Electron density evolution of post-pulse high power microwave plasma

Plasma generation along the dielectric interface between the vacuum medium of the source and the atmospheric environment of the antenna is one of the limiting factors in power thresholds of high power microwave, HPM, systems. The maximum repetition rate for HPM platforms are ultimately determined by the relaxation times of this low-temperature surface plasma. While the microwave scattering parameters can be determined during the HPM pulse from the transmitted and reflected pulse itself, additional diagnostics effort has to be expended to capture post-pulse plasma relaxation and microwave scattering parameters. For this, a Bethe hole-type multi-standard waveguide coupler was designed and fabricated to inject a continuous low power signal into the main waveguide structure that carries the main 3 MW, 3 µs pulse at 2.85 GHz in an S-band waveguide structure. To facilitate flashover the waveguide is terminated by a Lexan window into a controlled atmospheric chamber with absorbing walls that mimic radiation into free space. The coupler injects 1W of continuous power at 10 GHz to measure the scattering parameters many microseconds after the pulse extinguishes. Using a model developed from a 1D plane wave excitation, the plasma´s electron density can be inferred from these power measurements. Then, from the temporal analysis of the electron density, the kinematic and chemical behavior of the plasma in relation to the recombination and attachment properties of the gas are inferred. Tested conditions include three gas types: N2, air, Ar, ranging in pressure from 10 to 155 torr. The transmission coefficients range from −40 to −10 dB corresponding to an electron density of ∼10¹⁴ to ∼10¹¹ cm⁻³ for these pressures, respectively. The decay of this plasma has been experimentally determined to be on the order of tens of microseconds with the density falling off proportional to t^{−- 1}, t⁻², exponential, or a linear combination thereof, depending on the dominant electron loss path. Reported here, are the design parameters of the waveguide coupler along with the post-pulse evolution of the electron density and recombination physics that it entails.