IONIZATION-INDUCED FREQUENCY UP-SHIFT OF A HIGH-POWER MICROWAVE INTERACTING WITH A PLASMA

The propagation of an intense electromagnetic wave (EMW), of frequency 9 GHz, in an underdense, near critical, inhomogeneous plasma (n(e) le ss than or equal to 1.2x10(12) cm(-3), T-e similar or equal to 3 eV) l eads to an up-shift of the EMW frequency of a few MHz. The injected EM W with power, P-0 less than or equal to 250 kW, and with pulse duratio n of 1 mu s, produces additional ionization of the neutral argon gas ( n(0) similar or equal to 6.6x10(13) cm(-3)) through the heating of pla sma electrons. The increase of plasma density, which can reach during the EMW pulse several tens of percents in relative value, leads to a m odulation in time of the spatial phase of the EMW. Experimental result s for the evolution in space and in time of density, electric field, a nd frequency shift are compared to a simple nonlinear model of EMW pro pagation in a plasma, which takes into account the additional ionizati on due to the absorption of the EMW. (C) 1997 American Institute of Ph ysics.