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

Abstract

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, λ iz , and the width of the magnetic sheath rather than the ratio of λ 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 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.

Modeling turbulent impurity transport in the SOL of DIII-D with a reduced model

“…Prompt redeposition of sputtered W atoms in DIVIMP is necessary when considering absolute magnitudes of W density within the SOL. The probability of a W ion not promptly redepositing is described by the ERO scaling in [23] and experimental validation in [24]:…”

“…This is consistent with ERO simulations, which indicate that for DIII-D divertor conditions the re-deposition fraction is expected to be particularly large (see figures 9 and 10 in [23]). For core-edge integration, it is thus desirable to keep x = λ iz /λ sheath as as possible by using wall materials with a short λ iz or scenarios in which λ sheath is maximized [24].…”

Focus on plasma-facing materials in nuclear fusion reactors

“…While the materials science community has a long history of conducting fundamental and applied research on plasma-materials interactions [6][7][8], this collection focuses on the same type of physics with respect to a nuclear fusion environment. The topics covered are unique to 'Plasma-Facing Materials in Nuclear Fusion Reactors' such as: deuterium and tritium retention in PFCs [9][10][11][12][13]; fundamental processes at the plasma-surface interface [10,[14][15][16][17][18][19][20]; evolution of structure and properties under fusion-reactor-relevant heat loads [21]; material degradation under ion exposure [15,16,19]; material degradation under neutron irradiation [9,21]; material erosion, migration, and deposition [14,15,18,20,22]; plasma fueling [12]; and diagnostics for plasmamaterials interactions [23]. Although the details of the underlying mechanisms that govern the above phenomena remain largely unresolved, the results presented here will drive the emergence of engineering solutions to the amelioration of plasma-facing materials degradation.…”