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In a tokamak configuration, superposition of the magnetic field of resonant helical windings which surround the toroidal plasma current outside the first wall destroys the magnetic surfaces in the boundary layer (ergodization). A transport model is analysed, where convective flow of the plasma from the boundary layer to the first wall permits elevated particle densities in the boundary layer and leads to very high particle and energy transport. The convective flow is driven by the pressure gradient along the field lines which intersect the toroidal wall at an oblique small angle ϵ. The required thickness Δ of the boundary layer is around 1015 n−1·cm−2. As a result, the plasma temperature there can be reduced towards the threshold of critical plasma-wall-interaction processes, the plasma core can be shielded against impurities from the wall and, at the same time, a very short life-time of all particles in the boundary layer can be achieved (use of pumpholes and/or scrape-off-limiters for removing ash). Thus, this model also improves the concepts of edge radiation cooling. An estimate is given of the parameters of INTOR using only a weak helical perturbation field which conserves the magnetic surfaces in the plasma core: one can reach wall temperatures Tw between 20 and 30 eV in the presence of wall densities nw approaching 1014cm−3.

Solutions are given for axisymmetric MHD-equilibria obtained by superposing the magnetic field of the plasma currents and the field of current-carrying conductors situated outside the plasma. These equilibria were numerically calculated by several different iteration methods for solving the non-linear free boundary problem. They can describe experiments in which the discharge time is long compared with the diffusion time of the magnetic field through the external conducting walls. Equilibria were obtained both for approximately circular plasma cross-sections and strongly elongated (elliptical) ones. Such calculations are of interest for constructing axisymmetric tokamak divertors, as well as for producing elongated crosssections, which have been postulated to be stable for higher values of plasma pressure. The results presented here are restricted to the case of zero plasma pressure and a particular current distribution in order to reduce the number of free parameters and allow more detailed discussion of the dependence of the equilibrium configurations on the arrangement of the external conductors and the currents.

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