田畑 邦佳

This report describes an investigation by experimentation to elucidate atmospheric discharge plasma induced using a 50 kW millimeter-wave beam at 94 GHz. Millimeter-wave discharge plasma is useful for an ultraviolet light source, radioactive material detection, chemical decomposition, and beamed energy propulsion (BEP). The gyrotron used in this study is the “UT-94” with a frequency of 94 GHz and an output power of 100 kW, which was developed at the University of Tokyo specifically for BEP research. The 94 GHz frequency is promising for atmospheric energy beaming because of its low atmospheric attenuation, small beam divergence, and existing utilization track records in the atmosphere. This study experimentally investigated the relationship between the incident beam power density and propagation velocity of an ionization-wave front, which is particularly critical to thrust performance. In addition, the plasma structures were also clarified at 94 GHz and compared with other microwave and millimeter-wave frequencies, such as 28 GHz and those higher than 100 GHz. As a result, finer microscopic structure in the H–k plane was observed than those reported in earlier studies. Furthermore, we found a clear relation between structures and propagation velocities in terms of the electric field concentration of the incoming electromagnetic-waves.

© 2018 IEEE. Microwave Rocket is one of beamed energy propulsion systems, whose energy is wirelessly supplied by using a millimeter-wave beam. Although millimeter-wave discharge plasma in 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 gyrotron was conducted to confirm the physical modeling. Assuming that electron excitation temperature is higher than vibrational temperature, vibrational temperature was examined. As a result, the measured vibrational temperature of nitrogen molecules increased as peak intensity of millimeter-wave beam becomes lower. The result implies that electron excitation temperature is higher in those regions and highly excited molecules are important for ionization below critical intensity.