A SYSTEM TO MEASURE SUPRATHERMAL ELECTRON-DISTRIBUTION FUNCTIONS IN TOROIDAL PLASMAS BY ELECTRON-CYCLOTRON WAVE ABSORPTION

A two-chord, four-beam suprathermal electron diagnostic has been insta lled on TdeV (B > 1.5 T, R = 0.86 m, a = 0.25 m). Resonant absorption of extraordinary mode electron cyclotron waves is measured to deduce t he chordal averaged suprathermal electron distribution function amplit ude at the resonant momentum. Simultaneously counterpropagating beams permit good refractive loss cancellation. A nonlinear frequency sweep leads to a concentration of appropriately propagating power in a narro w range of time of flight, thus increasing the signal-to-noise ratio a nd facilitating the rejection of spurious reflections. Numerous measur ements of electron distribution functions have been obtained during lo wer-hybrid current-drive experiments. (C) 1997 American Institute of P hysics.