FREQUENCY UPSHIFTING AND PULSE-COMPRESSION VIA UNDERDENSE RELATIVISTIC IONIZATION FRONTS

When pulse of electromagnetic radiation impinges upon a moving plasma/ neutral gas boundary, i.e., an ionization front, its frequency and dur ation are altered. The largest frequency upshifts and pulse compressio ns are obtained when radiation is incident on an ionization front movi ng at close to the speed of light. Such an ionization front is created when a short laser pulse photo-ionizes a gas through which it propaga tes. We have done theoretical and experimental work on the interaction of radiation with a relativistically propagating, underdense ionizati on front in a waveguide. In the experiment, 35 GHz microwave pulses we re upshifted and compressed upon encountering a moving front. The freq uency spectrum of the upshifted radiation was determined independently using sections of cutoff waveguides and a microwave diffraction grati ng. These frequency upshifts were proportional to the plasma density o f the ionization front as predicted by the theory. The front density w as determined using microwave interferometery. The pusewidths of the u pshifted radiation were measured with fast diode detectors. These puls ewidth measurements were also in good agreement with the theory. Frequ ency upshifts and pulse compressions up to a factor of five were recor ded in this experiment.