Confinement and stability in ASDEX discharges close to the beta limit

Abstract

The highest β-values derived from diamagnetic flux measurements in H-discharges are found to be close to the β-limit due to kink and ideal ballooning modes. Both energy and particle confinement are degraded in these discharges and do not recover during the injection period. The energy confinement time decreases by a typical factor of two, the electron thermal diffusivity being correspondingly enhanced. Electron heat conduction is again found to be the dominant energy loss channel. The degradation of confinement close to the ideal-MHD limit suggests that kink and ballooning instabilities occur and are responsible for the enhanced transport. This conclusion is supported by the correlation with the pressure profile and by the result that global and local transport do not change in a reference discharge which is stable against ideal-MHD modes.