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Carine Giroud 1 IAEA, Chengdu 16.11.2006

Progress in understanding impurity transport at JET. C. Giroud 1 , C. Angioni 2 , G. Bonheure 3 , I. Coffey 4 , N. Dubuit 5 , X. Garbet 5 , R. Guirlet 5 , P. Mantica 6 , V. Naulin 7 , M.E. Puiatti 8 , M. Valisa 8 , A.D. Whiteford 9 , K-D. Zastrow 1 , M.N.A. Beurskens 1 , M. Brix 1 ,

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Carine Giroud 1 IAEA, Chengdu 16.11.2006

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  1. Progress in understanding impurity transport at JET C. Giroud1, C. Angioni2, G. Bonheure3, I. Coffey4, N. Dubuit5, X. Garbet5, R. Guirlet5, P. Mantica6, V. Naulin7, M.E. Puiatti8, M. Valisa8, A.D. Whiteford9, K-D. Zastrow1, M.N.A. Beurskens1, M. Brix1, E. de la Luna10, K. Lawson1, L. Lauro-Taroni8, A. Meigs1, M. O’Mullane9, T. Parisot5, C. Perez von Thun1, O. Zimmermann11 and the JET-EFDA Contributors. 1 2 3 4 5 7 6 8 10 11 9 Carine Giroud 1 IAEA, Chengdu 16.11.2006

  2. Picture of impurity transport in present devices • Inside core region • W accumulation observed • with peaked density profile without central wave heating in ASDEX • Global observation: • plasma with peaked density are prone to accumulation of highly charged impurities Rest plasma region Inward velocity V~Vneo Impurity D >> Dneo Iter physics group NF 99 • Within ITB region: • Transport can be close to neoclassical predictions

  3. Content • Observation of anomalous impurity transport at JET • Reduction of Nickel peaking by electron heating • Brief description of recent development in the • turbulent transport theory • Comparison of experiment with theoretical predictions • A transition of the dominant instability driving the transport could explain the difference in Nickel peaking • Experimental test of Z dependence predicted by turbulent transport theory

  4. Experimental determination of transport coefficients • Linear relationship assumed between impurity flux and density gradient • In steady-state conditions and with edge source the local impurity density gradient length: Convection coefficient Vz >0 outwards Diffusion coefficient Peaking factor R major radius device

  5. Experimental determination of transport coefficients • Intrinsic impurities such as C: direct measurement of density profile • measured density gradient determines –RV/D • Extrinsic impurities injected by laser ablation (Ni) or gas injection (Ne, Ar). • D and V determined individually by modelling of time evolution of spectroscopic data • Ni: soft x-ray and VUV • Ne and Ar: soft x-ray and VUV and also from charge exchange spectroscopy

  6. Effect of electron heating on Ni transport • Two similar ELMy H-modes: • Two heating schemes: • Different gradient lengths: ICRHdominant ion heating: 8 % 3He ICRH dominant electron heating: 20% 3He #58149, dominant electron #58144, dominant ion [M-E. Puiatti PoP 13 2006]

  7. Two very different Ni profiles ICRH dominant ion heating Peaked Ni profile ICRH dominant electron Slightly hollow Ni profile Steady-state profile calculated from D and V [M-E. Puiatti PoP 13 2006]

  8. Due to change in Ni transport ICRH dominant ion heating ICRH dominant electron heating Measurement Neoclassical x10 • Diffusion increased in centre • Convection reversed at mid-radius While neoclassical transport unchanged • Reduction in Ni peaking due to anomalous transport [M-E. Puiatti PoP 13 2006]

  9. Recent development in turbulent transport theory • Two main electrostatic micro-instability considered ITG/TEM

  10. Recent development in turbulent transport theory • Three main mechanisms have been identified 1[J. Weiland NF 29 1989] 1[X. Garbet PRL 91 2003] 1[M. B. Isichenko PRL 1996] 2[X. Garbet PoP 12 2005] 2,3[C. Angioni C PRL. 96 2006] 1[D.R. Baker PoP 5 1998] 1[V. Naulin Phys Rev. E 2005] 2[M. Frojdh NF 32 1992]

  11. Pinch mechanisms in theory of turbulent impurity transport propagation: ITG ion diamagnetic direction TEM electron diamagnetic direction All contribute to the total turbulent pinch

  12. Illustration of complex Z dependence of turbulent transport GS2 [R/LTi=7, R/LTe=6, Te/Ti=0.88] • D and V calculated with the linear version of the gyrokinetic code GS2: • - trace impurity considered. • only the fastest growing mode is taken in • the quasi–linear model • no neoclassical transport included. • Complex trend in Z of turbulent transport • specific calculation neededfor studied discharge [C. Angioni]

  13. Different dominant instability for peaked and flat Ni density Peaked Ni profile ICRH dominant ion Slightly hollow Ni profile ICRH dominant electron GS2 Steady-state profile calculated from D and V [M-E. Puiatti PoP 13 2006]

  14. Ni pinch reversal found for a ITG to R/LTe driven TEM transition ITG • Investigate transition from • ITG to R/LTe driven TEM • Stabilised R/Ln driven TEM: R/Ln=2. • gradually decreasing R/LTi towards stabilisation of ITG modes. • Reproduce a pinch reversal as observed experimentally Real frequency of most unstable mode (cs/R) TEM Te/Ti=0.95, R/Ln=2 V<0 V>0 [C. Angioni PRL. 962006] [M-E. Puiatti PoP 13 2006]

  15. First results on measured Z dependence of impurity peaking #66134 • Ne, Ar and Ni injected • in ELMy H-mode • q0>1, 0.1 <neff <0.2 • Bt=2.9T, q95=7, • 2MW ICRH, 8.6MW NBI r/a =0.15 Neoclassic Peaking lower than neoclassical measure- ment Stronger Z dependence of peaking in core than at mid-radius r/a =0.55 measure- ment Neoclassic Negative C peaking Hollow profile

  16. Discharge ITG dominated R/LTi~5.8, R/LTe~6.3, R/Ln~0.3 and Te/Ti~1.1, n*~0.10 GS2 w/o Thermodiffusion Neoclassic GS2 GS2 Anomalous part: -R(V-Vneo)/(D-Dneo) Measurement GS2 peaking in same range as measurements

  17. Summary • JET experiments confirm earlier observations that neoclassical transport is not sufficient to describe impurity transport in bulk plasma • First comparison between turbulent impurity transport theory and experiments show encouraging results: • A transition in the dominant instability driving the transport could explain the observed reversal of Ni convection • Same range of peaking as calculated by linear gyrokinetic calculation are measured : no strong increase of V/D as a function of Z.  Turbulent transport could give the means for controlling heavy impurity peaking in ITER • JET is set out to systematically compare theoretical predictions with experiment in coming campaign using JET upgraded CXRS capability. • .

  18. Spare slides

  19. Reduction of Ni peaking calculated with linear GS2 • For increasing R/Ln and R/LTe • reduction of peaking calculated Condition for discharge with peaked Ni densities Condition for discharge with flat Ni densities Transition from ITG to R/Ln driven TEM Reduction of the pinch predicted but no reversal of the pinch [M-E. Puiatti PoP 2006]

  20. r r time [s] LBO Data : inverted SXR emissivity profiles. 58143 58149 MC MH 58142 58144 58141 slower penetration and more peaked profiles

  21. MC norm. brightness MH Simulation: line & SXR brightnessess (core) (edge) (core) (edge) Solid: experimental; dashed:simulation

  22. Effect of electron heating on Ni transport #58144 #58149 • two similar discharges • q0>1 • low collisionality0.1 <neff <0.2 • 2 ICRH heating schemes were applied dominant ion heating: 8 % 3He conc. dominant electron heating: 20% 3He Bt=3.28T, q95=?, 3MW ICRH, 12-14MW NBI • Ni transport probed Pions (MW.m-3)

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