Yoichi Kagaya

Quasisteady magnetoplasmadynamic thruster with applied magnetic fields for near-earth missions

A quasisteady multimegawatt magnetoplasmadynamic (MPD) thruster with applied magnetic fields has been investigated to achieve the high-thrust performance at specific impulse levels between 1000 and 2000 s, which is demanded for space missions near the Earth. The applied field coils are connected in series with a power source (pulse forming network). Field application causes lower discharge voltages and greater thrusts than those for no applied field in the following special conditions: 1) the axial applied field intensity is equal to the azimuthal selffield intensity only in the main region for current conduction, where the Joule heating is enhanced, and outside the region is very small; 2) the magnetic field lines are parallel throughout the inside surface of the anode. Furthermore, from the viewpoint of cathode life, it is worthy to note that the cathode erosion rate with the applied field is reduced to about half that with only the self-field.

Applications of Quasi-Steady Magneto-Plasma-Dynamic Arcjets to Ceramic Coatings

For applications of quasi-steady magneto-plasma-dynamic (MPD) arcjets to material processing, an MPD arcjet with a cathode covered with ceramic materials was developed. In 100-shot operations at 5-10 kA, the ablation rates of the ceramic covers ranged from 2.5 to 5.0 mg/shot. The front velocities of ablated Al atoms inferred with a streak camera were much higher than velocities of 200-500 m/s for conventional plasma torches for material processing. This is effective for deposition of rigid films adhering strongly to substrate surfaces. The SEM photographs showed that a dense uniform film was deposited near the center of the substrate. From the XPS spectra, the peak area ratio of Ti/Al for the coating film almost equaled that for the cross section, and the valence numbers of Al and Ti did not change.

Material Spraying Using Electromagnetically Accelerated Plasma

In magneto-plasma-dynamic (MPD) arcjet generators, plasma is accelerated by electromagnetic body forces. The MPD arcjet generator can produce higher-velocity, higher-temperature, higher-density and larger-area plasmas than those of conventional thermal plasma torches. Two types of MPD arcjet generator were developed for applications to ceramic spray coatings. One generator was installed with a cathode covered with mullite or zirconia ceramics and the other with a titanium cathode. The former was operated with Ar for mullite or zirconia coating by an ablation process of the cathode cover and the latter with N2 for titanium nitride coating by a reactive process between ablated titanium particles and nitrogen plasma. The MPD spray process could successfully form dense, uniform and hard ceramic coatings. In titanium nitride reactive spraying, plasma diagnostic measurement and flowfield analysis were also carried out. A large amount of N and N+ was expected to be exhausted with a high velocity from the MPD generator. Both the electron temperature and the electron number density were kept high at a substrate position compared with those for conventional low-pressure thermal sprayings. A chemically active plasma with excited particles of N+, Ti, Ti+ and Ti2+ was expected to contribute to better titanium nitride coatings. All coating characteristics showed that the MPD arcjet generators had high potentials for ceramic spray coatings.

Development of electromagnetic acceleration plasma generator for titanium nitride coatings

Abstract Electromagnetic acceleration plasma generators, which are called magneto-plasma-dynamic (MPD) arcjet generators, can produce higher-velocity, higher-temperature and higher-density plasmas than those of conventional thermal plasma torches, because MPD arcjet plasma is efficiently accelerated by electromagnetic body forces in MW-class input power operation. These properties are effective for deposition of rigid films adhering strongly to substrate surfaces. In this study, we developed a reactive spray process of titanium nitride film deposition using MPD arcjet generators. The MPD arcjet generators had titanium cathodes, and their working gas was nitrogen. The phase structure and the composition of the titanium nitride films were analyzed by means of X-ray diffraction (XRD) and scanning electron microscopy, and the Vickers hardness of the titanium nitride films was also measured. XRD analysis showed that TiN ratio was highly sensitive to the cathode diameter and the discharge current. The Vickers hardness reached about 1000 at the substrate center.

Gas conditions after the passage of reflected shock waves and their application to torus‐like experiments

In this article we describe briefly recent results on basic processes concerning a crowbar method utilizing reflected shock waves by a pressure‐driven shock tube. After that, we propose an apparatus which is an application of this crowbar technique to a new method for producing plasmas.

Crowbar Experiments by Ionizing Shock Waves and New Methods to Produce Torus Like Plasmas

Various data on basic processes concerning crowbar methods by ionizing shock waves were gathered to apply this technique to the real and larger machines. A pair of crowbar electrodes were flush mounted at the end of a pressure driven shock tube which was terminated with a dielectric plate for shock reflection. Rather restricted shape of crowbar electrodes and use of ambient air in laboratory as a test gas did not decrease so much the insulating voltage of the crowbar gaps. To breakdown the crowbar gaps by reflected shock waves with the incident mach number of 7 ~ 16 in air at the initial pressure of 5 ~ 250 μHg, electrodes voltage must be applied by 700 ~ 400 V and the deduced conductivity of shocked plasmas was 10−11 ~ 10−9 Ω/cm. Two kinds of test circuits were tried to be crowbared. CRT traces of the discharge current with the crowbar gap closed by reflected shock waves showed successful and monotonous envelopes of the oscillating ones without the crowbarring. Various important improvements and another application of this type of experiments are proposed and shown briefly. Among them is a new method to produce torus-like plasmas, which is a different type of application of this crowbarring-idea, itself.