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

摘要

Presently, mostly advanced microwave pulse compressors with plasma switches provide hundreds megawatt power in nanosecond pulses. 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 3–10 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 ∼5–10 cm/s. For time-resolved spectroscopy, the system was filled with helium at 2–10 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 10 cm. 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.