COLLECTIVE THOMSON SCATTERING AT W7-AS

Collective Thomson scattering (CTS) of electromagnetic radiation from thermal plasma fluctuations in principle allows the velocity distribut ion of plasma ions and its composition in the plasma to be measured. T he use of powerful microwave radiation from gyrotrons opens new perspe ctives for the application of CTS, which is considered to be a promisi ng candidate for alpha-particle diagnostics in reactor-size tokamaks w ith D/T operation. We have performed the first experiments at W7-AS wi th different scattering geometries to prove the applicability of gyrot rons for CTS. The experiments were performed with a 140 GHz gyrotron w hich is routinely used for ECRH, delivering a power of 0.45 MW. The re ceiver antenna and detection system for the registration of CTS spectr a were especially designed for the scattering experiment. In backscatt ering experiments, which have inherently no spatial resolution, we hav e measured a transversely propagating, non-thermal lower-hybrid turbul ence, which is driven by perpendicularly injected fast particles from a diagnostic neutral beam. The instability is excited by the beam ions under double-resonance conditions, where the LH frequency coincides w ith some harmonic of the beam ion gyrofrequency. For scattering geomet ries with the scattering wavevector not perpendicular to the magnetic field, thermal density fluctuations in the plasma were experimentally detected. The ion temperatures derived from these thermal spectra agre e well with other diagnostics. A modified scattering geometry (90 degr ees scattering) allows local measurements of the ion temperature and i s considered a prototype for the design of a routine diagnostic for io n-temperature measurements.