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 were explored by computer simulations. It is shown that the observed improvement of the energy and particle confinement is due to a double or triple reduction of the electron heat diffusivity x(e) and the diffusion coefficient D that is correlated with the density scale length, while the heat diffusivity x(i) is constant. This result agrees with measurements of the diffusion coefficient and with the coupling between x(e) and D found in all confinement regimes. It is shown that there are lower limits for x(e) and D. Empirical relations for x(e) and x(i), D and the inward drift velocity v(in) and their scalings in different confinement regimes are presented. The enhancement factor for x(i) with respect to the neoclassical value is found to be 1.4. Modelling of the measured density profiles requires a time independent ratio v(in)/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.
Bibliographic Reference: Article: Nuclear Fusion, Vol. 30 (1990) No. 11, pp. 2285-2293
Record Number: 199110057 / Last updated on: 1994-12-02
Original language: en
Available languages: en