Beam emission spectroscopy measurement for density fluctuations in compact helical system

A system of beam emission spectroscopy (BES) which has been proposed as a method for the measurement of long wavelength plasma density fluctuations has been applied to compact helical system (CHS). Spatial channels of BES consist of 16 optical fibers with object lenses focused on the plasma minor radius in CHS. Spatial resolution which corresponds to the width of a sightline and the spatial pitch between sightlines are 1.0 and 1.1 cm on the imaging surface, respectively. The hydrogen neutral beam for heating with the acceleration voltage of about 28 keV is used for the probe beam. The avalanche photodiode detectors combined with a 100 kHz low-pass filter are used as photon detectors. The temporal evolution of the inverse density scale length Ln-1 and the density fluctuation were measured simultaneously at the edge region where the beam attenuation was negligible. As an initial result, the rapid increase of the local plasma density gradient near the plasma edge accompanied by a coherent-like density fluctuation of several kilohertz was detected by using this system when the edge particle transport barrier was formed. Measuring the density fluctuations at the inner regions considering the beam profile, its attenuation, and normalized radii of the sightlines with finite beta shift will be a subject of study in the future