Study of physics and practical applications of laser-driven in-tube accelerator

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Basic and application studies were conducted on the laser-driven, in-tube accelerator which had been developed by the head investigator. The operation principle was experimentally validated in 4-m-high vertical launch experiment. The impulse generation mechanisms were experimentally and numerically studied by highlighting the interactions among shock waves, expansion waves and laser-generated plasma and accompanying fluid-dynamics instabilities. A scaling law of its propulsion performance was established. The impulse enhancement using laser ablation was experimentally demonstrated. Those are useful for practical applications. In this device, the momentum coupling coefficient, which is defined as the ratio of an impulse to a laser energy, can be tripled owing to a confinement effect and with appropriate setting of the working gas species and its fill pressure. The momentum coupling coefficient scales with the reciprocal of the speed of sound, which was experimentally validated using thr … More ee kinds of monoatomic gases, argon, krypton and xenon. The secondary effect of their ionization potential on the impulse was also obtained. The impulse performance is uniquely characterized using a dimensionless momentum coupling coefficient, the dimensional momentum coupling coefficient multiplied by the speed of sound, and a dimensionless parameter, the ratio of the fill pressure multiplied by an effective projectile volume to the laser energy. From the experimental and numerical analysis, it is concluded that the impulse performance can be optimized from the effective expansion volume of the laser plasma. In the laser-ablative performance experiment, polyacetal exhibited the best performance ; its impulse is sensitive to the ambient pressure. At an appropriate laser fluence, the impulse becomes highest at ambient pressures below 100 Pa. This favorable performance can be useful not only for improving the propulsive performance of the device but also for other various space applications including space debris deorbiting. Less