Current density transport, confinement and fusion burn conditions

The coupled evolution of the plasma current density and the electron temperature in toroidal, magnetically confinement plasmas is investigated, special attention being devoted to regimes where D-T ignition can be attained. On the assumption that the electron temperature Te, and the current density, J, diffuse in response to each other's gradients as well as their own, a matrix equation for both quantities including a new thermal-viscous transport coefficient is introduced. Its symmetry properties are used to establish the form of the electron thermal and momentum balance equations. The forms of the thermal conductivity and the related thermal-viscous coefficient are deduced from the condition that the profiles of Te and Jparallel are well behaved and of the same type as those experimentally observed, while the electrical resistivity is assumed to be collisional. A model for the current density transport that is readily implemented in numerical transport codes has been used, as a supplement to the symmetry based theory, to reproduce existing experiments. The initial current rise phase of a representative, high field D-T ignition experiment is studied by using a numerical, free boundary transport analysis. This phase is shown to provide an important fraction of the ohmic heating in such experiments and also to impose important constraints on the further evolution of the discharge, in particular, on its macroscopic stability. Macroscopic m0=1 modes that depend on magnetic reconnection and finite resistivity are shown to be stable in a large parameter region that can apply to ignition experiments and present advanced experiments. High field D-T experiments with tight aspect ratio can operate within this region since they can maintain low values of beta poloidal up to ignition conditions. Then the ideal MHD stability threshold is approached only after a significant population of alpha particles has been produced, which renders both ideal MHD and resistive modes harder to excite. (author). 22 refs, 2 figs, 1 tab