of LSD-LSC (laser-supported combustion) transition, LSD can be sustained shorter than 0.9μs in the atmospheric pressure argon gas. In the region of the shorter period, LSD propagation velocity is sustained over 3.8×103 m/s, the level in the case of LSD-LSC threshold.

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Although expansion tubes have been used to simulate the atmospheric entry environment, the flow conditions have not been completely characterized. In our previous research, the sensitivity of laser absorption spectroscopy (LAS) was not high enough to diagnose ISAS/JAXA expansion tube flows. In this study, a multi-pass cell was formed to increase the sensitivity of LAS. Since the mirrors were set outside of the chamber, the window displacement by the shock arrival was found to decrease the sensitivity. From the ray trace calculation, the window angle should be less than 2.4 degree to keep the sensitivity enhancement of 44 times. As a result of multi-pass LAS diagnostics using the oxygen molecule line at 763.43 nm, it was found that the operation time of the expansion tube was 55 us, and the translation temperature was ranged from 4,400 K to 6,000 K.

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The effect of solution temperature on the generation of cavitation bubble plasma was investigated for the purpose of improving the plasma generation rate in aqueous sodium chloride solution with a high conductivity. The number of discharge starting points, the number of continuous discharges and the amount of bubbles were evaluated as a function of the solution temperature. When the solution temperature was increased from 30 to 56 ℃, the plasma generation rate increased from 19% to 41%, and the number of discharge starting points and the number of continuous discharges increased 1.5 times and 1.3 times, respectively. It was suggested that the increase in plasma generation rate was due to the increase in cavitation bubbles.

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In order to improve the thrust performance of ion thrusters with pulse width modulation control, optimized grid system by numerical simulation was used for the reduction of leak ion beam current. The effect of reduction of mass flow rate on thrust performance was also investigated for the improvement of propellant utilization. The leak current was decreased from 30% to 3.5%, achieving dramatic improvements. The propellant utilization was also improved from 0.25 to 0.48 by decreasing the xenon mass flow rate from 9.8 ug/s(0.1 sccm) to 4.9 ug/s(0.05 sccm).

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In this study, atmospheric pressure low temperature plasma was generated by using a high voltage pulse power supply (1 – 10 kV, 20 kHz) under the water. Helium gas and oxygen gas were flow into between parallel flat electrodes with glass plates to produce a dielectric barrier glow discharge. Using this plasma, the formaldehyde solution was decomposed. The decomposition rate of formaldehyde was up to about 33% when treated for 6 hours, with underwater plasma generated under the conditions of applied voltage: 10 kV, He gas flow rate: 80 sccm, O₂ gas flow rate: 15 sccm.

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