Comparison of fast-imaging and probe measurements

Langmuir-probe systems with high spatial and temporal resolution are well established for turbulence measurements in the stellarator experiment TJ-K. As a complementary, non-intrusive diagnostics, a Photron-FASTCAM-SA5 high-speed camera is used. In this work the comparability of probe measurements and fast imaging is investigated. The signal-to-noise ratio of the camera signal limits the dynamical range of detectable fluctuations. Meaningful signals are obtained for frame rates in the range of 150 up to 300 kfps. The light intensity of the 656,4 nm Helium emission line can be modelled as I∝ne1.6Te2.0. The intensity from the full spectrum has been found to depend linearly on the plasma density. Since in TJ-K temperature fluctuations are negligible, intensity and ion-saturation current fluctuations are directly proportional. Cross-correlation analyses of camera data with respect to a fixed reference probe in the focal plane show high correlation values and reveal poloidally propagating structures if performed with all pixels of the 2D-camera image. The two-dimensional structure of the fluctuations from both, the correlation and conditional averaging analysis, turns out to be similar for probe and pixel data taken as reference. To benchmark the results of measurements with a single reference probe, a direct comparison with 2D probe measurements using an 8 x 8 probe matrix are presented.