Radiation and spectroscopy analysis of divertor discharges with neon gas puff in JT-60U

Abstract

A radiative divertor experiment with neon gas puff was carried out with the aim of investigating impurity behaviour, after the open divertor was modified to the pumped W-shaped divertor. To evaluate neon radiation, we analysed lines from the neon ion measured with a VUV spectrometer in the divertor plasma. As a result, lines from Ne IV-Ne VIII were identified. By combining the measurement of an absolutely calibrated multichannel interference filter spectrometer with that of the VUV spectrometer, the radiation loss through neon line emission was estimated in the divertor plasma. In the case of pumping-off, the detached plasma evolved into multifaceted asymmetric radiation from the edge (MARFE). It was observed that the line intensities of highly ionized neon (Ne VII, Ne VIII) increased simultaneously with the formation of the MARFE and furthermore increased after the MARFE formation. It is considered that after the divertor plasma was detached, the plasma flow velocity and the friction force were weaker in the case of pumping-off than in the case of pumping-on and more impurities moved toward the X-point region. These observations imply impurity back flow from the divertor upstream by the reduction of friction force.