Barnard, H.S.

Lipschultz, B.

Whyte, D.G.

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                       June 2011  Plasma Science and Fusion Center Massachusetts Institute of Technology Cambridge  MA  02139  USA                 This work was supported by the U.S. Department of Energy, Grant No.  DE-FC02-99ER- 54512-CMOD.  Reproduction, translation, publication, use and disposal, in whole or in part, by or for the United States government is permitted. PSFC/JA-10-69     A study of tungsten migration in the  Alcator C-Mod divertor  H.S.Barnard, B.Lipschultz, D.G.Whyte  A study of tungsten migration in the Alcator C-ModdivertorH. S. Barnard, B. Lipschultz, D. G. WhytePlasma Science and Fusion Center, MIT, NW17-125, 175 Albany Street, Cambridge,MA 02139-4307, USAAbstractA toroidally-continuous row of bulk tungsten (W) tiles was installed nearthe typical location of the outer strike–point in the Alcator C-Mod divertorand was removed for analysis after two campaigns (3151 plasma discharges).External beam (in–air) particle induced X-ray emission (PIXE) analysis hasbeen used to observe campaign–integrated Wmigration patterns in the other-wise Mo divertor using the W as a tracer for divertor erosion and transport.With a set of high spatial resolution PIXE measurements (∼3 mm resolu-tion), poloidal profiles of W migration have been constructed. Redepositionof up to 1.5× 1021atoms/m2 (∼20 nm equivalent thickness) is observed onthe outer and inner divertor, and in the private–flux region. The campaign-integrated deposition on the analyzed tiles corresponds to net W erosion of∼60 nm ( > 0.014 nm/s). The migration profiles indicate sputtering andprompt redeposition in the outer divertor, neutral and ion transport throughthe private–flux region, and transport in the scrape off layer.Keywords:PACS: 52.40.Hf, 29.30.KvJNM keywords: Divertor Materials, Plasma–Materials Interaction, Erosion,MigrationPSI-16 keywords: Alcator C-Mod, Erosion Deposition, Surface Analysis,TungstenEmail address: hbar@mit.edu (H. S. Barnard)Preprint submitted to Journal of Nuclear Materials June 28, 20111. IntroductionThe core plasma performance in tokamaks is strongly affected by complexplasma surface interactions (PSI) at the edge. The plasma facing components(PFC) are exposed to extreme conditions including high mechanical stress,high heat loads, exposure to energetic particles, and radiation damage. High-Z refractory metals have high melting points and are among the most resilientmaterials against physical and chemical sputtering [1]. They are therefore adesirable choice for PFCs and are used in Alcator C-Mod and many othertokamaks. Plasma exposure still causes sputtering of high-Z surfaces whichresults in erosion and the introduction of impurities into the plasma. Sincehigh-Z impurities in the core plasma degrade energy confinement due en-hanced radiation, boron coatings are frequently used in C-Mod. Boronizationis often performed daily and deposits ∼100 nm of boron (B) which has beenshown to improve performance until its effectiveness is diminished, usuallyafter tens of plasma discharges, due to erosion. The introduction of B how-ever further complicates the PSI because B becomes the primary impurityin the edge plasma which contributes significantly to high–Z sputtering [2].Since net erosion and migration of material can, over time, cause macroscopicchanges and damage the integrity of PFCs, it is important to study these ef-fects so they can be minimized. A study of erosion and poloidal migration oftungsten (W) and molybdenum (Mo) is described in the following sections.2. ExperimentAlcator C-mod has a plasma facing first wall made from closely packedMo tiles with ∼ 2 × 2 cm surfaces. In 2007, a toroidally–continuous rowof bulk W tiles was installed in the outer lower divertor of C-Mod at onepoloidal location near the typical position of the outer divertor strike-point.Since all of the plasma–eroded W originated from one toroidally symmetricrow and no W melting was observed, the W could effectively be used as atracer to observe campaign-integrated W erosion and migration patterns [3]in the otherwise Mo divertor.After two experimental campaigns which exposed the PFCs to a totalof 3151 plasma discharges, a set of Mo tiles from a single toroidal location,spanning the poloidal extent of the lower divertor were then removed foranalysis. In addition, a W tile from this toroidal location and one adjacentwere removed for analysis. These tiles were examined for W and Mo us-ing external beam (in–air) particle induced X-ray emission (PIXE) analysis.2PIXE analysis was chosen because of its sensitivity to high-Z elements. SinceX-ray production cross sections for K and L emission are typically very largefor high-Z elements (10-1000 barns) and vary smoothly with energy PIXEcan be used to effectively measure average areal densities of trace amounts ofsurface deposition [4]. An external beam was used to simplify and expeditemeasurements and sample handling.Analysis was performed with the tandem accelerator at the MIT PSICenter using an external beam produced by a 2 MeV proton beam passingthrough 3 mm diameter aperture supporting a 7.5 µm thick vacuum tightkapton exit foil. The Mo-L, W-Lα, and W-Lβ X-ray peaks were measuredusing an Si(Li) detector. A detection geometry with 45o incidence and detec-tion angles was used to allow for beam access to the sample and line-of-sightaccess to the detector.3. Results and DiscussionTungsten migration due to erosion of W at the outer strike–point has im-portant implications for high Z impurity transport. The W that is depositedin the divertor originates from a toroidally continuous row of tiles at onepoloidal location in a high erosion region of the outer divertor. This effec-tively creates a toroidally symmetric tungsten source. Symmetry simplifiesthe analysis to two dimensions, allowing predictions to be made regardingthe mechanisms that cause W migration. In addition, it was shown withRutherford backscattering spectroscopy that the W was not localized at anydepth and was distributed throughout the B surface films, verifying that theW deposition resulted from sputtering over many plasma discharges.PIXE spectra were measured systematically at 160 locations across 22 Motiles spanning the lower C-Mod divertor in one poloidal plane. In addition,spectra were measured on a W tile from the same poloidal plane and asecond toroidally adjacent W tile. On the outer divertor and divertor dome,measurements were taken along the center of each tile every 3 to 5 mm inthe poloidal direction. The sides of outer divertor tiles showed visible signsof deposition where plasma entered the gap, so measurements were made onthe side of these tiles near the edge closest to the plasma facing surface. Theinner divertor tiles have sloped surfaces to shadow bolt holes in their centersfrom direct plasma exposure so measurements were made on all three of thesloped surfaces on these tiles.3The data presented are measurements of the W and Mo areal density[atoms/m2] derived from the PIXE spectra. For convenience, areal density isrepresented as an effective thickness: 1.59 nm ≈ 1020 W atoms/m2 and 1.55nm≈ 1020 Mo atoms/m2 assuming that each element is at its solid the densityat the surface (6.31× 1028 W atoms/m2, 6.45× 1028 Mo atoms/m2). Thesedeposition patterns are indicative of the possible mechanisms of plasma edgetransport and are described in the following sections.3.1. Global W MigrationTungsten is sputtered from the W tiles by plasma particles from thescrape off layer (SOL) which often have temperatures of > 10 eV. Thoughthe vast majority of the particles in the SOL are deuterons (D+), boron ionsB+1,+2,... are the major cause of W and Mo sputtering [2] since the W and Mosputtering yields are negligible for D+ ions from a 10eV plasma (Ei < 50eV).Significant amounts of this sputtered W were detected at specific locationsof the lower divertor, while W was not observed in others. These results aresummarized in the W profile shown in figure 1.>1410-137-95-63-41-2<1x 10 atoms/m 202ww~1.6 nmFigure 1: Eroded tungsten from the W row forms a distinct migration pattern with depo-sition on the C-Mod outer divertor, inner divertor, and divertor dome.3.2. W and Mo Deposition on the Outer DivertorThe largest quantities of W were observed on the outer divertor tileslocated directly below the row of W tiles while a negligible amount wasobserved above (figure 2). It appears that the sputtered W particles become4ionized in the SOL and are confined to the flux surface until they intercept aPFC surface where they are redeposited. This is consistent with the measurednet downward migration because the magnetic geometry is such that all ofthe flux surfaces with line-of-sight to the sputtering location intercept a lowerposition on the vertical target.Outer Divetor Tiles0 50 100 150 200 250 30001020Poloidal Proﬁle of Tungsten DepositionW thickness [nm]Poloidal location [mm]SurfaceEdge(a) (b) (c)(a) (c)(b)(a)WFigure 2: Sputtered W is ionized and redeposited causing net downward migration ofW (right). An image of analyzed tiles (top left). PIXE measurements of W depositionthickness(lower left).A similar quantity of Mo is also observed on the surface of both of theanalyzed W tiles (figure 3). A significant fraction of Mo was likely sputteredonto the W tiles above (in the poloidal dir.), and redeposited as in the modeljust described, however its is possible that the Mo originated elsewhere sincenearly all PFCs are Mo. The Mo deposition on W data shows a trend whichdecreases toward the top of the tile surface. This trend is not predicted by themodel, however it can be attributed to a higher net erosion towards the topof the tile due to longer plasma exposure at higher temperatures throughoutthe tile’s life time. This was confirmed by statistical data described in section3.5.3.3. Deposition in the Private Flux RegionDeposition in the private flux (PF) region is seen in two areas: on thevertical surface of the divertor dome, and adjacent to the inner divertor.The distribution pattern shows relatively large deposition on tiles adjacent50 10 20 3005101520Poloidal Position [mm]Mo Thickness [nm]Mo Deposition on W Tile 1SurfaceEdge0 10 20 3005101520Poloidal Position [mm]Mo Thickness [nm]Mo Deposition on W Tile 2SurfaceEdgeW Tile 1 W Tile 2Poloidal DirectionToroidal DirectionFigure 3: Sputtered Mo is ionized and transported from above tiles onto the tiles in theW row. An image of analyzed tiles (left). PIXE measurements of Mo deposition thickness(right).to the inner divertor, and less deposition the vertical surface which increasessmoothly toward the bottom (figure 4).The vertical surface of the divertor dome has line-of-sight access to theW tiles and is not connected by field lines. The W therefore must migrateas sputtered neutrals that pass through the plasma across the ∼25 mm gap.More W however, is observed toward the bottom of the dome rather thanon the surface directly across from the W row which intercepts the largestsolid angle. There are several possible explanations for this result. Since theions strike the PFCs with oblique incidence, the sputtering distribution mayhave a preferred ejection angle. This is likely because sputtering angulardistributions, usually approximated by a simple cosine distribution, tend toshift strongly in the forward direction with oblique incidence [5]. Anotherpossibility is that, due to the oblique angle made between the flux surfacesand the tiles, sputtered particles with an ejection angle normal to the surfacemust travel further through the SOL than those with downward angles thusproducing the observed deposition pattern.The tiles on the horizontal surface that are near the inner divertor donot have line-of-sight access to the W row. Therefore, the deposition mustoccur from ions that flow along the magnetic field in the PF region. Thesputtering however, occurs in the hotter SOL and must cross the separatrix.If the W ionization mean free path is long enough, it is possible that some Wcan cross the separatrix and enter the PF region before being ionized. It isalso possible that transport of ions across the separatrix into the PF regioncould be enhanced due to the ~E × ~B drift [6].6Divertor Dome Tiles0 50 100 150 2002468Poloidal Profile of Tungsten DepositionW Thickness [nm]Poloidal Location [mm](a) (b)(b)(a)(a) (b)WFigure 4: Tungsten was observed to migrate to the divertor dome to regions with line-of-sight access as neutrals to location (b) and as ions along flux surfaces to location (a). Animage of analyzed tiles (top left). PIXE measurements of W deposition thickness (lowerleft).3.4. Deposition on the Inner DivertorA significant amount of tungsten is observed to accumulate at or near theinner divertor strike-point but is negligible elsewhere (figure 5). Since theinner strike-point intercepts the divertor both above and below this regionof deposition during the campaign, there are several transport mechanismsthat may cause this migration.It is possible that this deposition originates from the W ions that flowalong the field across PF region through the same mechanism as the deposi-tion that is observed on the horizontal surface of the divertor dome (section3.3). The other possibility is that the W could be transported around thecore plasma in the SOL. Significant amounts of W in the SOL would likelydiffuse into the core plasma. Migration through the SOL would seem to be asmaller effect than PF transport since the C-Mod soft X-ray diagnostics donot typically observe W in the core plasma.3.5. Plasma StatisticsStatistical data was collected on the strike-point location and plasmaconditions from C-Mod data from the 2007 and 2008 campaigns to helpinterpret the erosion patterns on the outer divertor. The strike-point lo-cation was determined from the location of the radial flux coordinate ρ at7Inner Divertor Tiles0 50 100 150 200 25001020W thickness [nm]Poloidal Profile of Tungsten Deposition Poloidal Location [mm]Core WSOL WwPF W(a)(a)(a)(b)(b)(b)Figure 5: Tungsten was observed to migrate from the outer divertor to the location at/nearthe inner divertor strike-point through the private flux region and/or in the scrape-offlayer (right). An image of analyzed tiles (top left). PIXE measurements of W depositionthickness (lower left). Illustration of the sloped tile surface geometry (lower middle)the midplane from EFIT equilibrium reconstruction data, projected onto thedivertor. Plasma conditions throughout each plasma discharge such as den-sity ne, temperature Te, etc. were determined from flush mounted Langmuirprobe data. A histogram of the strike-point data binned for various thresholdtemperatures is shown figure 6.W TileFraction of Total Plasma Time Strike-Point Location On Outer Divertor Vertical TargetVertical Position [cm]-55 -54 -53 -52 -51 -50 -49 -48 -47All DataT>0eVT>2eVT>5eVT>10eVT>15eVT>20eV6420x 10-3Figure 6: Statistics on the outer divertor’s exposure to the strike-point were collected forseveral plasma temperature ranges for all available plasma data using EFIT and Langmuirprobe data.84. Summary and ConclusionsPIXE proved to be a very effective method for measuring trace amountsof tungsten deposition on PFCs. These measurements made it possible toobserve campaign integrated tungsten migration in the C-Mod divertor andinfer possible mechanisms for W and Mo migration including sputtering andprompt redeposition in the outer divertor, neutral and ion transport throughthe private-flux region, and transport in the scrape off layer.From the tungsten deposition profile an estimate was made for net tung-sten erosion assuming toroidal symmetry. The measured tungsten areal den-sity was integrated and normalized to the plasma facing area of the W tilesto give the net erosion ∆Wmeasured = 58 ± 3 nm during 3151 plasma dis-charges. From the net erosion, the erosion rate ∆W/∆t was estimated usingthe total lower divertor plasma exposure time ∆t. Since the plasma condi-tions vary substantially throughout each plasma discharge, ∆t was definedas the sum of time during which the strike-point was located on the lowerdivertor with strike-point temperature above a threshold value. The diver-tor in C-Mod was exposed to plasma with Te > 5 eV for 4218 s and 2317 swith Te > 10 eV corresponding to erosion rates 0.014 nm/s (Te > 5 eV) and0.025 nm/s (Te > 10 eV). These results represent a lower bound on the nettungsten erosion because it is possible that tungsten was transported else-where in C-Mod and it is also likely that the W is embedded deeply in theboron films where the sensitivity of the PIXE technique is decreased. Thesetwo effects suggest that the areal densities of deposition and net erosion areslightly higher than was measured with PIXE analysis.In a previous study by Wampler et al. [7] a net erosion of ∼100 nm ofmolybdenum was measured at the same poloidal location as the position ofthe W tiles. This measurement was from a campaign with 1090 discharges,meaning that the equivalent Mo erosion rates is roughly 0.09 − 0.1 nm/s.Therefore, the measured erosion rate for tungsten appears to be more afactor of ∼5 lower than the erosion rates for molybdenum which is consistentwith W’s lower sputtering yield.References[1] G. Federici et al 2001 Nuclear Fusion 41 1967[2] B. Lipshultz, Physics of Plasmas 13, 056117 (2006)9[3] Y. Ueda et al, Journal of Nuclear Materials 363-365 (2007) 66-71[4] S. A. E. Johansson, et al, Particle Induced X-Ray Emission Spectrome-try (PIXE), John Wiley and Sons, Inc., 1995[5] W. Eckstein, Journal of Nuclear Materials 248 (1997) 1-8[6] C. Boswell et al, Journal of Nuclear Materials 290-293 (2001) 556-560[7] W. R. Wampler, Journal of Nuclear Materials 266-269 (1999) 217-22110

A study of tungsten migration in the Alcator C-Mod divertor

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A study of tungsten migration in the Alcator C-Mod divertor

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