Observation of atmospheric millimeter-wave discharges at 94 GHz and comparison with other microwave and millimeter-wave frequencies

Abstract

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.