MHD STABILITY OF HIGH-BETA PLASMAS IN CHS

In the vacuum magnetic field configuration of CHS, the MHD stability depends on the position of magnetic axis (R(ax)). When R(ax) > 95 cm, the magnetic well in the central region and the strong magnetic shear in the boundary region give the MHD stability for the ideal interchange. For the inward shifted configuration, the Mercier unstable region comes out. The volume-averaged equilibrium beta 2.1 % was obtained in 1993 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 evaluate 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 strong MHD activities were observed for those discharges.