Plasma-Wall Interaction in the Open Divertor of Globus-M Spherical Tokamak

Composition, morphology and structure of the deposited (mixed) surface layers after preliminary carboboronization procedure (B/C:H layers deposition) and subsequent plasma-wall interaction in the different regions of Globus-M spherical tokamak have been analyzed. Globus-M to the best of our knowledge is unique in that its first wall is almost completely covered by RGTi protection tiles: currently about 90% of its inner vessel area facing to plasma, is covered by RGTi graphite tiles doped with 2 at.% of Ti and 0.3-0.7 at.% of Si. The mixed layer properties were examined by different diagnostic techniques (electron probe microanalysis, scanning electron microscope, X-ray diffraction, Rutherford backscattering, nuclear resonance reactions, thermal desorption spectroscopy, infrared Raman scattering, and secondary ion mass spectroscopy) after sample exposure to plasma during 3000-10000 tokamak pulses. It was found that mixed protective layers and/or deposits existed even in high flux regions (separatrix strike points). The initial composition of the protective layers formed during boronization is dramatically modified during longterm plasma-wall interaction and resulted in significant intermixing of its components with inner vessel materials. Properties of the mixed layers deposited onto the surfaces intensively interacting with plasma, as well as the composition of the mixed layers deposited in the “shadowed” zones are discussed. Deuterium retention in Globus-M tokamak was estimated relying upon the data on deuterium concentrations in the mixed layers that were measured by different methods. It is revealed that deuterium was absorbed only in the mixed layers and its concentration vanished in the bulk of the tiles. Conditions of deuterium desorption are analyzed and merits of Ti doped graphite are discussed.