Continuous laser illumination for in situ investigation of tungsten erosion under transient thermal loads

Abstract

Two types of in situ optical diagnostics of erosion processes on tungsten surface caused by transient heat loads were developed and applied on the BETA (Beam of Electrons for materials Test Applications) facility at Budker Institute of Nuclear Physics. Tungsten plates were exposed to an electron beam with a duration of up to 350 μs and with an absorbed heat flux factor (HFF) of 20-50 MJ/m2s0.5, which is below the melting threshold. The distribution of thermal radiation on the sample front surface was imaged by a CCD camera. In addition, the illumination of the target surface with a continuous wave laser was used to record various types of material modification: roughening, cracking, bending, and local melting. The tile bending is associated with thermal expansion and irreversible plastic deformation of the heated thin layer. The lift of edges of the cracks during their formation is detected by recording the intensity of reflected and scattered radiation, and the resulting structure of the cracks is visualized with a CCD camera. The latter system can also identify the appearance of molten grains on the irradiated surface due to their low thermal bond to the material bulk. The described optical systems are able to detect first wall damaging during a plasma discharge in devices with magnetic confinement.