Numerical simulation of electromagnetic-wave interference induced by ionization-front of millimeter-wave discharge at subcritical conditions and application to discharge structure identification.

This article focuses on the development and numerical validation of an interferometric discharge structure identification (IDSI) method for millimeter-wave atmospheric discharges, specifically at 170 GHz under subcritical conditions. The IDSI method analyzes the time-varying waveform of standing-wave intensity formed by interference between incident and reflected millimeter waves at the ionization-front, enabling distinction between continuous comb-shaped and discrete discharge structures without relying on high-speed cameras. Numerical simulations demonstrate that smooth waveforms correspond to continuous structures, while noisy waveforms with high-frequency components indicate discrete structures; in both cases, the peak frequency of the Fourier spectrum is proportional to the ionization-front propagation speed, validating the location method for speed measurement. Additionally, the high-frequency components in discrete structures arise from increased millimeter-wave absorption during new plasma spot formation, suggesting the IDSI method can also time plasma spot formation experimentally. The study further shows that off-axis observation points yield similar waveform distinctions, supporting practical experimental implementation.