Experimental results and modelling of ASDEX Upgrade partial detachment

Transcription

1 Experimental results and modelling of ASDEX Upgrade partial detachment M. Wischmeier 1 With thanks to X. Bonnin 2, P. Börner 3, A. Chankin 1, D. P. Coster 1, M. Groth 4, A. Kallenbach 1, V. Kotov 3, H. W. Müller 1, D. Reiter 3, M. Tsalas 5, J. G. Watkins 6, E. Wolfrum 1, DIII-D and ASDEX Upgrade team 1 IPP Garching, Germany 2 Université Paris 13, Villateneuse, France 3 Institut für Palsmaphysik, Forschungszentrum Jülich 4 LLNL, Livermore, USA 5 NCSR Democritos Inst. of Nucl. Rad. Protect., Attica, Greece 6 Sandia National Laboratory, Albuquerque, NM, USA This work was supported by the Intra-European fellowship (EURATOM) 10 th ITPA meeting, Avila, Spain 1 1

2 Introduction / motivation / knowledge: Definition, what we know, what we don t really know Examples of typical modeling problems Ohmic discharges in ASDEX Upgrade Experiment and modeling Conclusions 2 2

3 Motivation by ITER: Introduction Burning plasma operation relies on partial detachment (see A. Kukushkin) Design modification of divertor structures, identification of the operational window, development support of diagnostics rely on quantitative predictions using 2D fluidneutrals Monte Carlo codes Need of qualitative and quantitative validation of codes using experimental data 3 3

4 Detachment Definition Necessary condition: Loss of total (static + dynamic) plasma pressure along field line between upstream + target Sufficient condition: Reduction of (local/total) ion flux to target plate (required by ITER for reduction of power load, i.e. release of potential energy through recombination on target surfaces) 4 4

5 What is our current understanding? Mechanisms (quantitatively and qualitatively[?]) not really understood that lead to detachment else we d have no problems applying 2D modeling codes and match experiment Most (even all [?]) attempts of modeling high recycling and detached conditions of existing divertor configurations with 2D fluid - neutrals Monte Carlo codes do not reproduce experimental data stuck with Ohmic and L-mode, no attempt yet to model detachment in H-mode! However: we know which ingredients we need for the recipe: sufficient removal of power (recycling or (seed/intrinsic) impurities sufficient removal of momentum (e.g. CX) removal of charges (CX & volumetric recombination) 5 5

6 Examples of knowledge I Detachment depends on fueling species (D vs. He: DIII-D, JET, TCV) Deuterium discharges: probably know that detachment is stronger in vertical target configuration than horizontal usually (unless asymmetric configurations) inner target colder than outer usually (Fwd field) observed asymmetry between inner and outer target ion flux is reduced with field reversal we infer that drifts play a role (but to which degree and how? role of power? Role of particle sources?) 6 6

7 Examples of knowledge II Continued: Detachment stronger with reduced power and/or seed impurities with increased loss of plasma pressure, measured neutral pressure in divertor increases when outer target strongly detaches a MARFE is observed and soon after the density limit is reached Most of all: using our experimental experience we know/learn how to induce detachment (density, heating power, amount and location of seed impurities) 7 7

8 Unclear mechanisms and problems radial power and particle transport especially at high densities importance of main chamber recycling: divertor starved of power by radial transport or by impurity radiation due to interaction of plasma with the main chamber walls (e.g. TCV)? Power sources of SOL well defined, but particle sources? Molecular assisted recombination (MAR) via H 2+ or hydrocarbons Generalization might well be impossible, detachment depends on machine which information relevant to ITER? 8 8

9 Typical problems when modelling Typical problems when modeling: too high j sat and n e, too low T e along target plates too low neutral pressure (combination means too low momentum removal via CX) radiation confined to a layer along target plate or along separatrix towards X-point but not a large volume of radiation as shown by bolometers power flux to target plates can be easily adjusted in C machines applying the range of possible Y chem and don t even think of inter ELM dynamics for H-mode plasmas at high density 9 9

10 Modelling (partial) detachment Attempt to set up a well defined experiment to validate model: 1MA, -2.0T (Fwd. field), q 95 ~3.4 ohmic discharges with step wise density scan at ASDEX Upgrade (with active cryo pumps) and DIII-D (not active cryo pumps) DIII-D: full carbon machine with open divertor (inner partially vertical plate, outer horizontal plate, no dome) ASDEX Upgrade: carbon target plate 2006, tungsten target plate 2007, rest mostly tungsten; closed divertor structure with vertical plates and dome 10 10

11 Key observations DIII-D Lin. av. Density [10 19 m -3 ]: PFR SOL SOL PFR J.G. Watkins & M. Groth Asymmetry of j sat as function of line averaged density Steady decrease of j sat at inner target Roll over of j sat at outer target 11 11

12 j sat [m 2 s 1 ] j sat [m 2 s 1 ] distance from separatrix [m] 6 x x Key observations ASDEX Upgrade #21305 #21306 #21326 #21322 B25E Drifts B25E + C HFS B25E No Drifts #21305 #21306 #21326 #21322 B25E Drifts B25E + C HFS B25E No Drifts distance from separatrix [m] midplane separatrix density [m 3 ] peak j sat [m 2 s 1 ] 6 x Peak ion fluxes to targets outer target inner target roll over at outer target steady decrease at inner target asymmetric particle fluxes 12 12

13 16 x Modelling lowest density (2.6x10 19 m -3 ) n e upstream experiment no drifts drifts drifts Y chem T e upstream experiment no drifts drifts drifts Y chem 10 n e [m 3 ] 8 6 T e [ev] ptot = pdyn + pstat [Pa] R R [m] sep Total pressure partial detachment inner Divertor no drifts drifts drifts & Y chem above X-point parallel distance from inner target plate [m] RR [m] sep distance from separatrix [m] T e [ev] PET11 accepted for CPP 5 Outer target T e #21305 #21306 no drifts drifts drift+y chem 13 13

14 Set up of the model (ASDEX Upgrade) Scan of power, Y chem 48x18 Radially varying transport, atomic or 96x36 poloidally constant physics, gas puff, %He, transport coefficients density, drifts & no drifts Gas puff > 6.5x10 20 /s Ion surface interaction only at targets, neutrals everywhere Neutrals Ions Additional not self consistent C source 5x10x 18 /s and 1.3x10 21 /s (experiment max ~10 20/ s) Bohm chodura at targets or feedback on separatrix n e Chemical sput.: C not C x D y limited value experimentally! MAR but no vibrationally resolved molecules Pump (100m 120m 3 /s), R=0.91 Baffle for gas conductance (neutral pressure 7:1 to 4:1) low high density 14 14

15 Set up of the model (DIII-D) Most is same as ASDEX but: Not sufficient number of simulations yet converged; first results to expect in 2-3 weeks Main chamber walls R=0.98 Targets R=1.0 No sub-divertor structures for neutrals and no pumps! However: problems may arise from plasma MCW interaction on LFS (or elsewhere) 15 15

16 Density scan for ASDEX Upgrade..after some twiddling something not too catastrophic however in experiment heating power increases with density in simulation constant P SOL and j sat increases with power as well as symmetry of fluxes peak j sat [m 2 s 1 ] 8 x outer target inner target SOLPS drifts outer SOLPS drifts inner midplane separatrix density [m 3 ] Integral ion fluxes too high at outer target (and inner target!) Inner and outer target more symmetric compared to experiment, worse when no drifts with increasing density not very sensitive to moderate Y chem or C from HFS hard to assess any impact of MAR; steep gradients in front of targets 16 16

17 New information on neutral pressure (2007) Neutral flux 2.6x10 19 m -3 j sat Pressure gauge in outer target tile inside PFR: dome/(outer target): Low density ~3 High density ~0.9 courtesy R. Pugno J sat and neutral flux similar if target T e profile allows for neutrals to penetrate to pressure gauge Neutral flux j sat 5.5x10 19 m

18 A modified model of the divertor a guess Limited neutral conductance between sub-dome and outer target (pressure gradient), but how big? neutral source for sub-dome is likely inner target. Additional knowledge that leaks exist across dome and target plates but their size is unknown Introduced leak in dome and inner target & baffle between sub dome and outer target 18 18

19 Role of neutral pressure peak j sat [m 2 s 1 ] 8 x outer target inner target SOLPS drifts outer 550kW SOLPS drifts inner 550kW 900kW..even with Y chem = midplane separatrix density [m 3 ] Substantial asymmetry of peak j sat between inner and outer target Reduced peak j sat by more than factor 2! No increase of inner target peak j sat with density but when increasing P SOL closer to experiment for higher densities deviation from experiment integral flux higher than in experiment 19 19

20 Still more problems left e.g. H α z / m #21303 ; t in [3.5967,3.5997]s ε V,Hα / W/(m 3 sr) R / m z / m J. Harhausen (EPS 07) SOLPS model for AUG#21303 ε V,Hα / W/(m 3 sr) R / m Low density: O.K. High density:??? Detailed comparison needed with diagnostics 20 20

21 Next steps to investigate neutrals Apply EIRENE as stand alone without plasma ASDEX Upgrade What a simulation will look like y x FZ Juelich neutral mol. density (#/cm^3) R a p s 0.000E E E E E E E E+10 Preliminary working grid for testing Courtesy of P. Boerner & D. Reiter New pressure gauges installed under inner divertor (A. Scarabosio, G. Haas) 21 21

22 Conclusions Knowledge of correct description of conductance of neutrals is very important (however pressure gauges often neglected) The higher the power and ion fluxes to the targets the more difficult to break the symmetry in the modeling increased problem in H-mode expected (s. A. Chankin PPCF) Drifts enforce the asymmetry, but it seems other factors necessary to trigger a strong effect (detailed analysis needed) flatter profiles in front of target plates stage to re-test MAR, radial transport, Y chem DIII-D should be easier as no sub divertor structures and no pumping, but MCW interaction may pose problem Effect of complete detachment at inner target on partial detachment at outer target (correlations)? ITER: Need to use a (more) detailed description of the divertor? We can be more optimistic for the future wrt. Modeling! 22 22