Plasma-induced frequency shifts in microwave beams

The authors have been investigating microwave frequency shifts by two different processes. In the first, a microwave beam is reflected from a plasma jet produced by a Bostic gun. Frequency upshifts of about four MHz are produced on a beam at 3 GHz. The reflected signal at 3GHz is observed to drop in intensity during this process by about 30 dB. In the second process, microwaves are propagated through a waveguide in which plasma is suddenly created by a pulsed electrical discharge. Since the phase velocity is a plasma is higher than in a vacuum, we might expect the microwaves to be ejected from the plasma, producing a frequency upshift. Experimentally, we find that a frequency upshift of over 10 MHz in a signal of 2.6 GHZ is routinely observed, as shown. An interesting observation is that a frequency downshift is observed. This frequency downshift appears only in a system at low gas pressure, less than 10-4 Torr, and occurs later in time than the frequency upshift. The authors suspect that it is the result of plasma decay, and is the inverse of our second process described above