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A LOOK AT PULSED POWER RESEARCH

IN JAPAN

Observations from a recent visit to several Japanese industrial companies and organizations involved in pulsed power (especially rail gun) research are summarized. The number of Japanese organizations pursuing this line of research is quite surprising. Most of the work is similar to that in the United States, but in a few cases there were some unexpected and impressive performance parameters (e.g., v > 7 km/s for a rail gun and very high performance solid state power switches). The reason for the high Japanese interest in pulsed power research remains somewhat of an enigma.

INTRODUCTION

I was recently invited to present a 3-day series of lectures (14-16 November 1991) on pulsed power technology at Aso-Ikoinomura, Kumamoto Prefecture (Japan's largest national park). Before the lectures I had the opportunity to visit some Japanese universities and industrial laboratories engaged in related work. It is interesting that the lectures, which were sponsored by the Institute of Electrical Engineers of Japan and organized by Kumamoto University, were also supported by 13 large industrial firms. These were: Asahi Chemical Industry, Fuji Electric Corporation, Hitachi, Ishikawajima-Harima Heavy Industries, Japan Steel Works, Kawasaki Heavy Industries, Kobe Steel, Mitsubishi Electric Corporation, Mitsubishi Heavy Industries, Nichicon Co., Nissin Electric Co., Sumitomo Electric Industries, and Toshiba Corporation. I believe that it is very unlikely that any such set of lectures could be sponsored by such an impressive and varied group of U.S. industrial organizations, no matter who gave the lectures. U.S. firms would decide up front that this was a topic of no interest to

by M. Kristiansen

them, whereas Japanese firms want to make sure that they do not overlook a new field or opportunity. The lectures were attended by 59 people from 25 different organizations.

I was also surprised to learn that the Institute of Electrical Engineers of Japan has an active subcommittee on Applied Technology for Electromagnetic Mass Acceleration, and I had the pleasure to be hosted by them one evening in Tokyo. This committee has members from industry, universities, national laboratories, and the Japanese Defense Agency. There are strong Japanese interests and activities in various aspects of electromagnetic launchers (EMLs), and much of this report will be related to EML research. This research is mostly related to armor testing by companies, such as Japan Steel Works, in the hope that the Japanese Defense Agency will initiate a major program in this field.

TECHNICAL VISITS AND OBSERVATIONS

In the following I will summarize the technical observations I made at some Japanese industrial and university laboratories in the order in which I

visited them. Representatives from many of these institutions have visited our laboratories at Texas Tech over the past few years, some for extended periods of time, so I was already reasonably familiar with the main theme of their work.

Toshiba Corporation, Fuchu Works

This is, of course, one enormous corporation with a total of some 70,000 employees in the main company and over $30B in annual sales. The Fuchu Works employs some 7,500 people, with another 5,500 in the company's subsidiaries. The staggering fact is that 60% of the employees are engineers and 25% are assembly workers. I do not have the facts from similar industries in the United States or Europe, but I find it hard to believe that any of them has this high a percentage of engineers.

I had, on a previous occasion, visited Toshiba's Hamakawasaki Works and was familiar with some of its circuit breaker and vacuum interrupter work. I was met by representatives from both places and it is difficult for me to separate the exact responsibilities of each

place. My thanks go to Drs. K. Sugi, E. Kaneko, H. Ohashi, K. Hoshi, I. Ohshima, and K. Okamura.

Toshiba Corporation is a leading manufacturer of high voltage systems and switch gear and I was especially impressed by the latter aspect. Suffice it here to say that Toshiba's continued effort of reducing the size of the equipment is truly remarkable. Its vacuum interrupter work is also outstanding and it has been the leading organization in increasing the interruption current and voltage of these devices while continually reducing their sizes. This is achieved by clever electrode configurations and special electrode materials. The production of these devices increased by a factor of 14 in the period 1970-86, whereas U.S. industry basically dropped out of the competition.

Of special interest was the development of a wide range of high power semiconductor switches. Among these devices were 3-kV, 9-kA; 5-kV, 4-kA; and 6-kV, 3-kA gate turn off (GTO) transistors. For the first device a dI/dt of 8 kA/μs was reported. A nationally sponsored [Ministry of International Trade and Industry (MITI)] project on metal oxide semiconductor (MOS) assisted gate triggered (MAGT) thyristors has a design goal of 2.5 kV and 1011 A/s-cm2. They have achieved a current density of 2 KA/0.14 cm2 and operated at 5 kHz. These devices have low losses and require low trigger power. Figure 1 shows a convenient diagram for comparing the operating regimes of various types of high power semiconductor switches. Toshiba has done considerable, successful work on series-parallel connection of these devices. The company also reported the development of a new amorphous (cobalt based?) metal with much lower

loss, but lower saturation magnetic field, than Metglas (registered trademark of Allied Chemical). The new material is presumably available in 25-μm-thick foils. The greatly reduced losses in this new material may make it very attractive new material may make it very attractive for many repetitive pulsed power applications, such as isotope separation and NOX/SOX removal from flue stacks.

machines, bridges, etc. My visit was to its research institute. Of particular interest to me was the arcjet and EML work. The configurations of the magneto plasma dynamic (MPD) and arcjet thrusters for electric space propulsion were fairly conventional. The institute's published work on arcjet thrusters at recent international conferences has been concerned with low

Tokyo Institute of Technology power (0.5-1 kW) HN, and simulated at Nagatsuta

Dr. Koichi Kasuya has for many years worked on ion beam generation years worked on ion beam generation from cryogenically cooled (liquid nitrogen) anodes. The institute has a series gen) anodes. The institute has a series of PICA (particle beam inertial confinement fusion apparatus with cryogenically refrigerated anode driver) genically refrigerated anode driver) machines in use for this work. The goal is to obtain high purity, high current ion beams for inertial confinement fusion (ICF). The PICA-4 machine, for instance, is a 3-2, 120-ns, 1.5-MV machine. The anode is covered with D, or ammonia ice, and the best results seem to be a few kA current at a few

2

10s A/cm2 current density. The institute also does laser development work with equipment provided by industry. In all cases the work was sufficiently In all cases the work was sufficiently far from my own area of expertise that it was difficult to judge its quality.

Ishikawajima-Harima Heavy Industries (IHI) Co., Ltd., Yokohama

My hosts at IHI were Drs. K. Uematsu, S. Morimoto, T. Majima, and T. Tagaeto. This is again a very large company with many employees (~15,000 in 1988) and numerous activities in the heavy construction field, such as nuclear power plants, ships, oil drilling platforms, manufacturing drilling platforms, manufacturing

hydrazine (N2 + 2H2) devices. The reported performance has not been anything unusual. The researchers claim, however, that a barium oxide impregnated tungsten cathode has less erosion than other materials, e.g., thoriated tungsten, in their 8-kA, 600-μs, 1to 2-pps MPD generator. This is quite interesting since we have the opposite experience in continuous arcjets at 200 A. This could be due to variations in the impregnation process, but it needs further study.

They have also used the arcjet to produce diamond film with a very clean, narrow Raman spectrum. In this case they inject H2+ Ar in the conventional way from the rear of the device and inject CH, in the nozzle section. The film is deposited on a Mo (1,000 °C) substrate. The deposition rate is 1.5 μm/min for an arcjet power level of 2-5 kW. The back pressure in the chamber is 20-100 Torr and the plasma jet then does not expand enough to produce large area films. Five Japanese companies are apparently engaged in similar work.

IHI's EML (rail gun) was of a conventional design, superficially similar to the one described in the next section. In both cases the entire gun assembly was submerged in vacuum. Apparently the results were not as good as those reported in the next section of this report, and the reason for this was not obvious.

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Figure 1. Operating regions for various semiconductor devices [reprinted with permission from
K. Okamura, E. Kaneko, et al., "Pulsed power and industrial application," J.I.E.E. Plasma
Research EP-91-57, 35 (1991)].

The Institute of Space and Astronautical Science, Kanagawa

Professor N. Kawashima was my host during this visit. This institute has a very interesting rail gun in terms of its performance. The basic system appears fairly conventional, as shown in Figures 2 and 3. The 2-meter-long gun has a nominal bore of 1.3 cm. The rails are made of copper and the insulator of polycarbonate. The whole rail system is enclosed in a vacuum tank evacuated by a rotary vacuum pump. No preinjection is used. The interesting claim is that the researchers achieve 6 km/s velocity for a 0.9-g projectile “stably” and 7.5 km/s in "some cases." This is, indeed, quite impressive. In the United States 6 km/s is considered a "velocity barrier," which is under extensive investigation. There have been a few, unverified reports of a few shots with v > 7 km/s, but not on a repeatable basis.

It is not clear why this device has such excellent performance except that

Trigger Pulse

they disassemble and rebore the gun after each shot. Each time they bore off 0.5 mm. Extreme care in assembly and rebore seems to be the only explanation they could give.

The system operates at 2 shots/wk and is mostly used for micrometeorite and is mostly used for micrometeorite and space debris impact simulation studies.

Kumamoto University

Dr. Hidenori Akiyama has cooperated with our research group for several years and has spent considerable time in our laboratory. We have coauthored several publications and he was the main organizer of my lectures and laboratory visits. He has an excellent, small research group that utilizes lent, small research group that utilizes space and facilities to the fullest. His interaction with students and staff is most cordial and there is evidence of excellent group spirit and enthusiasm.

The university's ASO I and II machines have produced very interesting plasma opening switch results with very modest size facilities. This type of

opening switch is of considerable interest both in the United States and in Russia, where the latest nuclear weapons effects simulators under construction depend critically on the success of such switches. These switches are equally important to inertial confinement fusion devices. Akiyama and his group have shown that it is possible to do meaningful work in this field with modest, university-size machines. They have, for instance, demonstrated a factor of 12 voltage gain (from 30 to 400 kV) at 80 kA with 10-ns risetime using a twostage opening switch with a first-stage fuse followed by a plasma opening switch (POS).

Avery interesting concept is the use of a laser-produced plasma, on a rotating graphite target, to achieve high reproducibility and long life. They are in the process of upgrading the laser from 1 J to 30 J/pulse to get higher currents than the 20 kA they get with the 1-J laser. High reproducibility and long life are critical parameters for all the intended applications of such

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Figure 2. Schematic layout of ISAS rail gun "HYPAC" system. Adapted and reprinted with permission from A. Yamori et al., "Rail gun experiment (HYPAC) at ISAS," IEEE Transactions on Magnetics 27(1), 120-129 (1991). 1991 IEEE.

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