Microwave Interactions with Plasmas

: Microwave interactions with a cold, collisional plasma having gradual density gradients were studied. The plasma was created by the photoionization of tetrakisdimethylaminoethylene (TMAE) vapor seeded into atmospheric pressure helium. Photoionization was provided primarily by sparkboard Spatial scans of the absorption of a microwave probe beam along chords across the plasma, with subsequent Abel inversion, yielded three-dimensional plasma density profiles that showed a gradual decrease in the peak plasma density versus distance away from the sparkboard. When a 58cm-diameter reflector was illuminated with 10-GHz microwaves in an anechoic chamber, the plasma sorbed as much as 28 dB in direct reflection, with similar attenuation of the normally weak side-scattered and cross-polarized radiation.The attenuation was compared with model predictions. Detailed analysis of the temporal and spatial dependence of the electron density following ionization that both recombination and attachment processes influenced the plasma decay. The recombination rate of TMAE was found to be (9.0 +/- 1.1) x 10(-6) cm3 s(-1) for 300 K electrons. This work confirms the effectiveness of a cold, collisional plasma as a broadband, switchable wave absorber.