Candidate materials on the KTM tokamak first wall and divertor plates: study goals

Full text: One of the most complex and pressing problems of modern physics and controlled nuclear fusion technology is the selection of structural materials. The volume of modern material-testing studies being carried out at operating tokamaks and plasma radiation sources does not correspond to the scale of the problem inasmuch as these facilities are not designed to solve such tasks. For example, the TORSUPRA superconducting tokamak (France) allows only experiments to test the first wall materials to be carried out. At present, it is not possible at any facility to test divertor plate prototypes. Attempts to model the processes occurring under real thermonuclear reactor conditions by using impact or quasistationary plasma sources did not achieve suitable work conditions. The exception to this is large current disruptions modes, when there is a local interaction of plasma and the chamber wall in a short period of time (10-4 s). Thus, the main goal of creating the KTM experimental complex (the spherical tokamak with an aspect ratio of A=2) is the study and testing of materials to protect the first wall, receiving divertor plates, and divertor connections under modes of energy loads similar to ITER and future reactors; these are materials that are able to absorb modulated heat fluxes up to 10-20 W/m2. The main principle of the facility operation is creation and maintenance of a stationary regime of plasma column burning with temperature of >107 under additional RF heating and: other types of heating. The facility's design features include a movable divertor device and transport sluice device. They provide a means of decreasing loads and removing heat from the surface of divertor plates, to study the influence of divertor volume processes on a hot plasma column, to test the divertor device prototypes, determine optimal materials for first wall protection, as well as to test plasma disruption modes. One of the results of the work at the KTM facility will be the creation of a TNR candidate materials database. To ensure test conditions that simulate the real conditions of material utilization in thermonuclear reactors the following parameters are most important: the density of plasma flux on the protection screens of the first wall and divertor, the average energy of impinging particles, the duration of discharge and period between impacts, the temperature of the protection screens and samples. Practically all these parameters, except for discharge duration, can be realized in the KTM tokamak. The materials with a small Z (graphite, graphite composites, and beryllium) are intended to be used as candidate protection materials for the KTM tokamak first wall. Liquid metal divertors are planned to be tested together with wolfram, beryllium, graphite and C-C composites, which, as experience and calculations indicate, do not always ensure working and resource characteristics because of high erosion rate, plasma contamination with impurities, material code-position and tritium accumulation. Conception of lithium divertor (for example, on the basis of capillary-porous system or moving lithium film) needs elaboration and testing of various designs of heat removal, selection of compatible structural materials, and carrying out additional plasma-physical calculations