is raised. As a result the high surface pressure and laser pulse cause the plasma to move predominantly toward the laser [11], Such a sharp angular distribution is enhanced by an applied field. Expansion of the laser-induced plasma is restricted, for the most part, to the centre axis. That is, radial momentum is effectively converted into axial momentum in the magnetic nozzle. The electron temperature obtained from the probe characteristic is shown in Fig. 11. The measured electron temperature ranges from 0.5 to 2 eV and decreases slightly with applied field. This may be explained by increased conversion of thermal energy into axial energy through expansion in the magnetic nozzle. From probe measurements the axial momentum obtained in magnetic nozzle B was calculated to be about 1 /zN- sec without the applied field and increases up to 4 /zN-sec with the applied field. This result indicates that the application of the magnetic nozzle is effective for pulsed laser propulsion. Conclusion We have constructed a relatively simple and low-cost experimental set-up for laser propulsion research. In a vacuum chamber, metal pellets were injected one by one, their position and velocity were detected by photosensors, and a laser-induced plasma was generated. This system enables us to obtain high accuracy, reproducibility and flexibility under various experimental conditions. Moreover, with this device, laser- induced plasmas were seen to expand in a magnetic nozzle in which the conversion of radial to axial momentum was confirmed from probe measurements. REFERENCES [1] Jahn, R.G. (1968) Physics of Electric Propulsion (New York, McGraw-Hill). [2] Kantrowitz, A. (1972) Propulsion to orbit by ground-based lasers, Astronautics and Aeronautics, 10, pp. 74-76.
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