Controlled low-Z melting experiment in DIII-D tokamak

Controlled melting of aluminum (Al) was performed in the lower divertor of DIII-D, and the molten surface deformation was modeled with the MEMENTO code [1]. The experiment was largely motivated by using Al as a proxy for beryllium in ITER. Three Al blocks sized 1×1 cm in the toroidal and poloidal directions, with the top surface angled at ~32 degrees towards the incident plasma heat fluxes, were exposed under steady L-mode discharge conditions using the Divertor Material Evaluation System (DiMES) manipulator. During the exposure, Al samples were imaged by visible and infra-red (IR) cameras, and the current through them was measured via a shunt with resistance of ~0.15 Ω. Heat fluxes incident on the angled surfaces were inferred from IR data aided by SMITER field line tracing. The block exposed to the incident heat flux of ~10 MW/m² for ~1 sec achieved the intended shallow melting. MEMENTO code was able to qualitatively reproduce the surface deformation profile and the melt onset timing, but detailed comparison was impeded by the unknown thermo-physical properties of the sample due to strong oxidation. Cross-sectional analysis of the molten surface for improved comparison is in progress.

[1] S. Ratynskaia, et al., Nucl. Mater. Energy 33 (2022) 101303