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Title: Characterizing low-Z erosion and deposition in the DIII-D divertor using aluminum

Here, we present measurements and modeling of aluminum erosion and redeposition experiments in separate helium and deuterium low power, low density L-mode plasmas at the outer divertor strike point of DIII-D to provide a low-Z material benchmark dataset for tokamak erosion-deposition modeling codes. Coatings of Al ~100nm thick were applied to ideal (smooth) and realistic (rough) surfaces and exposed to repeat plasma discharges using the DiMES probe. Redeposition and re-erosion in all cases was primarily in the downstream toroidal field direction, evident from both in-situ spectroscopic and post-mortem non spectroscopic measurements. The gross Al erosion yield estimated from both He and D plasma exposures was ~40-70% of the expected erosion yield based on theoretical physical sputtering yields. However, the multi-step redeposition and re-erosion process, and hence the measured net erosion yield and material migration, was found to be influenced by the surface roughness and/or porosity. On rough surfaces, the fraction of the eroded Al coating found redeposited outside the original coating area was 25x higher than on smooth surfaces. The amount of Al found redeposited on the rough substrate was in fact proportional to the net eroded Al, suggesting an accumulation of deposited Al in surface pores and other areasmore » shadowed from re-erosion. In order to determine the fraction and distribution of eroded Al returning to the surface, a simple model for erosion and redeposition was developed and fitted to the measurements. The model presented here reproduces many of the observed results in these experiments by using theoretically calculated sputtering yields, calculating surface composition changes and erosion rates in time, assuming a spatial distribution function for redepositing atoms, and accounting for deposit trapping in pores. The results of the model fits reveal that total redeposition fraction increases with higher plasma temperature (~30% for 15-18eV plasmas, and ~45% for 25-30eV plasmas), and that 50% of the atoms redepositing on rough surfaces accumulated in shadowed areas.« less
Authors:
 [1] ;  [2] ;  [3] ;  [4] ;  [2] ;  [5] ;  [6] ;  [6] ;  [3] ;  [6] ;  [1] ;  [7] ;  [1] ;  [1] ;  [4] ;  [7] ;  [4] ;  [2]
  1. General Atomics, San Diego, CA (United States)
  2. Univ. of California San Diego, La Jolla, CA (United States)
  3. Univ. of Toronto, Toronto (Canada)
  4. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
  5. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
  6. Oak Ridge Associated Univ., Oak Ridge, TN (United States)
  7. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Publication Date:
Grant/Contract Number:
FC02-04ER54698
Type:
Published Article
Journal Name:
Nuclear Materials and Energy
Additional Journal Information:
Journal Volume: 12; Journal ID: ISSN 2352-1791
Publisher:
Elsevier
Research Org:
General Atomics, San Diego, CA (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; erosion; deposition; migration; roughness; aluminum; mixed materials
OSTI Identifier:
1341466
Alternate Identifier(s):
OSTI ID: 1372469