Doppler velocimetry of moving ionization fronts in free space plasmas generated by continuous high power microwave beams.

The Air Force Research Laboratory (AFRL) is studying the stability of plasmas generated and sustained indefinitely in high power microwave beams. The plasmas exhibit two typical discharge modes: fully stable (typically at lower power) and quasi-stable (typical at higher power). The quasi-stable mode consists of a series of highly repetitive ionization fronts propagating from near the beams focus towards the broadcast antenna. This phenomenon is closely related to similar behavior observed in pulsed experiments and in descending ionization layers overserved by the HAARP facility. The motion of the reflective ionization front illuminated by the drive beam provides a natural opportunity for the use of Doppler velocimetry to measure the speed of the ionization front. This velocity can be related to key plasma parameters such as the electron temperature and diffusion rates. Plasma parameters estimated using this technique are compared to measurements taken with conventional diagnostics to quantify velocimetry’s utility in circumstances where direct diagnostic access may not be practical. DISTRIBUTION A. Approved for public release: distribution unlimited. PR number AFMC-2018-0104.