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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan Edge impurity transport study in stochastic layer of LHD and scrape-off layer of HL-2A 1 M. Kobayashi, S. Morita, C.F. Dong, Y. Feng*, S. Masuzaki, M. Goto, T. Morisaki, H.Y. Zhou, H. Yamada and the LHD experimental group National Institute for Fusion Science, Toki 509-5292, Japan *Max-Planck-Institute fuer Plasmaphysik, D-17491 Greifswald, Germany Z.Y. Cui, Y.D. Pan, Y.D. Gao, J. Cheng, P. Sun, Q.W. Yang and X.R. Duan Southwestern Institute of Physics, P. O. Box 432, Chengdu 610041, China 1.Introduction 2.Edge impurity transport and energy transport 3.Modelling results of LHD & HL-2A Effects of magnetic field geometry 4.Summary Contents

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan Introduction : Impurity transport in SOL/Divertor Understanding of impurity transport in SOL/Divertor region is important for control of Impurity influx to core, Radiation distribution & intensity in SOL, Material migration. 2 Effects of the different magnetic field geometry on the edge impurity transport? Divertor optimization for fusion reactor is ongoing in tokamaks (2D), stellarators, non-axisymmetric tokamaks (3D). e.g. Ergodic divertor in TEXT, Tore Supra, TEXTOR-DED, DIII-D etc. Stochastic field as a tool for controlling edge plasma 3D divertor configuration intrinsic edge stochastization Helical devices with non-axisymmetric configuration ITER: Tokamak 2D axi-symmetric SOL (Closed, V-shaped divertor)

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan //-Impurity transport model ↔ Ion energy transport Momentum : // - Classical force balance Friction Ion thermal force Electric field Impurity pressure gradient force s core SOL LCFS target TiTi dT i /ds V iII Recycling Thermal Friction 3 Electron thermal force In a steady state The force balance is closely related to ion energy transport. Extra function is added to energy transport module of EMC3-EIRENE to analyze the energy transport.

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan 4 D z is same as the value of bulk plasma (which is deduced from experiments). D z has No charge dependence. No drift. Mass : ⊥ - Diffusion with anomalous coefficient Ionization, recombination Diffusion Source at PFC : Divertor plate or first wall with 0.05 eV ejection energy. Simplification for perpendicular transport model

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan 5 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)

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan 6 divertor plate Helical coils R=3.90 m a~0.70 m LHD HL-2A LHD heliotron and HL-2A tokamak : distinct magnetic flux tube topology R=1.65 m a= 0.40 m 10 5 10 4 10 3 10 2 10 1 10 0 Connection length (m) 1 m Stochastic layer L C =10 m~1km Scrape-off layer L C ~40 m Divertor plate Divertor legs Separatrix R Z Enhanced ⊥ interaction between flux tubes Dominant // transport divertor entrance

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan 7 Low density (collisionality) case : HL-2A Strong thermal force ( ) Impurity buildup at upstream Coordinate s

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan 8 High density (collisionality) case : HL-2A Strong friction force near divertor ( ) Impurity screening (against divertor source) Residual thermal force at upstream ( ) Divertor plate Coordinate s

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan 999 Magnetic field structure in LHD (Large Helical Device) LHD Helical coils Connection length (L C ) distribution in poloidal cross section 3.9 m radial poloidal 10/8 10/7 10/6 10/2 10/3 10/4 10/5 Mode structure n/m Edge surface layers* Stochastic region* r eff (m) 0.70 0.65 0.60 Divertor Core plasma

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan 10 radial poloidal Edge surface layers* Stochastic region* r eff (m) 0.70 0.65 0.60 Divertor Core plasma r eff =0.72 m (MW/m 2 ) r eff =0.65 m (MW/m 2 ) L C (m) 10 4 10 3 10 2 10 1 10 0 r eff =0.60 m (MW/m 2 ) Upstream Downstream Low density (collisionality) case : LHD Strong thermal force ( ) Impurity buildup at upstream Radial impurity profile Divertor Core plasma

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan L C (m) 10 4 10 3 10 2 10 1 10 0 r eff =0.72 m 11 radial poloidal Edge surface layers* Stochastic region* r eff (m) 0.70 0.65 0.60 Upstream Downstream r eff =0.65 m r eff =0.60 m (MW/m 2 ) High density (collisionality) case : LHD Strong friction force @ downstream ( ) Impurity screening Suppression of thermal force at upstream ( ) Radial impurity profile Divertor Core plasma Divertor Core plasma

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan 12 Impurity screening as a function of and impurity source location HL-2A: very strong screening against divertor source, but weak against first wall source LHD: modest screening against divertor source, but effective against first wall too Geometrical effect

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan 13 ion 10 -3 10 -2 10 -1 HL-2A LHD q // =q ⊥ for =10 -4 (b) Edge plasma parameter range of LHD and HL-2A (effects of perpendicular transport) HL-2A: high density and low temperature at downstream because of strong up-down coupling through SOL flux tubes LHD HL-2A Preferable for increasing ratio LHD: enhanced perpendicular transport in stochastic layer weakens the up-down coupling Modest change of at downstream

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan 14 ion (b) Enhanced perpendicular transport in stochastic layer of LHD can replace with suppression of thermal force at upstream Radial profiles of ion energy transport ratio between // and ⊥ components in LHD

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan 15 312 0-2-3 Friction – Thermal force (10 4 m/s) Friction force dominant Thermal force dominant n LCFS =5.0x10 19 m -3 LHD n LCFS = 0.59x10 19 m -3 HL-2A Distribution of screening region in LHD and HL-2A Residual thermal force Flow acceleration No flow acceleration LHD: Screening region distributed poloidally screening effect is independent of impurity source location HL-2A: Screening region localized near divertor plate screening effect is very sensitive to impurity source location Inclusion of large upstream flow observed in experiments might alter the results

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6th Japan Korea workshop 28-29 July 2011, NIFS, Toki-city Japan Summary Modelling results The modelling shows impurity screening for the both devices. The screening effect is different between the both devices against collisionality (density) and impurity source location. HL-2A: Very strong screening against divertor source, but not for first wall source LHD: Modest screening against divertor source, but screening effective against first wall too Magnetic field geometrical effect Interpretation HL-2A: Strong up-downstream coupling Preferable for increasing ratio at downstream, i.e. screening. No flow acceleration at upstream weak against first wall source Inclusion of large upstream flow acceleration in experiments might alter the results LHD: Enhanced ⊥ transport in stochastic layer weaken up-down coupling modest screening effect compared to HL-2A can replace with reduction of thermal force ( ) Screening region is distributed poloidally screening effect independent of impurity source location Validation of the model results in experiments is ongoing …….. 16 Edge impurity transport has been analyzed in LHD and HL-2A based on fluid impurity transport model using 3D edge transport code EMC3-EIRENE.

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