Plasma-induced frequency shifts in microwave beams

Summary form only given, as follows. Microwave frequency shifts have been investigated 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 4 MHz are produced on a beam at 3 GHz. The reflected signal at 3 GHz 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 in a plasma is higher than in a vacuum, one might expect the microwaves to be ejected from the plasma, producing a frequency upshift. Experimentally, it is found that a frequency upshift of over 10 MHz in a signal of 2.6 GHz is routinely observed. 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. It is suspected that it is the result of plasma decay and is the inverse of the second process described above