Edge and internal transport barrier formations in CHS

Abstract

Edge transport barrier (ETB) formation was observed in a compact helical system. A sharp decrease in Hα emission indicates the quick transition of edge particle transport. The increase in the density gradient at the edge was measured by using various profile diagnostics. The transition process has a slowly developing pre-phase, and a quick transition for which the fastest case is less than 0.2 ms. There exists a heating power threshold which is roughly proportional to the density and the magnetic field. The transition and back transition is controlled by the heating power. The magnetic configuration effect on ETB formation was also studied. The local density measurement by beam emission spectroscopy shows intermittent bursts of low frequency fluctuations during the ETB formation phase. When the initial density profile is very hollow, ETB formation together with the electron temperature increase (electron internal transport barrier) in the core region, was observed for the NBI discharges without ECH.