Speaker
Description
We examined the effects of divertor plasmas on 14 distinct tungsten and ultra-high temperature ceramic (UHTC) materials, providing insight into how combined high heat and particle fluxes affect their surface composition and structure. The experiments were carried out using the Divertor Materials Evaluation System (DiMES) at DIII-D. The test matrix included commercially available tungsten alloys doped with (20 ppm and 30 ppm) K and (10 %) Re, along with UHTC materials produced at Stony Brook University, including NbC, (Nb+Ta)C, ZrC, WC, (W+Si)C, and SiC. In each case, the samples were exposed to 6-7 H-mode plasma shots, with an average steady-state perpendicular heat flux of 2.4 MW m$^{-2}$, an ELM heat flux of 6 MW m$^{-2}$, and an ELM frequency of 75 Hz. Select samples were cut to a 10° angled geometry to increase the intercepted heat flux to > 10 MW m$^{-2}$.
Postmortem characterization revealed that all tungsten alloys survived exposure to divertor plasmas well, with modest surface morphology changes, near-surface cracking, and leading-edge melting observed as the main effects of the plasma exposure. Spectroscopic ellipsometry, obtained before and after plasma exposure over a wavelength range of 245 – 1000 nm, was consistent with nm-scale roughening of the surface. This was confirmed with scanning electron microscopy, which also revealed evidence of grain boundary grooving commonly observed with high-temperature annealing of tungsten. Slight changes in fiducial marker geometry (including rounding of corners) due to erosion were also noted. All samples showed evidence of minor leading-edge melting (over regions spanning 50 – 100 μm in width), highlighting the sensitivity of tungsten materials to slight misalignment. The flush-mounted UHTC specimens also demonstrated promising performance, with minimal surface morphology changes observed following exposure. Additional X-ray and Auger spectroscopies are underway to assess preferential sputtering of the UHTC surfaces, as well as the use of grazing incidence X-ray diffraction and electron microscopy techniques to study grain growth and crystal structure stability. These results are expected to provide insights needed for further optimization of doped W and UHTC materials and provide guidance on materials selection for upcoming materials testing campaigns in DIII-D.
SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility, under Award(s) DE-FC02-04ER54698, DE-FG02-07ER54917, DE-SC0019256, DE-AC05-00OR22725, and DE-AC52-07NA27344.