Millimeter-Wave Discharge below Critical Intensity Using a 28 GHz Gyrotron

Microwave Rocket is one of beamed energy propulsion systems, whose energy is wirelessly supplied by using a millimeter-wave beam. Although millimeter- wavedischarge plasmain under-critical conditions is necessary for its thrust generation, it has not been clarified why discharge is sustained below the critical intensity. One of the possible mechanisms is the one in which the plasma is composed of many excited molecules which can be ionized by lower-energy electrons than those in a ground state. Therefore, experiments using a 28 GHz gyrotronwas conducted to confirm the physical modeling. Assuming that electron excitationtemperature is higher than vibrational temperature, vibrational temperaturewas examined. As a result, the measured vibrational temperatureof nitrogen molecules increased as peak intensity of millimeter-wave beam becomes lower. The result implies that electron excitationtemperature is higher in those regions and highly excited molecules are important for ionization below critical intensity.