laser beams are increased in power intensity. If highly energetic plasmas are produced by laser energy, damage to a solid nozzle will be unavoidable. In this case it will be necessary to use a magnetic nozzle in the expanding plasma region to protect the wall surface of a solid nozzle. Another problem in RP laser propulsion is how efficiently propellant is utilized. Low propellant utilization efficiency not only leads to low specific impulse but also degrades overall efficiency. It is thought that the use of a pellet-shaped propellant with a magnetic nozzle can help to solve the above-mentioned problems. For this reason, we have constructed a fundamental experimental device for RP laser propulsion research and investigated plasma expansion processes in a magnetic nozzle. Experimental Apparatus A schematic of the experimental set-up is shown in Fig. 1. This set-up is composed of vacuum chamber (~1.5 mPa), propellant feed and photo-detection systems, a ruby laser with kryptocyanine Q switching, magnetic coils, and plasma diagnostic instruments. All of them are controlled by a microcomputer. The output pulse of the beam has a duration of 0.1 to 2 //sec half-width, is focused by a simple 30 cm lens and irradiates the target with a spot diameter of approximately 0.5 mm. Alignment of the target at the focal point of the lens on the optical axis was accomplished by means of a 500 mW CW He-Ne laser. The targets - spherical pellets made of copper with a diameter of 0.5 mm-are successively injected towards the focal point of the laser beam by the pellet feed system as shown in Fig. 2. Several hundred pellets are stocked in the reservoir and extracted one by one through the pinhole of a thin plate driven by a computer-controlled stepping motor. They free-fall approximately 30 cm to the focal point of the laser beam. The time
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