Akira Yamori

Stable and reproducible production of high velocity projectile in ISAS railgun (HYPAC)

The ISAS (Institute of Space and Astronautical Science) railgun HYPAC (power supply: 6 mF, 10 kV, 300 kJ; rail: 13 mm phi *2 m bore) has been developed with a particular emphasis on stable and reproducible production of a high-velocity projectile so that it can actually be used for scientific impact experiments. A maximum velocity of 7.45 km/s has been obtained for a 0.87 g projectile in 4 mF, 288 kJ operation. The stability and reproducibility of HYPAC are so good that a velocity greater than 6 km/s can always be obtained with a prediction within +or-3% before the experiment. HYPAC is now extensively used for impact experiments for laboratory simulation in the field of planetary science. >

Characteristics of high efficiency 300 kJ railgun

The characteristics of a 300 kJ railgun is described. It has very efficient energy conversion characteristics and for this kind of high velocity railgun (7.8 km/s), a maximum of nearly 12% efficiency has been attained. The dependence of the efficiency on the stored energy of the power supply, projectile mass and the gun caliber is studied. The efficiency strongly depends on the caliber and increases as the caliber is increased. Measurements using an array of B-dot probes indicate that a growth of the secondary arc has a vital role in reducing the efficiency. >

Development of a lightweight space debris shield using high strength fibers

In order to protect space structure against space debris impacts, it is indispensable to develop a shield with high strength materials. A high strength fiber is one of potential materials from a viewpoint of strength, lightweight, and flexibility. The purpose of this study was to develop a new lightweight shield composed of high strength fibers against medium size debris impacts. We developed four kinds of shields using Vectran fibers, and hypervelocity impact tests were carried out by a railgun accelerator. The experimental results showed that the developed shield could stop the polycarbonate projectile with 13 mm in diameter, 1 gram in weight, and 6.9 km/sec in velocity. Adoption of the high strength fiber in the bumper materials may reinforce the protection capability and reduce the weight drastically.

Direct‐Display of Plasma Density and Temperature

A direct‐display plasma density and temperature meter was developed using a floating double‐probe method. A small ac voltage was superposed on the dc voltage, and the third harmonic component of ac current was detected. The phase reversal of the third harmonic component was observed at Vdc = V* = 1.30 kTe, and an experiment was carried out to fix the dc voltage automatically at this voltage V* detecting the phase reversal of the third harmonic component. The plasma density and electron temperature were obtained directly from the dc voltage V* and the dc current corresponding to the voltage V*.

Experimental results of a two-stage plasma armature railgun

We have been developing a two-stage plasma armature railgun as the first step to produce a multistage railgun system, which is considered as a promising tool to achieve hypervelocities. For successful operation of the two-stage railgun, three technical problems are addressed: the insulation between the two segmented rails, the formation of, and correct timing of, the ignition of the plasma armature in the second-stage railgun. Of these problems, the correct timing of the ignition of the plasma armature of the second-stage railgun will most significantly affect the performance of the two-stage railgun. A large increase in the projectile velocity has been obtained in the second-stage railgun by solving these technical problems, from 1.7 to 5.8 km/s.

Preliminary experimental results of a novel induction type railgun

This paper describes the concept of an induction type railgun and the results of preliminary experiments with such a railgun. The induction type railgun may surmount the barrier which has been created by secondary arcs in the operations of plasma armature railguns. The induction type railgun is operated as a conventional railgun during the early stage of acceleration and is designed to accelerate a projectile by the plasma armature which is enhanced by the induction from a set of external current loops during the late stage. The external current loops are configured just outside of the two rails at the middle segment of the railgun bore, and are shorter than the railgun. The induced voltage generated by the external current loops is applied to the circuit consisting of the plasma armature, the secondary arc and the two rails between the two arcs. The induced voltage increases the plasma armature current, and simultaneously reduces the secondary arc current. The authors have performed preliminary experiments of the induction type railgun with a relatively low energy stored in a capacitor bank. An increase in the plasma armature current and projectile velocity have been observed in their preliminary experiments.

Optimization of the power circuit for most efficient performance of ISAS railgun (HYPAC)

A study of the optimization of the Institute of Space and Astronautical Science (ISAS) railgun (HYPAC) power circuit has been performed to increase the efficiency of the railgun performance. Capacitance and the stored energy in the capacitor have been changed and the conversion efficiency of electrical to kinetic energy has been obtained. Experimental results show that the most efficient energy transfer is obtained when the projectile leaves the muzzle of the railgun at the end of the first half cycle of the discharge current. If the projectile leaves the muzzle earlier or later than this timing, the conversion efficiency decreases. >

Controlled experiment in the ionosphere with a rocket-borne plasma gun

A controlled active experiment was performed by a plasma gun on-board a rocket to study the plasma stream across the magnetic field in space. A mother and daughter system was employed. An 8 kV 0.8 μF (25 J) capacitor bank on the daughter rocket was discharged every 12 s and about 1016 ion and electron pairs were ejected. A plasma signal was detected on the electron temperature probe. The propagation speed of the plasma stream is estimated to be 106 cm/s, considerably lower than that obtained in the prelaunch test. The amplitude of the plasma signal decays as the inverse square of the mutual distance between the mother and daughter rockets and this decay is much slower than the free streaming. The importance of the geomagnetic field on the plasma propagation in space is discussed.

Development of a plasma armature railgun with two distributed power supplies

We have been developing a plasma armature railgun with two distributed power supplies as the first step to produce a railgun with multiple distributed power supplies, which is considered as a promising tool for hypervelocities. The railgun has almost the same system and operational principle as those of the two-stage plasma armature railgun, which had been developed in our laboratory, except that the two power supplies are distributed along segmented rails or continuous rails. To date, we have performed 11 shots to study the railgun. For the operation of the railgun with two distributed power supplies, to control the generation of a compact plasma armature in the 2/sup nd/ railgun bore is the most important. In 8 shots the compact structure of the plasma armature was not generated due to a large amount of leakage of the plasma armature around the joint between the two railguns. However in the remaining 3 shots, the deformation of the railgun bore around the railgun joint, which is considered to be the main reason for the leakage of the plasma armature, was able to be suppressed. As a result, the compact structure of the plasma armature in the 2/sup nd/ railgun bore was generated. Based on the experimental results, we are now planning to perform the next-step experiment with a higher power supply energy.

High quality railgun HYPAC for hypervelocity impact experiments

Abstract The characteristic of a very high quality railgun HYPAC (10 kV, 6,000 μ F, 300 kJ) at the Institute of Space and Astronautical Science (ISAS) which is capable of accelerating a 1–2 gram projectile of 10–20 mm diameter made of polycarbonate to a hypervelocity of 7.8 km/sec and its utilization in hypervelocity impact experiments are reviewed. Also shown are further efforts to increase the velocity, to improve the quality of the projectile such as the acceleration of metal powder and to explore a new application of this facility.