Frequency up-shift in the interaction of high power microwave with underdense inhomogeneous plasma

The frequency up-shifts around several megahertz and other higher frequency components have been observed in the interaction of a high power microwave with an unmagnetized inhomogeneous argon plasma, while there is no ionization front in the present experiments. The dependencies of the up-shifts and higher frequency components on plasma density, incident microwave power and spatial position have been clearly demonstrated in our experiments. The time evolution of these frequency components has also been investigated. The observed frequency up-shifts are considered to be related to a rapidly expanding plasma which is created by the ponderomotive force of a strong standing wave due to the incident microwave. The theoretical calculations have been carried out, and the results are in fairly good agreement with the results observed in the present experiments. (author)