Fast-frame optical imaging and time-resolved spectroscopy of plasma in a gas discharge-based switch of a microwave pulse compressor

Summary form only given. Presently, mostly advanced microwave pulse compressors with plasma switches provide hundreds megawatt power in nanosecond pulses¹. Nevertheless, in spite of significant progress in their development, no data existed on the nanosecond dynamics of the plasma formation under strong microwave fields in pressurized gases that ultimately determine a compressor´s output power. In this work, the evolution of the plasma formed in the S-band compressor was studied using fast-frame (2 ns) imaging and time-resolved spectroscopy. The compressor represented a rectangular waveguide-based cavity connected to an H-plane waveguide tee with a shorted side arm. The plasma discharge in the tee side arm was triggered by a Surelite laser. In experiments with optical imaging, the system was filled with dry air at up to 3·10⁵ Pa pressure. It was found that the plasma appears as filaments with diameters of <;0.6 mm expanding along the RF electric field with the typical velocity of ~5·10⁷ cm/s. For time-resolved spectroscopy, the system was filled with helium at 2-10⁵ Pa pressure. The nanosecond dynamics of plasma density was obtained by analyzing the shape of He I spectral lines: triplet 2s-3p (3888.65 Å) and triplet 2p-4d (4471.5 Å). Experimental data showed an evident correlation between the rise time of the plasma density and the peak power of the microwave output pulse: the density rise is steeper when the compressor output power is higher. The density reaches values of the order of 10¹⁶ cm³. Numerical simulations of the microwave energy release from the cavity with the appearance of the plasma yield a good agreement with measured output pulse peak power and waveform.