Measurement of coherent temperature fluctuations on the stellarator Wendelstein 7-AS

Coherent modes of temperature fluctuations have been observed and radially localized with a multichannel heterodyne radiometer of the ECE diagnostics. The modes are observed during NBI discharges with edge values of the rotational transform mainly in the range between 0.32 and 0.36. The mode numbers are m=3, n=1. The oscillations in the frequency range 18 to 55 kHz are driven by the fast beam ion component. This can be concluded comparing the rapid decay (<500 μs) of the mode after switch-off of the beams with the slowing down time and the energy confinement time. The plasma still further exists for several ten ms. Critical parameters for the occurrence of the modes seem to be the density and the profile of the rotational transform. No direct impact on the confinement properties of the plasma is observed. (author) 5 refs., 4 figs