Ion Temperature-Measurements in a Tokamak by Collective Thomson Scattering

Collective Thomson scattering, using a high-power pulsed D2O laser at 385-mu-m and a heterodyne receiver system, has provided local ion temperature (T(i)) measurements of the plasma in the tokamak chauffage Alfven (TCA) tokamak. Recent improvements in the noise-equivalent power of the Schottky barrier diode mixers permitted us to achieve a typical precision of +/- 12% for a single shot measurement at densities above 10(20) m-3. Even at densities of standard TCA discharges (5 x 10(19) m-3) the uncertainty is better than +/- 25%. For the interpretation of the measured spectra and the evaluation of T(i) the local value of the electron temperature (T(e)) is an important parameter. Therefore, T(e) was measured simultaneously by incoherent Thomson scattering at 0.694-mu-m during a series of shots. The density was obtained from a far-infrared interferometer. An independent measurement of T(i) on TCA can be obtained from a neutral particle analyzer. Comparison of the results from the two methods showed good agreement. The precision of a T(i) measurement depends strongly on the plasma density. Since an uncertainty of +/- 25% at standard densities may still not be regarded as satisfactory, further investigations using a numerical simulation code have been carried out to find ways of improvement.