Reheat Mode Discharges in Search of Attainable High Stored Energy and Density Limit of Compact Helical System

The results of experiments to attain high stored energy and density in the Compact Helical System (CHS) are reported. The experiments have been carried out under the maximum neutral beam heating power and highest magnetic field strength of the CHS. With the help of the reheat mode, we have so far reached a stored energy of 9.4 kJ and a density limit expressed as n_c = 0.65(P_absB_t /V_p)^0.5 for the CHS. In the high-density regime, the confinement of CHS plasma is limited by radiation collapse. A multichannel H light detector system shows an asymmetric feature in the poloidal cross section and indicates that confinement degradation in the high-density regime begins at the inboard side where the CHS plasma is close to the vacuum vessel wall.