Probability, Induction Time and Size of a Microwave Discharge with Laser Sparkless Initiation for Various Values of the Gas Pressure and the Laser Pulse Energy

Experimental data on the ignition probability, the induction time, the length, and the diameter of a subcritical microwave discharge initiated by a laser pulse without laser spark are given. The studies were carried out in air at the pressures from 50 to 80 Torr, the laser pulse energies of 10–200 mJ, and the pulse supply times from –100 to 0.5 μs relative to the microwave pulse. An MI-505 magnetron, that operates at the radiation frequency of 9.6 GHz and has the pulse duration of 2.5 μs, and a focusing system were used to create microwave radiation. In the plasma zone, the microwave radiation intensity was estimated at the level of 2.0 kV/cm. An Evergreen 200 laser that creates pulses of the 10 ns half-width and the 532 nm wavelength was used to initiate the microwave discharge. The laser pulses were focused by a lens with the focal length of 250 mm. Owing to laser initiation, the threshold pressure of the microwave discharge was increased from 50 Torr (without initiation) to 80 Torr (with laser initiation). Synchronous supply of the microwave and laser pulses gives the best result, namely, discharge ignition occurs without misfires and the discharge characteristics (geometry, induction time) have a minimum spread. Premature supply of the laser pulse also makes it possible to obtain the discharge with an increased ignition threshold, but its characteristics become worse. Laser initiation also has positive effect on the supercritical microwave discharge. The results obtained can be useful in developing a plasma generator in aerodynamics and other problems with the plasma energy deposition.