Correlation spectroscopy diagnostics in the H-l heliac

A simple diagnostic arrangement is suggested for measuring density and temperature fluctuations. The diagnostic uses a cross-correlation technique to extract local frequency spectra, spatial distribution or (in the case of low mode numbers) the mode structure from the chord-average fluctuating intensities of the visible light in the H-l heliac. For low temperature H-l plasmas (T, x 20 eV, n, z 2 x lo’* crrP3), the spectral line emission from neutral atoms is used. For strong turbulence, where the coherence length of the fluctuations is much shorter than the plasma radius, the amplitude of the cros#s-correlation between two crossed-sightline fluctuating intensities is proportional to the fluctuation amplitude in the intersection volume. The optical arrangement on the H-l heliac uses two orthogonal views with mirrors that allow the intersection volume to be scanned over the full plasma poloidal cross-section. Under certain discharge conditions, 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 (because, in this case, ,the fluctuation correlation length is of the order of the plasma diameter), the measurements allow the poloidal mod.e number and spatial localization of the fluctuations to be determined after comparison with numerical modelling results; these show good agreement with the Langmuir probe results.