The operation of fusion reactor on the base of tokamak geometry of magnetic field is expected to be accompanied by pulsed heat loads to divertor. The loads lead to formation of plastic deformations and residual mechanical stresses. The stresses may result in mechanical destruction of tungsten armor of divertor. Typically it appears as a crack formation. The cracks decreases the heat removal from the surface, spoils the geometry of plasma flow to divertor, make easier the formation and ejection of micro particles, etc. The optical and X-ray diagnostics are applied to studies of the stresses and deformations in tungsten caused by pulsed heat loads in Budker Institute.
The optical diagnostics are applied at the facility BETA (Beam of Electrons for material Test Application). The measuring of thermal radiation and scattered on the surface laser radiation during pulsed heating gives information about the surface temperature, roughness and curvature of the surface, etc. Among the most significant discoveries made using these diagnostics, we can note the detection of a delay between the end of the pulse heating and the formation of cracks by several orders of magnitude compared to the expected values. Recently, a new optical diagnostic has been developed, which allows determining the radius of curvature of the surface by changing the focal length. The results of the diagnostics confirmed the previous results on the delay. In addition, measuring the dynamics of surface curvature allows calculating the distribution of deformations and stresses along the depth of the sample. Of course, the results of such calculations depend on the applied strain models.
The more direct methods for measuring residual stresses are used at stations of X-ray scattering «Precision diffractometry and anomalous scattering» and «Diffraction "movie"» at the VEPP-3 source of synchrotron radiation. The residual stresses may be calculated using the dependence of the scattering angle on the inclination of the sample. The station «Precision diffractometry and anomalous scattering» was used to make measurements of residual stresses with the spatial distributions. The decreasing of residual stress during slow heating simulating the result of continual heating of divertor was measured at the station «Diffraction "movie"».
The station of X-ray radiation «Plasma» at the VEPP-4 source of synchrotron radiation is oriented to measurements of deformations and mechanical stresses in during pulsed heating simulating by laser radiation. The previous experiments demonstrated the possibility of the measurements of the depth distribution of deformations in single-crystal tungsten with the time resolution about 10 μs. The first demonstrative experiments on the measurements of the diffraction on polycrystall tungsten were carried out. The acceptable magnitude of signal were measured with the whole exposure 150 ms. The time resolution is admissible for measurements of the dynamic of the stress between the pulsed heating and crack formation.