Propagation of microwave breakdown in argon induced by a 28 GHz gyrotron beam

An atmospheric argon discharge plasma was induced by a high-power microwave beam using a 28 GHz gyrotron and investigated at pressures of 40 kPa–100 kPa and Gaussian peak intensities of 0.115 GW/m² (0.204 MV/cm) and 0.168 GW/m² (0.246 MV/cm). According to high-speed imaging results, the propagation velocity of the discharge front increased with the backpressure to maintain a range of 600 m/s–1000 m/s. The propagation velocity was 25 times larger in argon than in air. Applying the collisional-radiative (CR) model to the spectroscopic results showed that the electron density increases with the pressure from $5.0×1021 m−3$ at 40 kPa to 5$.0×1022 m−3$ at 100 kPa and lies on the critical density curve. The electron temperature decreases as the background pressure increases from 2 eV to 0.5 eV, and the gas temperature was 300–400 K. According to CR analysis, the population densities of 3d_n and 2s_n excitation levels in the fast velocity condition are much lower than that in any other conditions. The results revealed that the energy transfer from electrons to ionized particles is more remarkable in the high background pressure and the fast velocity condition.