OBSERVATIONS OF STRONG MICROWAVE-ABSORPTION IN COLLISIONAL PLASMAS WITH GRADUAL DENSITY GRADIENTS

Strong microwave absorption was observed when a 10 GHz source illuminated an underdense collisional plasma that had a density gradient scale length several wavelengths long. Significant reductions in angular scattering and cross-polarized components were also observed. These experiments confirm that absorption was the dominant process. The plasma was created by the photoionization of tetrakisdimethylaminoethylene molecules seeded into atmospheric pressure helium. Sparkboard arrays provided the intense vacuum ultraviolet ionizing radiation. Plasma density profiles were measured using transverse scans of 9.7 GHz probe microwaves and were found to approximate an Epstein profile. The absorption at 10 GHz by this plasma was as large as 28 dB in direct backscattering and 15-20 dB when orthogonally polarized microwaves were launched and detected. The peak absorption scales with sparkboard energy in a way that suggests that electron-ion recombination is the dominant electron-loss mechanism at high plasma densities.