Particle and energy transport studies of TFTR and implications for helium ash in future fusion devices

Local thermal particle and energy transport studies of balanced-injection L-mode and Supershot deuterium plasmas with the same toroidal field, plasma current, and neutral beam heating power have been performed on TFTR. The particle transport of He2+ and electrons following a small helium gas puff and Fe24+ introduced by laser ablation has been examined and compared to the local energy transport characteristics inferred from power balance analysis. All particle perturbation diffusivities are radially hollow and are similar in magnitude and shape to the effective thermal conductivities found by power balance analysis. All particle diffusivities are 1-2 order of magnitude larger than neoclassical values, except near the magnetic axis. A reduction in the helium diffusivity DHe in the Supershot as compared to the L-mode is accompanied by a similar reduction in the effective single fluid thermal conductivity χfluid. Also, the helium core convective velocity VHe is found to increase in the Supershot over the L-mode for r/a < 0.5. A quasilinear model of electrostatic drift waves has been used to calculated ratios between particle and energy fluxes in the Supershot. The measured ratios of the helium and iron particle diffusivities are in good accord with predictions, as are predicted ratios of VHe/DHe. Modelling indicates that the similarity in magnitude and profile shape of DHe and χFluid has generally favorable implications for helium ash content in a future reactor. The core convection found in the Supershot increases the helium concentration on axis but does not reduce the plasma reactivity significantly. (author). 24 refs, 4 figs 1 tab