Frequency downshift in rapidly ionizing media

Recently, theory, computer simulation and experiment have demonstrated that the frequency of an electromagnetic (EM) wave interacting with a rapidly created plasma is upshifted. The interest in frequency upshifting phenomenon can be attributed to its many potential applications including the enhancement of the frequency tuning capability of existing sources of EM waves and improvement of the performance of high-power radar and direct energy systems by enabling reflectionless propagation of very powerful microwave pulses through the air, etc. Moreover, there are no technical difficulties in producing dense plasma rapidly. It was observed, however, that in some experiments a frequency downshift also exists. Since the created plasma decayed much slower than its growth and since the frequency downshift lines did not appear in the results of computer simulations using a lossless unmagnetized plasma model, the frequency downshift phenomenon has not been discussed. In this work, experimental evidence and theory is presented for the simultaneous creation of a downshifted wave. There is excellent agreement between experimental results and computer simulation based on a simple theory.