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 duration are altered. The largest frequency upshifts and pulse compressions are obtained when radiation is incident on an ionization front moving 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 propagates. We have done theoretical and experimental work on the interaction of radiation with a relativistically propagating, underdense ionization front in a waveguide. In the experiment, 35 GHz microwave pulses were upshifted and compressed upon encountering a moving front. The frequency spectrum of the upshifted radiation was determined independently using sections of cutoff waveguides and a microwave diffraction grating. These frequency upshifts were proportional to the plasma density of the ionization front as predicted by the theory. The front density was determined using microwave interferometery. The pusewidths of the upshifted radiation were measured with fast diode detectors. These pulsewidth measurements were also in good agreement with the theory. Frequency upshifts and pulse compressions up to a factor of five were recorded in this experiment.