Speaker
Description
Tungsten heavy alloys (WHA) consisting of tungsten with small amounts (few weight %) of nickel (Ni) and iron (Fe) or copper (Cu) are ductile and tough materials, which could in some cases replace brittle bulk tungsten as a plasma-facing material. Therefore, such materials are considered for moderately loaded areas in current and future fusion reactors, e.g. the stellarator W7-X, the high-field tokamak SPARC and the temporary first wall of ITER.
The high heat flux test facility GLADIS was used to study the behavior of these alloys under high heat flux loading. Mock-ups designed for SPARC made of Ni (2%) – Fe (1%) tungsten heavy alloy were quasi adiabatically loaded under various loading conditions to assess their behavior and the operational limits.
Using a continuous beam with a heat flux of 30 MW/m² the effect of extreme overload was evaluated. Since the melting points and vapor pressures of W and the alloying elements are strongly different, the purpose was to investigate the sample behavior in the temperature range, where the Ni-Fe matrix exceeds its melting point. Due to the high vapor pressure noticeable evaporation of Ni and Fe occurred. This was investigated in a time- and temperature-resolved manner by optical spectroscopy. Despite considerable material loss and strong morphological changes, the thermomechanical integrity of the component was maintained. This was confirmed by post-exposure metallographic and microscopic analysis.
Using a much lower power density of 5 MW/m² the surface temperature increases much more gradually allowing for a higher time and temperature resolution up to the melting point of the Ni-Fe matrix. In this setup, pulses with a length on the order of 15 s can be employed and detailed spectroscopic data correlated with the tile temperature measured by pyrometry and thermocouples can be extracted. At the same time witness samples catching part of the evaporated elements were installed and investigated by surface analysis techniques for a semi-quantitative evaluation of the material loss. The results from this work provide a parameterization of ejected material as a function of temperature, and are used to define operational limits for SPARC.
Finally, the surface morphology of the WHA tiles is compared to the results observed after earlier exposure of similar material in AUG and DIII-D to link the results to reactor conditions.