The possible nature of hot spots on tokamak walls

The authors consider the mechanism of the contraction of the thermal contact area between the plasma and the wall in a tokamak. This mechanism is attributed to the strong dependence of the thermionic emission of electrons on the temperature of the wall surface. A simple, largely analytical model for the description of the heat balances of the plasma and the wall is developed. An instability of small perturbations of homogeneous stationary states is investigated. The possibility of the existence of 'autosolitons', i.e. long living solitary hot spots with a surface temperature of 3000 degrees C, being an intensive source of impurities in a discharge, is shown. The theoretical results are compared with experimental data from TFTR and JET