Measurements of thermal electron heating and the formation of a non-Maxwellian energy distribution due to ion acoustic turbulence

The interaction of intense microwaves with an inhomogeneous plasma is studied in the U.C. Davis Prometheus III Device. P-polarized microwaves (f = 1.2 GHz, P0 less than or equal to 5 KW) are incident on an essentially collisionless plasma with a long scale length in an oversized waveguide. For modest powers, large amplitude ion acoustic turbulence is observed on the underdense plasma shelf due to a combination of the parametric decay and the electron drift instabilities. Suprathermal and thermal electrons are strongly heated in this region with the thermal heating due to scattering with the ion turbulence. Since the cross section for interaction decreases rapidly as the electron energy increases, the low energy electrons are preferentially heated. The electron distribution function is measured and agrees with theory; the power absorption is reduced by up to a factor of two compared to a Maxwellian distribution. After the microwaves have been measured to decay, the electron distribution function is seen to relax back to its initial Maxwellian form. This occurs, as theory predicts, roughly on the electron-electron collision time scale