Self-consistent plasma density evolution during RF energy extraction from a microwave pulse compressor

Abstract

Numerical simulations of the high-pressure plasma discharge in a switch of a microwave pulse compressor resulting in extraction of the compressor output pulse were carried out. The compressor comprised a rectangular waveguide-based cavity and an H-plane waveguide tee with a shorted side arm filled with helium. For simulations, the 3-D version of the PIC code MAGIC1 was used; the plasma was represented by the gas conductivity model provided by MAGIC. Simulations started from the preset RF fields (corresponding to the standing wave pattern in the cavity and H-tee), seeding electrons in a volume around the E-field antinode in the tee side arm (in the center of the waveguide cross-section), and ∼104 cm3 plasma density (cosmic background). The plasma density is then determined self-consistently by electron ionization cross-sections and avalanche rate, which depend on the E-field that decreases with the rise of the density. It was found that the plasma extends along the E-field forming a filament whose transverse size is set by dimensions of the volume initially populated by seeding electrons. There are three stages of the plasma density evolution: first, it grows exponentially up to the value at which the E-field within the plasma region begins to decrease because of the skin-effect; then, the avalanche rate decreases but the density still rises until the RF energy begins to rapidly release from the cavity; finally, when the E-field becomes insufficient to support the avalanche, the density is saturated. The simulated peak power and waveform of output pulses showed good agreement with those obtained experimentally in the S-band compressor with laser triggering of the plasma discharge at different levels of input microwave power. The behavior of the plasma density also agrees satisfactorily with experiments.