Materials Research Express (Jan 2023)

Recent DIII-D progress toward validating models of tungsten erosion, re-deposition, and migration for application to next-step fusion devices

  • T Abrams,
  • J G Guterl,
  • S Abe,
  • D C Donovan,
  • I Bykov,
  • C A Johnson,
  • J H Nichols,
  • J D Elder,
  • D A Ennis,
  • S D Loch,
  • D L Rudakov,
  • G Sinclair,
  • C H Skinner,
  • P C Stangeby,
  • D M Thomas,
  • E A Unterberg,
  • W R Wampler

DOI
https://doi.org/10.1088/2053-1591/ad0f43
Journal volume & issue
Vol. 10, no. 12
p. 126503

Abstract

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Fundamental mechanisms governing the erosion and prompt re-deposition of tungsten impurities in tokamak divertors are identified and analyzed to inform the lifetime of tungsten plasma-facing components in ITER and other future devices. Various experiments conducted at DIII-D to benchmark predictive models are presented, leveraging the DiMES removable sample exposure probe capability and the Metal Rings Campaign, in which toroidally symmetric rows of tungsten-coated tiles were installed in the DIII-D divertor. In tokamak divertors, the width of the electric sheath is of the order of the main ion Larmor radius, and a vast majority of sputtered tungsten impurities are typically ionized within the sheath. Therefore, W prompt redeposition is mainly governed by the ratio of the characteristic ionization mean-free path of neutral tungsten to the width of the sheath. In-situ monitoring of the prompt redeposition of tungsten impurities in divertors is demonstrated via the use of WII/WI line ratios and the ionizations/photon (S/XB) method in L-mode discharges. Even with this relatively limited set of emission measurements, net erosion measurements were found to be a consistent upper bound to an analytic scaling based on the ratio of the W ionization length, ${\lambda }_{{iz}},$ and the width of the magnetic sheath rather than the ratio of ${\lambda }_{{iz}}$ and the W ^+ gyro-radius. In the far-scrape-off layer (SOL) of the ITER divertor, however, it is calculated that the measurement of photon emissions associated with the ionization of tungsten impurities up to ${{\rm{W}}}^{5+}$ may be required. Finally, W deposition patterns on DiMES collector probes, interpreted via DIVIMP-WallDYN modelling, reveal the key roles of progressive W erosion/re-deposition staps and E × B drifts in regulating long-range high-Z material migration.

Keywords