Jacob, W.

Krieger, K.

Litnovsky, A.

Philipps, V.

Rudakov, D.

West, W.

Wong, C.

English

MPG.PuRe

D Rudakov et al PFMC-11 - 1Recent Results on Carbon Erosion, Migration and Re-deposition in DIII-D Tokamak using DiMESPresented at the PFMC-11 Workshop October 10-12, 2006, Greifswald, GermanyD. Rudakov, J. Boedo, E. Hollmann, R. Moyer, A. Pigarov University of California, San DiegoW. Jacob, K. Krieger Max-Planck-Institut fuer PlasmaphysikA. Litnovsky, V. Philipps, P. Wienhold Forschungszentrum JülichW. West, C. Wong, N. Brooks General AtomicsA. McLean, P. Stangeby University of Toronto W. Solomon Princeton Plasma Physics LaboratoryG. De Temmerman Institute of Physics,  University of BaselR. Bastasz , J. Whaley Sandia National Laboratories, LivermoreJ. Watkins, W. Wampler Sandia National Laboratories, AlbuquerqueS. Allen, M. Fenstermacher, M. Groth, C. Lasnier Lawrence Livermore National LaboratoryD Rudakov et al PFMC-11 - 2SummaryPart I: Temperature dependence of carbon erosion and re-deposition At 200º C we observed net carbon erosion at a rate of ~ 3 nm/sec from a plasma-facing surface under detached conditions, where normally net deposition is observed At 200º C carbon deposition down a simulated tile gap was reduced by about a factor of 2 -4 and D co-deposition by an order of magnitude compared to those at room temperature Carbon deposition was observed on molybdenum mirrors recessed below the divertor floor at room temperature and was suppressed at elevated temperature between 90-175ºCPart II: Migration of artificially introduced carbon dust in the divertor Micron size carbon dust introduced in the lower divertor of DIII-D penetrated core plasma raising the core carbon density by a factor of 2-3 The amount of C that penetrated the core plasma following the dust injection was equal to 1.5-2 % of the total dust carbon content (equivalent to a few million of dust particles) Experimentally observed dust trajectories can be explained by combination of ion drag force, E×B drifts, and reflections from PFC surface corrugationsD Rudakov et al PFMC-11 - 3 DiMES system is used to insert material samples in the lower divertor of DIII-D for erosion and deposition studies A newly developed in situ sample heating capability allows us to study the temperature dependence of erosion/depositionDivertor Material Evaluation System - DiMESD Rudakov et al PFMC-11 - 4Temperature dependence of carbon erosion and re-deposition in DIII-D divertorMotivation:¾ Chemical erosion rate of carbon and hydrocarbon films by hydrogen/deuterium is known to increases with surface temperaturePart I¾ Experiments with thermal ion beams have shown that the chemical erosion rate of carbon peaks at about 400°C¾ Transition from net erosion to net depositrion was observed in low-temperature methane plasmas between 200-300°CW. Jacob, J. Nucl. Mater. 337-339, 839 (2005)Erosion by atomic H and D ( 0.2 eV)1E-41E-31E-21E-1Total Erosion Yield [C/Ho]T, [K]Ho on C1,2Do on a-C:H2200 400 600 8001J.W. Davis et al, JNM 155-157(1988), 2342E. Vietzke et al. Fus. Technol. 15 (1989), 108softhardD Rudakov et al PFMC-11 - 5Motivation Tritium co-deposition/retention is one of the most critical issues for ITER High-priority ITPA topic One of the most troublesome carbon deposition regions for trapping tritium are the narrow tile gaps since such regions are not accessible to many of the proposed T-recovery methods In DIII-D co-deposition of deuterium (as a proxy for tritium) can be studied in a simulated tile gap using DiMES Altering the tile temperature may affect C deposition and D co-deposition ratesOriginally proposed by Wolfgang Jacob Max-Planck-Institut fuer Plasmaphysik, GermanyStudies of C deposition and D co-deposition in a simulated tile gap - Tile Gap DiMESD Rudakov et al PFMC-11 - 6Tile Gap DiMES experimental concept and designBgraphitesiliconwafersConcept features:• radially oriented gap• deposition on the silicon wafers• defined geometry for modeling of the deposition profileBuilt in heater and thermocouple for in situtemperature controlheaterD Rudakov et al PFMC-11 - 7Two exposures at different temperatures were performed  Lower Single Null Simple-As-Possible Plasma (SAPP) shape DiMES located near the detached Outer Strike Point (OSP) Two exposures were performed, first at ~ 30º C, second at 200º C Each exposure was to 9 highly reproducible high-density L mode discharges for a total exposure time of about 32 secondsD Rudakov et al PFMC-11 - 8Non-heated versus heated exposures: plasma-facing surface• There were visible signs of plasma contact on the sample face upon removal, most likely deposits• No net erosion/deposition measurements were availableHeatedNon-heated• No visible signs of plasma contact on the sample face upon removal• A graphite button with implanted Simarker was built into the sample face to measure net erosion/deposition on the plasma-facing surfaceRBTD Rudakov et al PFMC-11 - 9A high erosion rate was measured on the heated sample• Ion Beam Analysis (IBA) at SNL Albuquerque has shown a total net erosion of about 90 ± 4 nm on the depth marked button from heated exposure• This corresponds to a net erosion rate of ~ 3 nm/sec = 1.2 µm / ITER shot = 9 cm / burn year – rather high!• We did not have erosion/deposition measurements on the non-heated sample, but it looked like there was some net depositionIBA locationsIBA data courtesy of Bill WamplerPotentially bad news for ITER: surface erosion of C is strongly increased at elevated temperatureD Rudakov et al PFMC-11 - 10No net erosion observed with  detachment at room temperature• A depth-marked DiMES sample exposed later to 7 comparable high-density SAPP L-mode discharges (but with OSP sweeps) showed no measurable erosion• Previous DiMES experiments in detached H-mode showed net deposition around OSP and in the PF zone02.04.06.08.01.01.212096512070501234561209651207050 1000 2000 3000 4000 50001.401.451.501.551.601.65 120965120705DiMESNon-heated HeatedIp(MA)R_OSP (m)ne(x1013cm-3)Whyte et al., Nucl. Fusion 41 (2001) 1243Total time at OSP:  Non-heated ~ 4 secHeated ~ 32 secErosion on heated sample must be due to the elevated temperatureTime (ms)-40-30-20-10010-2 -1 0 1 2Toroidal position (cm) Up B Down BCarbon erosion (nm) D Rudakov et al PFMC-11 - 11Carbon deposition inside the gap  was a factor of 2 - 4 lower in the heated exposure• C deposition inside the gap was ~ 2 - 4 times lower in the heated exposure• The deposit thickness profiles from ellipsometry are in good agreement with the carbon areal density from IBA• Ellipsometry failed to resolve the deposit thickness in the heated exposure • Some C may have been absorbed into the wafers to form silicon carbideNon-heatedHeatedDistance from top (mm)nC(x1016cm-2)105 10 15 20IBAEllipsometryLayer thickness (nm)024681002468D Rudakov et al PFMC-11 - 12¾ Potentially good news for ITER: by controlling the PFC temperatureit may be possible to control WHERE the T co-deposition would occurD co-deposition inside the gap  was an order of magnitude lower in the heated exposureDistance from top (mm)nD(x1016cm-2)105 10 15 20Heated x 10!02468Non-heatedIBA¾ However, increased carbon impurity production at high temperature may offset the advantage of higher re-erosion rates in the gapsD Rudakov et al PFMC-11 - 13Motivation Optical mirrors are foreseen in ITER for many diagnostics, and will be used in infrared, visible and ultraviolet wavelength ranges High-priority ITPA topic Mirrors in the ITER divertor will likely suffer from deposition, and dedicated experiments in tokamak divertors are urgently needed Using DiMES, we had a chance to perform first ever tests of ITER-relevant mirrors in a tokamak divertor under well-diagnosed plasma conditionsTests of ITER-relevant diagnostic mirrors in a tokamak divertor - Mirror DiMESOriginally proposed by Andrey LitnovskyForschungszentrum Jülich, GermanyD Rudakov et al PFMC-11 - 14Mirror DiMES experimental concept and designHeaterThermocoupleMolybdenum MirrorsStainless steel holder45oProtective flapD Rudakov et al PFMC-11 - 15Mirror DiMES exposure geometryLower tangential camerasDivertor TSIRTVMDSFilterscopesDiMESTVPFR Lower Single Null SAPP-like shape DiMES located at the Private Flux Region Highly reproducible ELMing H-mode dischargeswith detached outer strike point1cm Divertor floorB Leading edgeDownstreammirrorUpstreammirrorRD Rudakov et al PFMC-11 - 16Three exposures at different temperaturesNote: in 1st and 2nd exposures the thermocouple measured the bulk temperature of the holder.In the 3rd exposure thermocouple measured temperature at the back of a mirror0204060801001201401601801 3 5 7 9 11 13 15 17Temperature (°C)Number of exposure discharges 1st set2nd set3rd setOpen symbols = before dischargeSolid symbols = after discharge• 1st set of mirrors was exposed at ambient temperature for a total of ~ 25 s• Visible deposits were found on both mirrors and holder upon removal• 2nd mirror set was exposed for a total of ~70 sec at elevated temperature changing from 140º C to 90º C (planned exposure at 400º C but heater failed)• Upon removal, practically no deposits were visible on the mirrors• Recently 3rd mirror set was exposed at ~150 º C for a total of ~36 sec.• No visible deposits were found on the mirrorsD Rudakov et al PFMC-11 - 17Deposition is suppressed at elevated temperatureUpstream DownstreamUpstreamSet #1 non-heatedSet #2 heatedDownstreamSecondary Ion Mass Spectrometry (SIMS) measurements locationsD Rudakov et al PFMC-11 - 18SIMS shows significant C deposition on the cold mirror and virtually no deposits on the heated mirror0 100 200 300 400 500101102103104105 C2 keV Cs+ beamCold mirrorHeated mirrorIntensity, a. u.Sputter time, s (measure of film thickness)~93 nm** Estimations from the spectral ellipsometry.Deposition rateof ~ 4 nm/sec at room temperaturePotentially very good news for ITER: it may be possible to mitigate carbon deposition by moderate heating of diagnostic mirrors Negligible deposition at elevated temperatureD Rudakov et al PFMC-11 - 19Elevated temperature mitigates the reflectivity drop0 500 1000 1500 2000 2500102030405060708090100Total reflectivity (%)Wavelength (nm)Heated mirrorBefore exposureAfter exposureCold MirrorBefore exposureAfter exposureMeasurements made on downstreammirrors are shownT• Elevated temperature mitigates the reflectivity drop• The reflectivity of the heated mirrors was essentially preserved in the wavelength range above 500 nm• Between 250-500 nm the reflectivity of the heated mirror was slightly degraded due to a thin (<15 nm) oxide film formed on the mirrors, presumably from long term storage in airNote that heated exposure was almost 3 times longerD Rudakov et al PFMC-11 - 20Part I Summary At 200º C we observed net carbon erosion at a rate of ~ 3 nm/sec from a plasma-facing surface under detached conditions, where normally net deposition is observed At 200º C carbon deposition down a simulated tile gap was reduced by about a factor of 2 - 4 and D co-deposition by an order of magnitude compared to those at room temperature Carbon deposition was observed on molybdenum mirrors recessed below the divertor floor at room temperature and was suppressed at elevated temperature between 90 - 175º CD Rudakov et al PFMC-11 - 21Motivation• Micron size dust is commonly found in tokamaks• Dust can be a serious problem for ITER for a number of reasons:- Tritium retention and co-deposition with Carbon dust - Accumulation of toxic and radioactive materials- Be dust in the divertor may cause hydrogen explosion hazard- Dust can cause core contamination and degrade performance• In DIII-D dust is commonly found during vents in ports, between tiles etc. Possible sources include:9Flakes from deposited a-C:D films9Blistering9Leading edges9Monopolar arcsMigration of artificially introduced carbon dust in the DIII-D divertorPart II9Thermal Stress Induced Fracture9Particles left over from entry vent9Degradation of grafoil under tiles9Volume condensationD Rudakov et al PFMC-11 - 22 Recent analytic work [S. Krasheninnikov et al, Phys. Plasmas 11 (2004) 3141, Plasma Phys. Control. Fusion 47 (2005) A339] and modeling [A. Yu. Pigarov et al., Phys. Plasmas 12 (2005) 122508] show that in a tokamak edge plasma dust particles can move with high speed and traverse distances comparable to tokamak radii  Due to acceleration by plasma flows (ion drag) and reflections prom PFC surfaces micron size dust particles can acquire velocities of  10-100 m/s Transport of dust particles can be an important mechanism of core plasma contamination by impurities Estimates from Thomson scattering (TS) data show that in DIII-D under normal operation dust is not a significant contributor to core contamination [W.P. West et al., presented at PSI-17 (2006), submitted to J. Nucl. Matter.] Naturally occurring dusts in DIII-D are hard to diagnose – typically 1-2 or less events per shot both from TS and TV cameras  Size is not resolved by TS for particles larger than 0.25 microns in diameter We decided to artificially introduce pre-characterized micron-size carbon dust in DIII-D divertort to see whether any of the dust would make it into the core plasmaDust can be a source of core impuritiesD Rudakov et al PFMC-11 - 23Dust DiMES experiments• Three individual experiments were performed• About 1 mg of dust was used in the first experiment, not enough to cause any effect• 25-30 mg of dust was used in second and third experiments• A suspension of dust in alcohol was prepared and applied to a shallow dip in the holder• Upon drying the dust formed a rather uniform layer clinging to the holder10 µm• Carbon dust supplied by Toyo Tanso Company, Ltd., Japan• 0.5–10 µm diameter flakes, ~6 µm median diameter from volume count• ~1013 C atoms per median size dust particleD Rudakov et al PFMC-11 - 24Diagnostic setup for the dust DiMES experiments1 8070 90015012021 0240300330 3 060FilterscopesMDS chordsTangential TV ( 2 cameras)BTIpDiMES TVDiMESMDS05, Fs04DiMES• DiMES TV and one of tangential TVs equipped with Kodak Wratten89B IR filters with less than 1% transmission below 680 nm• The second tangential TV equipped with CIII filterD Rudakov et al PFMC-11 - 25Dust DiMES experiment No.2• High-power ELMy H-mode discharge with OSP swept inward and back• Significant increase in C radiation during the first pass of OSP over DiMES!• Cameras observed dust tracks1500.002000.00DiMESShot 1224281.26e131.01.21.41.61.80 2000 40000t (ms)(cm-3)(MA)R (m)a.u.a.u.a.u.DiMESip 122428density  122428rvsout 122428rvsin 122428spred_c3  122428phd04c3  122428prad_tot 122428prad_divl 122428prad_divu 122428Dust injectionD Rudakov et al PFMC-11 - 26CER data show strong increase of core carbon content after OSP passes over DiMES around 2 secShot 1224270.000.02Shot 1224280.0 0.2 0.4 0.6 0.80.000.020.040.06t=1950 mst=2000 mst=2100 msρnC/ ne1224280.0 0.2 0.4 0.6 0.8100020003000400050000.000.020.040.060.08ρnC/ neTime (ms)Courtesy of W. SolomonRough estimate of the amount of C that got into the core:5×1019m-3 × 0.02 × 20m3 = 2×1019atoms = 4×10-4g ~ 106 dust particlesAbout 1.5% of the dust carbon made it into the core!D Rudakov et al PFMC-11 - 27• From DiMES TV we can only make a low-bound estimate since most tracks go off screen2010 ms• Tangential camera view is inclined, but we can still make an estimateDust velocities are 10 – 100 m/sv ~ ltrack / texp ~ 10 – 20 m/sFrame #146, 2430 msv ~ ltrack / texp ~ 100 m/sTrack length: ltrack ~ 20 – 40 cm Exposure time: texp = 16.7 msTrack length: ltrack ~ 10 cm Exposure time: texp = 1 msB RD Rudakov et al PFMC-11 - 28Dust trajectories are consistent with ion drag force • Toroidal direction is what one would expect – same as that of the plasma flow into the divertor• Dust particles are accelerated by the plasma flow (ion drag)• Apparent high field side inclination of trajectories may be either a projection effect or a drift effect• Another mechanism that may accelerate the dust particles is the “rocket force” from asymmetric ablation due to the plasma heat flux coming preferentially from upstream sideBt RBzFlowBDiMESQ• Both the flow and the rocket force should push dust particles towards the plate• Particles can bounce from surface corrugations and fly towards the core         S. Krasheninnikov and T. Soboleva, PPCF 47 (2005)RNote: according to DustTmodeling rocket force does not play a big roleD Rudakov et al PFMC-11 - 29Comparison to modeling: Dust Transport DustT code• DustT code solves equations of motion for dust particle in 3D self-consistently with ablation model given by equations for dust temperature and radius• The code uses magnetic equilibrium mesh and plasma background from UEDGE code• Based on UEDGE data, the forces acting on dust particle from plasma are calculated • DustT employs Monte Carlo method for incorporating the dust collisions with walls and micro-turbulence• DustT is capable of reproducing important features of tokamak experiments:9Dust particles preferentially move in the direction of plasma flow9Dust trajectories are “elongated” in toroidal direction9The “cruise” velocity of dust in plasma is 10-100 m/s[A. Yu. Pigarov et al., Phys. Plasmas 12 (2005) 122508]D Rudakov et al PFMC-11 - 30Part II Summary Micron size carbon dust introduced in the lower divertor of DIII-D penetrated core plasma raising the core carbon density by a factor of 2-3 The amount of C that penetrated the core plasma following the dust injection was equal to 1.5-2 % of the total dust carbon content (equivalent to a few million of dust particles) Experimentally observed dust trajectories can be explained by combination of ion drag force, E×B drifts, and reflections from PFC surface corrugations 3D modeling of dust dynamics using DustT code is capable of reproducing experimentally observed dust velocities and trajectory shapes

Recent results on carbon erosion, migration and redeposition in the DIII-D tokamak using DiMES

Recent results on carbon erosion, migration and redeposition in the DIII-D tokamak using DiMES

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