Plasma density evolution in a microwave pulse compressor (Vol. 46 No. 2)

Plasma density vs. time after the laser triggering for different microwave output.

Microwave plasma discharges have been widely investigated for many years; there is, however, a subject that is insufficiently studied. It is the plasma formation at the initial – nanosecond time-scale – stage of the high-pressure discharge in a resonant cavity and its interrelation with the process of microwave energy release from the cavity that goes out of resonance during the plasma generation. This subject directly concerns the operation of microwave compressors using commercial magnetrons and klystrons for short-pulse high-power microwave generation. In this work, for the first time, spectroscopic measurements were performed to investigate nanosecond dynamics of the plasma density in the S-band compressor with laser triggering. For pressurized helium filling the compressor cavity and switch, the plasma density was evaluated from the shapes of the 3888.65 Å and 4471.5 Å He I spectral lines. The measured evolution of the density was found to correlate with the peak power of the compressor output pulse and efficiency of the stored microwave energy extraction. With increasing microwave output, the plasma appears earlier in time after the laser beam enters the system, the plasma density rises more steeply, and it reaches higher values.

L. Beilin, A. Shlapakovski, M. Donskoy, T. Queller and Ya. E. Krasik, “Plasma density temporal evolution in a high-power microwave pulse compressor switch”, EPL 109, 25001 (2015).
[Abstract]