A calculation model for density limits in auxiliary heated, gas fueled tokamaks and application to DIII-D model problems

摘要

A model for the calculation of density limits in high confinement (H-mode) tokamaks is described. The model consists of coupled calculations of (1) the power, particle and momentum balances for the core plasma and for the divertor/SOL (scrape-off layer) plasma; (2) the transport of fueling and recycling neutrals; (3) pedestal gradient scale lengths and MHD (magnetohydrodynamic) limits; (4) confinement degradation due to thermal instabilities in the edge transport barrier; (5) divertor and core MARFE (multifaceted asymmetric radiation from edge) onset; (6) radiative collapse leading to disruption, and (7) power threshold for high-to-low mode transition. The model is applied to study the effects of different operational parameters (e.g., P, I, B) on the density limit for auxiliary heated, gas fueled DIII-D [J. L. Luxon, F. Batty, C. B. Baxi , Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] model problems. Predicted values of densities, temperatures, and edge gradient scale length are generally consistent with measured values, and many of the phenomena observed experimentally in association with density limits are predicted. It is found that the MARFE density limit increases strongly with increasing auxiliary heating power per unit plasma surface area (i.e., power flux exiting the core into the SOL) and with increasing plasma current, and decreases weakly with increasing magnetic field and with increasing carbon impurity concentration. (C) 2001 American Institute of Physics. [References: 43]