1. Model prediction of impurity retention in ergodic layer and comparison with edge carbon emission in LHD (Impurity retention in the ergodic layer of LHD) M. Kobayashi, Y. Feng a, S. Morita, M.B. Chowdhuri, M. Goto, K. Sato, S. Masuzaki, I. Yamada, K. Narihara, H. Funaba a, N. Tamura, Y. Nakamura, T. Morisaki, N. Ohyabu, H. Yamada, A. Komori, O. Motojima and the LHD experimental group National Institute for Fusion Science,322-6 Oroshi-cho, Toki 509-5292, Japan a Max-Planck-Institute fuer Plasmaphysik, Wendelsteinstrasse 1, D-17491 Greifswald, Germany 18th PSI 26-30 May 2008, Toledo, Spain

2. 2 Implication of impurity screening at high density operation Even at very high density operation with more than 10 21 m -3 & peaked density profile, no significant impurity accumulation Z eff < 2 Implication of a possible mechanism of impurity retention (screening) in the edge region. Neo classical transport model in core region can not interpret it.

3. 3 1D impurity transport model along B field lines >1  impurity retention Introduction : Condition for impurity retention in 1D model Condition for impurity retention s core SOL LCFS target TiTi dT i /ds V iII Recycling friction ion thermal force dominant terms In force balance, impurity velocity becomes, Impurity retention : dense and cold plasma, flow acceleration, suppress //-grad T. Z-independent

4. Outline of the talk 1. Introduction : IDB plasma with Z eff < 2 2. 3D impurity transport modelling in LHD Geometrical effect on impurity transport 3. Comparison with experiments Density dependence of carbon emission 4. Summary

5. 5 Basic field structure: island overlapping  stochastic poloidal radial 10/8 10/7 10/6 10/2 10/3 10/4 10/5 Low-order islands m/n Confinement Region* Stochastic Region* Edge surface Layers* *Terminology: N. Ohyabu et al., Nucl. Fusion, Vol. 34, No.3 (1994) Connection length / m radial poloidal

6. 6 3D modelling of LHD edge region (EMC3-EIRENE) Boundary conditions Bohm condition at divertor plates  Power entering the SOL  Density on LCMS  Sputtering coefficient SOL P SOL Core plasma LCMS wall target EMC3 Divertor legs C0C0 H,H 2 Computational mesh, configuration and installations Core, CX-neutral transport, particle source SOL, EMC3 simulation domain Vacuum of plasma* Cross-field transport coefficients  e =  i =3D roughly holds  spatially constant (global transport)  determined experimentally Example of LHD helical divertor Physics model Standard fluid equations of mass, momentum, ion and electron energy Trace impurity fluid model (Carbon) Kinetic model for neutral gas (Eirene)

7. 7 From thermal-force to friction-dominated impurity transport Plasma ion flow nv II Thermal force dominant inward flow friction dominant outward flow Carbon density distribution Impurity flow n LCMS =2×10 19 m -3 3×10 19 4×10 19 (10 18 m -3 ) 1.0 0.1 0.01 Increase n e

8. Edge surface layers : effective retention Stochastic region : flat profile (suppression of thermal force) Edge surface Stochastic stochastic edge surface 2e19 m -3 3e19 m -3 4e19 m -3 Carbon density thermal friction

9. 9 poloidal radial T e /eV 0 190 Island structure affects plasma transport m/n=10/3 10/5 10/7 R / cm T e / eV * Thomson (t=1.24,1.27,1.3) EMC3/EIRENE * EMC3-EIRENE

10. 10 Reduction of II-conductive heat flux by  -transport 1D radial energy transport for ions: If  << 1 (long connection length),  -heat conduction makes significant contributions. Condition for significant reduction of parallel ion heat conduction: (Y.Feng, Nucl. Fusion 46 2006)  II r core SOL LCMS target TiTi w  w/o  q II upstream downstream qq  ~10 -4 (LHD)

11. 11 n up =4e19 m -3 n up =2e19 m -3 edge surface layer accumulation retention Clear separation of profile between C +1 ~C +3 & C +4 ~C +6 Impurity retention : C +1,C +2,C +3 C +4,C +5,C +6 Impurity density profiles of different charge states Ionization potential : CIII (C +2 ) : 48 eV CIV (C +3 ) : 65 eV CV (C +4 ) : 392 eV CVI (C +5 ) : 490 eV big gap

12. Spectroscopy Density dependence of carbon emission : indication of impurity retention CIII: 977Å, 1S 2 2S 2 -1S 2 2S2P (VUV monochromators) CIV: 1548Å, 1S 2 2S-1S 2 2P (VUV monochromators) CV: 40.27Å, 1S 2 -1S2P (EUV spectrometer) Absolutely calibrated CV/ne CIII/ne CIV/ne CIII/ne w/o fric. CIV/ne w/o fric. CV/ne w/o fric.

13. Other works : Impurity transport analysis of ergodic layer D.Kh. Morozov et al., POP 2 (1995) 1540. Importance of friction of background plasma to exhaust impurity. M.Z. Tokar, POP 6 (1999) 2808 Core carbon content (C 6+ ) reduction with decreasing edge Te in Tore Supra. Y. Corre et al., NF 47 (2007) 119 Y. Feng et al. NF 46 (2006) 807 (W7-AS) M. Kobayashi et al. CPP 48 (2008) 255 (LHD) Prediction of impurity retention @ high density with 3D modelling Viscosity terms related to thermal flux.

14. 14 Summary The impurity transport in the ergodic layer of LHD is analyzed, using the edge transport code (EMC3-EIRENE) in comparison with edge carbon emission.  Magnetic field structure of the ergodic layer in LHD : Edge surface layer : short and long flux tubes coexist. Stochastic region : remnant islands with Lc > ~ 100 m.  Plasma transport, particle and energy, follows the island structure as confirmed by the 3D modelling as well as experiments.  3D code predicts impurity retention in the ergodic layer : Flow acceleration in edge surface layers Cross-field transport prevents large // T drop  suppress thermal force.  Good agreement in carbon emission btw 3D modelling & exp. :  Indication of impurity retention in ergodic layer by friction force.  The retention model could apply also for high Z impurity : Balance btw friction & thermal force is Z-independent High Z impurity has shorter neutral penetration length than low Z impurity.