Evolution of plasma density generated by high power microwaves

The relaxation time of pulsed rf-generated plasma is investigated. A 3 MW, 3 μs width, 50 ns risetime HPM pulse is transmitted through a dielectric window that terminates a WR-284 waveguide filled with insulating gas. The investigated plasma is formed across the dielectric window on the atmospheric side. This produces electron densities on the order of 10¹³ to 10¹² cm⁻³ for 60 to 145 torr in air, respectively. In the same pressure range, initial attenuation (∼ 0.5 dB) of the microwaves is observed after tens to hundreds of nanoseconds with final attenuation values approaching −40 to −10 dB, respectively. To determine plasma relaxation times after the HPM pulse terminates a multi-standard waveguide coupler (X/S-band) was designed to inject a low power 10 GHz signal used for probing the surface plasma. The coupler was designed to have high coupling coefficients (> −5 dB) for the specific narrowband around 10 GHz (BW ∼ 10 MHz) along with negligible insertion loss of the HPM propagation. From the measured attenuation and reflection of the 10 GHz probe signal, the evolution of the electron density is inferred. Furthermore, in an air environment, the density falls exponentially implying attachment as the dominate electron loss mechanism.