CORRELATION SPECTROSCOPY DIAGNOSTICS IN THE H-1 HELIAC

A simple diagnostic arrangement is suggested for measuring density and temperature fluctuations. The diagnostic uses a cross-correlation tec hnique to extract local frequency spectra, spatial distribution or (in the case of low mode numbers) the mode structure from the chord-avera ge fluctuating intensities of the visible light in the H-1 heliac. For low temperature H-1 plasmas (T-e approximate to 20 eV, n(e) approxima te to 2 x 10(12) cm(-3)), the spectral line emission from neutral atom s is used. For strong turbulence, where the coherence length of the fl uctuations is much shorter than the plasma radius, the amplitude of th e cross-correlation between two crossed-sightline fluctuating intensit ies is proportional to the fluctuation amplitude in the intersection v olume. The optical arrangement on the H-1 heliac uses two orthogonal v iews with mirrors that allow the intersection volume to be scanned ove r the full plasma poloidal cross-section. Under certain discharge cond itions, the plasma is dominated by strong, low frequency oscillations with frequencies in the range 15-20 kHz. Although such discharges are not very suitable for implementing the main idea of the diagnostic (be cause, in this case, the fluctuation correlation length is of the orde r of the plasma diameter), the measurements allow the poloidal mode nu mber and spatial localization of the fluctuations to be determined aft er comparison with numerical modelling results; these show good agreem ent with the Langmuir probe results. (C) 1997 Elsevier Science S.A.