Examination of energy confinement in DIII-D at small values of the plasma safety factor

Abstract

The effect on H-mode energy confinement of operating at low values of the plasma safety factor q has been investigated experimentally on the DIII-D tokamak (see Plasma Physics and Controlled Nuclear Fusion Research 1986 (Poroc. 11th Int. Conf. Kyoto, 1986), Vol.1, IAEA, Vienna (1987) 159). Operationally, the linear increase of H-mode energy confinement with plasma current ceases when the ratio of the plasma current to the toroidal magnetic field is high than approximately=1 MA/T. Experimentally, it has been determined that this saturation of confinement is not the result of a ceiling imposed by saturated Ohmic confinement, by the plasma reaching a toroidal beta limit, by enhanced plasma radiation or by edge localized modes. The observed saturation of confinement is found to be quantitatively consistent with a model of global energy confinement degradation resulting primarily from the macroscopic phenomena of sawteeth. The experimental data are also qualitatively consistent with an alternative explanation for the confinement saturation which involves enhanced transport brought about by ion temperature gradient driven turbulence