Measurements of electron densities and collision frequencies using microwaves

Summary form only given. SRI has developed and used various microwave systems to characterize the electron density and collision frequency in high-pressure and low-pressure plasmas. Systems at 10 GHz, 35 GHz and 94 GHz have been employed to characterize photoionized plasmas, electron-beam-produced plasmas, microwave-breakdown, and shock-perturbed plasmas. An interferometer employing a CO/sub 2/ laser (1=10.6 mm) has also been constructed to probe plasmas in the 1016 cm/sup -3/ range. In addition, lower frequency systems using microwave cavity perturbation methods have been developed at operating frequencies below 2 GHz. The analysis of microwave-interferometer or perturbed-cavity phase and amplitude signals yields the electron density and collision frequency. The measurements are integrated over the probe pathlength in the case of interferometers, or over the plasma volume in the case of cavities. Abel inversion and related methods are employed to determine the spatial variation of density or collision frequency. This presentation will describe SRI´s use of such methods to characterize high-pressure plasmas and their potential use in characterizing APR plasmas.