MHD STABILITY OF HIGH-BETA PLASMAS IN CHS

In the vacuum magnetic field configuration of CHS, the MHD stability d epends on the position of magnetic axis (R(ax)). When R(ax) > 95 cm, t he magnetic well in the central region and the strong magnetic shear i n the boundary region give the MHD stability for the ideal interchange . For the inward shifted configuration, the Mercier unstable region co mes out. The volume-averaged equilibrium beta 2.1 % was obtained in 19 93 with R(ax) = 92 cm configuration. The self-stabilization effect of high-beta plasma gave Mercier stable equilibrium while the R(ax) = 92 cm configuration has the Mercier unstable region for low-beta plasmas. The measurements of magnetic fluctuations and the soft X-ray signals did not show strong instabilities in these discharges. In order to eva luate the stability boundary for ideal interchange instabilities, the efforts of producing high-beta plasmas have been made for more inward shifted magnetic axis configurations (89 cm < R(ax) < 99 cm). The stro ng MHD activities were observed for those discharges.