An experiment on microwave beaming propulsion under reduced pressure has been conducted. 170GHz millimeter-wave of up to 0.5MW was provided by a gyrotron. Although measured impulsive thrust has a weak dependency on ambient pressure, propagation velocity of plasma front observed using a high-speed framing camera was increased with the decrease in ambient pressure. Plasma-front propagation velocity was predicted based on photo-ionization in a pre-shock region. The predicted propagation velocity dependency on pressure showed a good agreement with measured one.
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