Modeling of a-Particle Redistribution by Sawteeth in TFTR using FPPT Code
Authors: N. N. Gorelenkov , R. V. Budny, H. H. Duong , et al.
Results from recent DT experiments on TFTR to measure the radial density profiles of fast confined well trapped a-particles using the Pellet Charge eXchange (PCX) diagnostic [PETROV M. P., et.al. Nucl. Fusion, 35 (1995) 1437] indicate that sawtooth oscillations produce a significant broadening of the trapped alpha radial density profiles. Conventional models consistent with measured sawtooth effects on passing particles do not provide satisfactory simulations of the trapped particle mixing measured by PCX diagnostic. We propose a different mechanism for fast particle mixing during the sawtooth crash to explain the trapped a-particle density profile broadening after the crash. The model is based on the fast particle orbit averaged toroidal drift in a perturbed helical electric field with an adjustable absolute value. Such a drift of the fast particles results in a change of their energy and a redistribution in phase space. The energy redistribution is shown to obey the diffusion equation, while the redistribution in toroidal momentum Pj (or in minor radius) is assumed stochastic with large diffusion coefficient and was taken at. The distribution function in a pre-sawtooth plasma and its evolution in a post-sawtooth crash plasma is simulated using the Fokker-Planck Post-TRANSP (FPPT) processor code. It is shown that FPPT calculated a-particle distributions are consistent with TRANSP Monte-Carlo calculations. Comparison of FPPT results with Pellet Charge eXchange (PCX) measurements shows good agreement for both sawtooth free and sawtoothing plasmas.