Ionization-induced frequency up-shift of a high-power microwave interacting with a plasma

Abstract

The propagation of an intense electromagnetic wave (EMW), of frequency 9 GHz, in an underdense, near critical, inhomogeneous plasma (ne⩽1.2×1012 cm−3, Te≃3 eV) leads to an up-shift of the EMW frequency of a few MHz. The injected EMW with power, P0⩽250 kW, and with pulse duration of 1 μs, produces additional ionization of the neutral argon gas (n0≃6.6×1013 cm−3) through the heating of plasma electrons. The increase of plasma density, which can reach during the EMW pulse several tens of percents in relative value, leads to a modulation in time of the spatial phase of the EMW. Experimental results for the evolution in space and in time of density, electric field, and frequency shift are compared to a simple nonlinear model of EMW propagation in a plasma, which takes into account the additional ionization due to the absorption of the EMW.