Self-consistent evolution of plasma discharge and electromagnetic fields in a microwave pulse compressor

Abstract

Nanosecond-scale evolution of plasma and RF electromagnetic fields during the release of energy from a microwave pulse compressor with a plasma interference switch was investigated numerically using the code MAGIC. The plasma was simulated in the scope of the gas conductivity model in MAGIC. The compressor embodied an S-band cavity and H-plane waveguide tee with a shorted side arm filled with pressurized gas. In a simplified approach, the gas discharge was initiated by setting an external ionization rate in a layer crossing the side arm waveguide in the location of the electric field antinode. It was found that with increasing ionization rate, the microwave energy absorbed by the plasma in the first few nanoseconds increases, but the absorption for the whole duration of energy release, on the contrary, decreases. In a hybrid approach modeling laser ignition of the discharge, seed electrons were set around the electric field antinode. In this case, the plasma extends along the field forming a filament and the plasma density increases up to the level at which the electric field within the plasma decreases due to the skin effect. Then, the avalanche rate decreases but the density still rises until the microwave energy release begins and the electric field becomes insufficient to support the avalanche process. The extraction of the microwave pulse limits its own power by terminating the rise of the plasma density and filament length. For efficient extraction, a sufficiently long filament of dense plasma must have sufficient time to be formed. V

Improved operation of a microwave pulse compressor with a laser-triggered high-pressure gas plasma switch

“…One can see that, for the focused beam, the peak power is more than two times higher, the rise time of the pulse is about two times shorter, the "flattop" is longer, and the delay with respect to the triggering event is dramatically shorter than for the unfocused beam. There is a good qualitative agreement between the output pulse waveforms and delays and the results of the simulations, 8 in which the seed electrons set in the filamentlike volume differed in density by six orders of magnitude. Thus, the focused beam evidently creates a much higher density of seed electrons, so that plasma filaments having sufficient length and density for efficient microwave reflection are formed in a time much shorter than the output pulse duration.…”

“…The discharge originates from the center of the waveguide cross-section, and the plasma in the switch appears as a filament expanding along the RF electric field. 6 This process was simulated numerically, 8 and it was shown that a low extraction efficiency is inevitable when the time of the filament formation with sufficiently high plasma density is longer than the round-trip time of the traveling wave along the cavity (output pulse length). In this case, the extraction of the output pulse limits its own power, since the RF electric field in the switch decreases to the level at which it no longer supports the electron avalanche.…”

“…The simulations described in Ref. 8 showed that such a scenario can be avoided (an extraction efficiency of $75% was obtained) when seed electrons with sufficient density are set in a filament-like volume parallel to the RF electric field. In practice, such electron seeding can be achieved by using a laser beam directed along the RF electric field in the switch waveguide and focused with a long-focus lens to enhance its intensity in an extended region across the waveguide.…”

“…An increase in the intensity beyond the level accessible in the present experiments is of interest, as it may provide the increase in power extraction efficiency up to $75% that is shown in numerical simulations. 8 Ya. E. Krasik and J. G. Leopold, "Initiation of vacuum insulator surface high-voltage flashover with electrons produced by laser illumination," Phys.…”

“…E. Krasik and J. G. Leopold, "Initiation of vacuum insulator surface high-voltage flashover with electrons produced by laser illumination," Phys. Plasmas 22 (8), 083109 (2015).…”