Плазменно-тепловая нагрузка в диверторе термоядерного реактора-токамака

Abstract

The problem of the limiting plasma heat load on the first wall in a tokamak fusion reactor is considered. It is pointed out, based on a review of available experimental data, that the major part of the heat flux from the plasma falling on the wall is absorbed by the surface in a narrow (a few millimeters wide) radial layer in the zone where the separatrix comes in contact with the diverter plates. The revealed empirical regularity relating to existence of an upper limit for the averaged heat load on the tokamak first wall is a factor generating the need to solve the challenging matters of ensuring stability of a stationary plasma discharge in a tokamak fusion reactor. The mechanisms imposing limits on the stationary discharge duration in a tokamak have not been investigated as yet. In the literature, only a few mechanisms are reviewed, including the influence of erosion material films formed on the tokamak first wall surface. In the article, the heat load on the critical area of the contact between the material surface and a plasma separatrix or the last closed magnetic surface is analyzed. Such analysis makes possible to elaborate additional criteria for estimating the limiting plasma heat load on the walls during steady-state operation of a tokamak fusion reactor: the main contribution in this effect is due to the heat flux limit on the diverter plates in a narrow (of a millimeter scale) zone of interaction between the separatrix and material surface. The limiting heat flux to the diverter plates depends on the conditions on the surface, including the degree of its roughness, porosity, and arcing effects. For a stationary tokamak fusion reactor, it should be expected that the plasma-wall interaction is mainly governed by the collective effects developing on space-time scales that vary by 6--12 orders of magnitude. The multiscale nature of the plasma-wall interaction in a tokamak reactor implies the need of using power laws to describe the effects. An interrelation (in power law form) between the electron temperature and plasma density near the separatrix is proposed proceeding from the results of analyzing the limiting heat fluxes on the wall. Such dependence should be expected in the H operation modes of a stationary tokamak fusion reactor with the limit discharge parameters, in which significant changes occur in the diverter plate surface profiles and properties influencing the heat transfer through the plasma sheath layer, including arcing effects. It is advisable to further extend the base of experimental data for carrying out a generalizing analysis of the interrelation between the electron temperature and plasma density near the separatrix. With such data at hand, it will become possible to develop approaches to control the plasma-wall interaction with a view to achieve the optimal conditions for maintaining stationary discharge in a tokamak reactor.