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Current understanding of divertor detachment: experiments and modelling O-25

Current understanding of divertor detachment: experiments and modelling O-25.

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Current understanding of divertor detachment: experiments and modelling O-25

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  1. Current understanding of divertor detachment: experiments and modelling O-25 M. Wischmeier1, M. Groth2, A. Kallenbach1, A. V. Chankin1, D. P. Coster1, R. Dux1, A. Herrmann1, H. W. Müller1, R. Pugno1, D. Reiter3, A. Scarabosio1, J G. Watkins4, DIII-D team and ASDEX Upgrade team 1Max-Planck-Institut für Plasmaphysik, EURATOM-Association, Garching, Germany 2Lawrence Livermore National Laboratory, Livermore, California, USA 3Institut fuer Energieforschung, Plasmaphysik, Euratom Association, Juelich, Germany 4Sandia National Laboratory, Albuquerque, NM, USA This work was performed with support by the Intra-European fellowship (EURATOM) and the auspices of the U.S. Department ofEnergy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and by Sandia National Laboratory under Contract DE-AC04-94AL85000

  2. Detachment Definition Necessary condition: • Loss of total (static + dynamic) plasma pressure along field line between ‘upstream’ + target

  3. Detachment Definition Necessary condition: • Loss of total (static + dynamic) plasma pressure along field line between ‘upstream’ + target • Sufficient condition: • Reduction of (peak/total) ion flux to target plate (required by ITER for reduction of power load, i.e. release of potential energy through surface recombination on target)

  4. A detachment recipe • Ingredients: • “sufficient” removal of power (recycling or seed/intrinsic impurities)  Te < 5eV • “sufficient” removal of momentum (e.g. CX) • removal of ions (CX & volumetric recombination) loss of pressure, reduction of surface recombination

  5. A detachment recipe • Ingredients: • “sufficient” removal of power (recycling or seed/intrinsic impurities)  Te < 5eV • “sufficient” removal of momentum (e.g. CX) • removal of ions (CX & volumetric recombination) loss of pressure, reduction of surface recombination • Ideal: • Partial detachment  reduction of peak heat & particle flux close to strike point + attachment in far SOL • Risks: • Complete detachment at outer target  Density Limit / Control issue • Influx of neutrals on high field side (density limit?, fuelling?, H-L transitions?)

  6. Assumption on understanding • Assumption: • SOLPS5.0 (B2.5 coupled to EIRENE) includesall known physicsassumed important for detachmentinside the computational domain/grid(no photon transport, no neutral-neutral collisions); use similar physics as for ITER modelling

  7. Assumption on understanding • Assumption: • SOLPS5.0 (B2.5 coupled to EIRENE) includesall known physicsassumed important for detachmentinside the computational domain/grid(no photon transport, no neutral-neutral collisions); use similar physics as for ITER modelling • Deficiencies are inherent to the model: • quantitative radial and poloidal variation of perpendicular transport, processes outside grid • time averaged quantities  no effect of fluctuations

  8. The test beds ASDEX Upgrade & DIII-D • Compare devices of similar size • Use similar discharge parameters (1MA, ohmic/L-mode, q95~3.4-3.7 (through ITPA) PFC: MCW: W Divertor: C or W ASDEX Upgrade

  9. The test beds ASDEX Upgrade & DIII-D • Compare devices of similar size • Use similar discharge parameters (1MA, ohmic/L-mode, q95~3.4-3.7 (through ITPA) PFC: Graphite (C) PFC: MCW: W Divertor: C or W DIII-D ASDEX Upgrade

  10. ASDEX Upgrade: C versus W (I): upstream ne ne=2.6 ne=5.5 • Very similar upstream densities in C and W • Quality of Te profiles not good enough (Te~50eV -30eV) • M. Groth showed @ EPS 07 low density profiles identical in DIII-D W,C ne=6.2-6.5

  11. ASDEX Upgrade: C versus W (II): outer target ne=6.2x1019m-3 detached ne=2.6x1019m-3 ne=5.0-5.5x1019m-3 low roll over • Similar maximum fluxes in C and W • Tendency to earlier detachment with C • both target materials: inner target complete detachment

  12. Set up of the model (ASDEX Upgrade) 48x18 or 96x36 • Radially varying poloidally constant transport coefficients

  13. Set up of the model (ASDEX Upgrade) 48x18 or 96x36 • Radially varying poloidally constant transport coefficients • Ion surface interaction only at targets, neutrals everywhere • Chemical sput.: C not CxDy – limited value experimentally! • Additional not self consistent C source 5x1018/s - 2.0x1019/s from experiment

  14. Set up of the model (ASDEX Upgrade) • Scan of power, Ychem transport, atomic physics, gas puff, %He, density, drifts & no drifts 48x18 or 96x36 • Radially varying poloidally constant transport coefficients • Ion surface interaction only at targets, neutrals everywhere • Chemical sput.: C not CxDy – limited value experimentally! • Additional not self consistent C source 5x1018/s - 2.0x1019/s from experiment

  15. Set up of the model (ASDEX Upgrade) • Scan of power, Ychem transport, atomic physics, gas puff, %He, density, drifts & no drifts 48x18 or 96x36 • Radially varying poloidally constant transport coefficients Gas puff > 6.5x1020/s or feedback on separatrix ne • Ion surface interaction only at targets, neutrals everywhere • Chemical sput.: C not CxDy – limited value experimentally! Neutrals  Ions • MAR (w. and w.o. vibrational resolution) • Additional not self consistent C source 5x1018/s - 2.0x1019/s from experiment Pump (100m – 120m3/s), R=0.91

  16. Set up of the model (ASDEX Upgrade) • Scan of power, Ychem transport, atomic physics, gas puff, %He, density, drifts & no drifts 48x18 or 96x36 • Radially varying poloidally constant transport coefficients Gas puff > 6.5x1020/s or feedback on separatrix ne • Ion surface interaction only at targets, neutrals everywhere • Chemical sput.: C not CxDy – limited value experimentally! Neutrals  Ions • MAR (w. and w.o. vibrational resolution) • Additional not self consistent C source 5x1018/s - 2.0x1019/s from experiment Pump (100m – 120m3/s), R=0.91 Baffle for gas conductance, Exp. pressure ratio dome vs. outer target 3:1 @ low density (P2-25 A. Scarabosio)

  17. Effect of conductance below dome Simulated outer target peak ion flux Modelled for PSOL=560kW • In experiment at low density it is found that conductance for neutrals below dome is limited pdome/pot~3 [P2-25]  • Introduction of baffle below dome limiting conductance for neutrals has (with drift terms activated) strong effect on jsat

  18. Modelling effect of Drifts Simulated outer target peak ion flux Modelled for PSOL=560kW Increasing importance of drift effects at high density, if impurities (C ) and baffle below dome are included.

  19. Comparison with Experiment Outer target peak ion flux • Start with successful cases for low density (M. Wischmeier CPP 44 2008) •  Density ramp • Psol 560kW at lowest density, else 900kW • most limited neutral conductance below dome (1cm gap) - Outer Exp. W - Outer Exp. C --Inner Exp C. SOLPS5.0

  20. Comparison with Experiment Outer target peak ion flux - Outer Exp. W - Outer Exp. C --Inner Exp C. SOLPS5.0 • Modelling: • NO decreasing jsat at inner target • Target fluxes symmetric • No MAR effect (similar to U. Fantz JNM 290-293 (2001)) • Assumed neutral conductance too low (real value?)

  21. Enhanced radial transport an option? Simulated Inner target ion flux density @ ne(sep)=2.0x1019m-3 From experiment high particle fluxes along inner heat shield [P1-27 McCormick, P2-25 Scarabosio] – not reproduced by the model • Possible reduction of jsat due to transport  radial extension of the grid to the wall needed

  22. Density scan DIII-D Inner target Lin. av. Density [1019m-3]: 2.6 3.0 3.9 Steady decrease of jsat at inner target  complete detachment Ion flux Outer target Roll over @ outer target Earlier onset of detachment in DIII-D vs. ASDEX Upgrade (nsepe =1.0 vs. 1.8 x1019m-3 ) 2.6 3.0 3.9 J. Watkins

  23. Experiment and modelling DIII-D Experimental and simulated peak ion fluxes SOLPS Experiment • Simulations for DIII-D without drifts, preliminary results including drifts do not reverse the trend • Possible important role of MCW interaction • Stronger impact of wrong solution at inner divertor on solution at outer divertor through neutrals cross-talk across private flux region

  24. Conclusions • Experiment: • In Ohmic discharges detachment in ASDEX Upgrade similar for C and W coated strike point area • ASDEX Upgrade and DIII-D: roll over at outer target, inner target steadily detaches •  strong in-out asymmetry • Modelling: • unable to reproduce asymmetry – despite drifts • Drifts have a stronger effect @ high density if combined with neutral conductance and impurities • Strong radial transport for HFS needed – first principle model? • jsat within factor 2 of experiment for ASDEX Upgrade • Larger discrepancy for DIII-D despite simpler geometry: role of main chamber wall impurity influx? Higher cross talk of divertors via neutrals (inner divertor  neutrals  outer divertor)

  25. Call We could be answering a lot more questions if we were able to self consistently detach the inner divertor in the modelling as we see it in existing experiments

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