Numerical analysis of structural change process in millimeter-wave discharge at subcritical intensity

Abstract

Plasma-front propagation processes of 170 GHz millimeter-wave discharge were investigated under subcritical incident electric field intensity by using a one-dimensional model. The discharge structure was numerically reproduced at more than 0.2 MV/m by introducing the detailed chemical reaction and radiation transport processes into the conventional model. The results revealed that the propagation mechanism of the plasma front in the millimeter-wave discharge changes depending on the incident electric field intensity. At intensities greater than 1.4 MV/m, the plasma front propagated at supersonic speed, while forming a discrete structure, which has intervals of 1/4 wavelength of the millimeter wave. This structure was generated by electron-impact ionization and photoionization processes. At the intermediate intensities, the plasma front propagated continuously rather than discretely because the gas expansion increased the reduced electric field and induced electron-impact ionization. The dominant heating process at the plasma front was fast gas heating. At intensities less than 0.3 MV/m, the plasma front propagated continuously, but the dominant heating process changed to vibrational–translational relaxation. The discharge was maintained by thermal ionization and associative ionization. The simulation results were in good agreement with the past millimeter discharge experiments at this intensity.