Cavity diagnostics on fluctuating discharges

Abstract

Resonance cavity diagnostics are applied to the magnetized plasma of a reflex discharge as well as an inductive RF discharge. Special attention is focused on the evaluation of the collisional frequency in the cavity Q and the underlying physical mechanism. When the conventional transmission method is used, a strong broadening of the frequency response curve is observed: the time-averaged transmission profile turns out to be a Voigt profile that arises from the Lorentzian profile of the instantaneous transmission curve and the Gaussian probability distribution of the density fluctuations. Obviously, the resonance broadening is not caused by collisional damping, but mainly by large scale density fluctuations. To measure the real absorption of the microwave power, the temporal decay of the electromagnetic wave energy in the cavity is detected. Two characteristic features are observed: (i) At magnetic field strengths well below the electron cyclotron resonance field, the measured (effective) collisional frequency nu eff shows a threshold behaviour as a function of the electron density. Below 7*1015m-3 the measured collisional frequency fits the electron-neutral collisional frequency, whereas above 9*1015m-3, nu eff tends to a value twice as high. (ii) Near the cyclotron resonance a strongly enhanced absorption is observed at low densities, though the TM010 cavity mode used has no electric field components perpendicular to the constant magnetic field. Neither effect can be attributed to the observed density fluctuations in the discharges. Various absorption processes are discussed to explain the findings.