Frequency upshifting and pulse compression via underdenserelativistic ionization fronts

Theoretical and experimental work on the interaction of radiation with a relativistically propagating, underdense ionization front in a waveguide was performed. 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 interferometry. The pulsewidths 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 of up to a factor of five were recorded in this experiment