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Edge impurity transport study in stochastic layer of LHD and scrape-off layer of HL-2A

Edge impurity transport study in stochastic layer of LHD and scrape-off layer of HL-2A. 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

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Edge impurity transport study in stochastic layer of LHD and scrape-off layer of HL-2A

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  1. Edge impurity transport study in stochastic layer of LHD and scrape-off layer of HL-2A 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 Contents • 1. Introduction • Edge impurity transport and energy transport • 3. Modelling results of LHD & HL-2A • Effects of magnetic field geometry • 4. Summary

  2. 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. Divertor optimization for fusion reactor is ongoing in tokamaks (2D), stellarators, non-axisymmetric tokamaks (3D). ITER: Tokamak Helical devices with non-axisymmetric configuration 2D axi-symmetric SOL (Closed, V-shaped divertor) 3D divertor configuration intrinsic edge stochastization Stochastic field as a tool for controlling edge plasma e.g. Ergodic divertor in TEXT, Tore Supra, TEXTOR-DED, DIII-D etc. Effects of the different magnetic field geometry on the edge impurity transport?

  3. //-Impurity transport model ↔ Ion energy transport LCFS Ti Thermal dTi/ds Recycling target ViII Friction core s SOL Momentum : // - Classical force balance Electron thermal force Ion thermal force Impurity pressure gradient force Friction Electric field 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.

  4. Simplification for perpendicular transport model Mass : ⊥- Diffusion with anomalous coefficient • Dz is same as the value of bulk plasma (which is deduced from experiments). • Dz has No charge dependence. No drift. Diffusion Source at PFC : • Divertor plate or first wall with 0.05 eV ejection energy. Ionization, recombination

  5. 3D modelling of LHD edge region (EMC3-EIRENE) Computational mesh, configuration and installations Core, CX-neutral transport, particle source SOL, EMC3 simulation domain Vacuum of plasma* C0 wall LCMS PSOL Divertor legs EMC3 SOL Core plasma target H,H2 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) • Boundary conditions • Bohm condition at divertor plates • Power entering the SOL • Density on LCMS • Sputtering coefficient Cross-field transport coefficients • e=i=3D roughly holds • spatially constant (global transport) • determined experimentally

  6. LHD heliotron and HL-2A tokamak : distinct magnetic flux tube topology LHD divertor plate Helical coils R=3.90 m a~0.70 m Divertor plate Stochastic layer LC=10 m~1km Z Divertor legs R Enhanced ⊥ interaction between flux tubes 1 m HL-2A R=1.65 m a= 0.40 m Scrape-off layer LC~40 m Connection length (m) 105 104 101 103 100 102 divertor entrance Separatrix Dominant // transport Divertor plate 1 m

  7. Low density (collisionality) case : HL-2A Strong thermal force ( )  Impurity buildup at upstream Coordinate s

  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

  9. Magnetic field structure in LHD (Large Helical Device) radial Mode structure n/m 10/2 Edge surface layers* 10/3 10/4 10/5 10/6 Stochastic region* 10/7 10/8 poloidal LHD Connection length (LC) distribution in poloidal cross section Helical coils 3.9 m reff (m) Divertor 0.70 0.65 0.60 9 9 Core plasma

  10. Low density (collisionality) case : LHD Strong thermal force ( )  Impurity buildup at upstream radial Edge surface layers* Stochastic region* poloidal reff=0.72 m (MW/m2) Divertor Downstream 100 101 102 103 104 LC (m) reff=0.65 m (MW/m2) reff (m) Core plasma Upstream 0.70 Radial impurity profile reff=0.60 m 0.65 (MW/m2) 0.60 Divertor Core plasma

  11. High density (collisionality) case : LHD • Strong friction force @ downstream ( ) • Impurity screening • Suppression of thermal force at upstream • ( ) radial Edge surface layers* Stochastic region* poloidal reff=0.72 m reff=0.60 m (MW/m2) (MW/m2) (MW/m2) Divertor Downstream 100 101 102 103 104 LC (m) reff=0.65 m reff (m) 0.70 Core plasma Upstream Radial impurity profile 0.65 0.60 Divertor Core plasma

  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

  13. Edge plasma parameter range of LHD and HL-2A (effects of perpendicular transport) LHD HL-2A HL-2A: high density and low temperature at downstream because of strong up-down coupling through SOL flux tubes Preferable for increasing ratio LHD: enhanced perpendicular transport in stochastic layer weakens the up-down coupling q//=q⊥ for Q=10-4 ion 10-3 LHD Modest change of at downstream 10-2 HL-2A 10-1 (b)

  14. 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 ion (b)

  15. Distribution of screening region in LHD and HL-2A LHD nLCFS=5.0x1019 m-3 HL-2A nLCFS= 0.59x1019 m-3 Friction – Thermal force (104 m/s) 0 -1 -2 -3 3 2 1 Friction force dominant Thermal force dominant No flow acceleration Residual thermal force Flow acceleration Flow acceleration HL-2A: Screening region localized near divertor plate  screening effect is very sensitive to impurity source location LHD: Screening region distributed poloidally  screening effect is independent of impurity source location Inclusion of large upstream flow observed in experiments might alter the results

  16. Summary Edge impurity transport has been analyzed in LHD and HL-2A based on fluid impurity transport model using 3D edge transport code EMC3-EIRENE. 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 ……..

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