High-flux plasma exposure of ultra-fine grain tungsten. (November 2016)

• Record Type: Journal Article
• Authors: Kolasinski, R.D.; Buchenauer, D.A.; Doerner, R.P.; Fang, Z.Z.; Ren, C.; Oya, Y.; Michibayashi, K.; Friddle, R.W.; Mills, B.E.
• Is Part Of: International journal of refractory metals & hard materials. Volume 60(2016:Nov.)
• Page Start: 28
• Page End: 36
• Publication Date: 2016-11
• Subjects: Heat resistant alloys -- Periodicals; Refractory materials -- Periodicals; Metallography -- Periodicals; Alliages réfractaires -- Périodiques; Matériaux réfractaires -- Périodiques; Métallographie -- Périodiques; Heat resistant alloys; Metallography; Refractory materials; Periodicals; Electronic journals; 669.73
• DOI: 10.1016/j.ijrmhm.2016.05.006
• Languages: English
• ISSNs: 0263-4368

Abstract

Abstract: In this work, we examine the response of an ultra-fine grained (UFG) tungsten material to high-flux deuterium plasma exposure. UFG tungsten has received considerable interest as a possible plasma-facing material in magnetic confinement fusion devices, in large part because of its improved resistance to neutron damage. However, optimization of the material in this manner may lead to trade-offs in other properties. We address two aspects of the problem in this work: (a) how high-flux plasmas modify the structure of the exposed surface, and (b) how hydrogen isotopes become trapped within the material. The specific UFG tungsten considered here contains 100 nm-width Ti dispersoids (1 wt%) that limit the growth of the W grains to a median size of 960 nm. Metal impurities (Fe, Cr) as well as O were identified within the dispersoids; these species were absent from the W matrix. To simulate relevant particle bombardment conditions, we exposed specimens of the W-Ti material to low energy (100 eV), high-flux (> 10 22 m − 2 s − 1 ) deuterium plasmas in the PISCES-A facility at the University of California, San Diego. To explore different temperature-dependent trapping mechanisms, we considered a range of exposure temperatures between 200 °C and 500 °C. For comparison, we also exposed reference specimens of conventional powder metallurgy warm-rolled and ITER-grade tungsten at 300 °C. Post-mortem focused ion beam profiling and atomic force microscopy of the UFG tungsten revealed no evidence of near-surface bubbles containing high pressure D2 gas, a common surface degradation mechanism associated with plasma exposure. Thermal desorption spectrometry indicated moderately higher trapping of D in the material compared with the reference specimens, though still within the spread of values for different tungsten grades found in the literature database. For the criteria considered here, these results do not indicate any significant obstacles to the potential use of UFG tungsten as a plasma-facing material, although further experimental work is needed to assess material response to transient events and high plasma fluence. Highlights: A new ultra-fine grain (UFG) tungsten material was developed for use in magnetic fusion experiments with Ti dispersoids to inhibit grain growth. We evaluated the response of UFG material to high flux hydrogen plasmas with post-mortem microscopy and thermal desorption techniques. Electron microscopy of focused ion beam profiles revealed no hydrogen precipitates present near the plasma-exposed surface of the UFG W. Trapping of hydrogen was moderately higher in the UFG W compared with reference materials.