ANALYSIS OF ENERGY AND PARTICLE-TRANSPORT AND DENSITY PROFILE PEAKING IN THE IMPROVED OHMIC CONFINEMENT REGIME

The local energy and particle transport and density profile peaking in improved Ohmic confinement (IOC) discharges of ASDEX have been explored by computer simulations. It is shown that the observed improvement of the energy and particle confinement is due to a two to three times reduction of the electron heat diffusivity χe and the diffusion coefficient D that is correlated with the density scale length, while the ion heat diffusivity χi is constant. This result agrees with measurements of the diffusion coefficient and with the coupling between χe and D which was found in all confinement regimes. It is shown that there are lower limits for χe and D. Empirical relations for χe, χi D and the inward drift velocity vin and their scalings in different confinement regimes are presented. The enhancement factor for χi with respect to the neoclassical value is found to be 1.4. Modelling of the measured density profiles requires a time independent ratio vin/D and a feedback controlled decline of the fuelling rate. Simulations with a constant high particle source yield no density profile peaking (saturated Ohmic confinement, SOC). At a given line averaged density, two steady state solutions for the density profile are obtained which correspond to high (SOC) and low (IOC) refuelling rates. © 1990 IOP Publishing Ltd.