谷貝 剛

A higher specific impulse and a larger thrust are required for a manned interplanetary space thruster. Prior to a realization of a fusion-plasma thruster, a magneto-plasma-dynamic arcjet (MPDA) powered by a fission reactor is one of the promising candidates for a manned Mars space thruster. The MPDA plasma is accelerated axially by a self-induced j x B force. Thrust performance of the MPDA is expected to increase by applying a magnetic nozzle instead of a solid nozzle. In order to get a much higher thruster performance, two methods have been investigated in the HITOP device, Tchoku University. One is to use a magnetic Laval nozzle in the vicinity of the MPDA muzzle for converting the high ion thermal energy to the axial flow energy. The other is to heat ions by use of an ICRF antenna in the divergent magnetic nozzle. It is found that by use of a small-sized Laval-type magnetic nozzle, the subsonic flow near the muzzle is converted to be supersonic through the magnetic Laval nozzle. A fast-flowing plasma is successfully heated by use of an ICRF antenna in the magnetic beach configuration.

Characteristics of wave excitation in a high speed plasma flow are investigated experimentally. Axisymmetric, azimuthal mode number in = 0 shear Alfven waves (SAW) are excited by a Rogowski-type antenna in a high density (more than 10(13) cm (-3)), high speed, magnetized helium plasma flow with ion Mach number (M-i similar to 1). The Rogowski-type antenna with Faraday-shield for the SAW excitation consists of a solenoid coil, so the magnetic field is not generated around the antenna in vacuum. Dispersion relations of excited SAW are in good agreement with the theoretical curve which is obtained by taking into account the effect of a Doppler shift due to a high speed plasma flow. Ratio deltaB(y)/ B-0 of the excited magnetic perturbation deltaB(y) to the uniform magnetic field B-0 is as large as 1%. Spatial profiles and damping rate of the SAW are also obtained experimentally.

A supersonic plasma is produced quasi-steadily by use of a magneto-plasma-dynamic arcjet (MPDA) in various shapes of an external magnetic field configuration. An ion acoustic Mach number M-i of the plasma flow is limited to be nearly unity in a uniform magnetic field configuration, while it increases up to almost 3 in a divergent magnetic nozzle configuration. Spatial variations of Mi is well predicted by an isentropic model for a compressible gas. The Mach number decreases in the far downstream region due to charge-exchange collisions between flowing ions and neutral atoms which are produced through surface-recombination on the end wall. Ion heating of the fast flowing plasma has been successfully demonstrated for the first time. This success is mainly due to the plasma density is high enough to reduce the penetration of neutral gases which cause the charge-exchange energy loss. It is found that an asymmetric RF wave with an azimuthal mode number m = +/- 1 is most effective to heat the ions.