Numerical Simulation of Frequency Dependence of Millimeter-wave Discharge at Subcritical Condition

A high-power millimeter-wave irradiation induces air breakdown under atmospheric pressure. An ionization front propagates toward the beam-source direction at high speed while forming various types of discharge structures such as comb-shaped, diffusive, and filamentary patterns. The millimeter-wave discharge can be classified into overcritical and subcritical conditions according to the electric-field intensity of the millimeter-wave. Past subcritical discharge experiments reported that the propagation speed, the discharge structure, and the structural transition threshold have frequency dependence on the millimeter-wave frequency and the electric-field intensity. However, the frequency dependence has not been examined by numerical simulations because those simulations at subcritical condition could not capture the ionization process. The authors have recently developed a new model that can simulate the ionization-front propagations of millimeter-wave discharge at subcritical condition by coupling millimeter-wave propagation, plasma fluid, compressible neutral fluid, detailed chemical reaction, and radiation transfer models. The calculation results changing the beam frequency are in agreement with the trends of past discharge experiments.