Post pulse recovery of HPM generated plasma at close to atmospheric pressure

Summary form only given. We study the recovery of the low temperature plasma generated by a microsecond high power microwave, HPM, pulse in different gases at pressures approaching one atmosphere. The experiment mimics the electrical breakdown at the interface between the vacuum HPM-source environment and the radiating environment (generally held at atmospheric pressure). This low temperature plasma is generated along the surface on the atmospheric side where the high E_eff/p, or reduced effective electric field with pressure, causes high levels of ionization leading to the formation of a highly conductive plasma within the duration of the microsecond pulse. With the maximum HPM pulse repetition rate being highly dependent on the relaxation of this plasma, it is the goal of this study to determine the kinematics and chemistry of this low temperature plasma as it pertains to ion, electron, and excited species densities. For this study, a 2.85 GHz magnetron operating in the TE₁₀ mode of a WR-284 standard waveguide is used to generate a 3 MW, 3 microsecond pulse with a rise time of less than 50 ns that is incident on the dielectric window separating the two environments. The atmospheric side is enclosed in a structure that mimics an open radiation pattern similar to that of current HPM systems. To understand the post-pulse features, two custom multi-standard waveguide couplers were designed to implement and extract a low power 10 GHz CW source into the main waveguide structure while keeping a low insertion loss for the HPM pulse. The results of this power signal (max attenuation values range -10 to -40 dB) along with a 1D plane wave excited plasma model is used to infer the temporal average electron density (specifically the longitudinal integral of the surface plasma density) at a range of pressures and different gases, typically 10-400 torr for air, N₂, Ar, and He. The peak electron density and loss rates are then correlated with diff- sion lengths, recombination and attachment rates given in literature to ascertain the dominant plasma relaxation path and species along with a means to extrapolate the time required to relax to a nominal background electron density. For instance, the dominant electron loss process in 90 torr air is attachment with a frequency of 121 kHz and peak electron density of ~10¹³ cm^-3 resulting in relaxation times of a few hundred microseconds while in N₂, the dominant process soon after the pulse is determined to be 2-body dissociative recombination.