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A summary of some recent edge physics research on TCV and JET

A summary of some recent edge physics research on TCV and JET. R. A. Pitts Centre de Recherches en Physique des Plasmas Ecole Polytechnique Fédérale de Lausanne, Switzerland Association EURATOM-Swiss Confederation and Leader, Task Force E, EFDA-JET. Outline. TCV

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A summary of some recent edge physics research on TCV and JET

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  1. A summary of some recent edge physics research on TCV and JET R. A. Pitts Centre de Recherches en Physique des Plasmas Ecole Polytechnique Fédérale de Lausanne, Switzerland Association EURATOM-Swiss Confederationand Leader, Task Force E, EFDA-JET R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  2. Outline • TCV • Brief overview of the machine: first wall, heating systems, edge diagnostics • Divertor detachment • Turbulent transport • Parallel flows • Not covered: ELMs, Infra-red investigations, SOLPS5 H-mode modelling • JET • Retarding field energy analyser • Flows and far SOL ELM ion energies • A lot more not covered! R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  3. TCVTokamak à Configuration Variable R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  4. The TCV tokamak • R= 0.88m; a= 0.25m • BT ≤ 1.5T; Ip ≤ 1.2MA • 0.9< k <2.8; -0.6< d <0.9 • X3: 118GHz • 3  0.5MW, 2s • Top launch ECH • ncut-off = 11.51019m-3 • X2: 82.7GHz • 6  0.5MW, 2s • Side launch ECH, ECCD • ncut-off = 4.21019m-3 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  5. TCV first wall • All graphite machine • Upgrade to ~90% coverage in 1998 • First wall tiled with polycrystalline graphite (~1700 individual elements) • Cold walls (during operation) • Regularly boronised (~220C, Glow with 10% B2D6/90% He) • Pulse length typically ~1.2 s • All tiles being B4C blasted this summer R. A. Pitts, R. Chavan, J-M. Moret, Nucl. Fus. 39 (1999) 1433 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  6. TCV: configurational flexibility • 16 independently powered poloidal field coils • Enormous scope for flexibility in plasma shape • Nightmare for edge physics and PSI however! R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  7. TCV: Edge diagnostics • 80 tile embdedded Langmuir probes • IR cameras • Fast reciprocating probe (flows and turbulence) • In-vessel pressure gauges • Fast AXUV diode cameras R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  8. Divertor detachment • Mandatory for successful ITER (and reactor) operation • Without (partial) divertor detachment in the separatrix region, power fluxes will be beyond the design power handling capacity • SOLPS5 (B2.5-Eirene) solutions show that this will be possible • But has the code been sufficiently benchmarked on today’s machines for us to have confidence? ISP OSP ITER Divertor DDD 17 (SOLPS5 runs by A. Kukushkin) R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  9. Divertor detachment on TCV (1019m-3) • Studies always made in simple ohmic discharges • Isolate physics, obtain best possible data • X2 ECR heating system precludes high density L-mode operation • Studied effect of geometry on detachment – “plasma plugging” Zeff (kW) PW PRAD,TOT PRAD,DIV Da,divertor Jsat(Acm-2) ISP OSP R. A. Pitts et al., J. Nucl. Mater., 290-293 (2001) 940R. A. Pitts et al., IAEA-CN77/EXP4/23 (2000) Time (s) R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  10. “Anomalous” detachment • TCV outer divertor does not detach like in other tokamaks • Divertor densities too low • Neutral baffling insufficient • SOLPS4 simulations (with A. Loarte) unable to reproduce observed detachment ne(1019m-3) Te(eV) • 3 year study with SOLPS5 tracked the problem down (probably) • Strong outward convective transport  main chamber recycling  increased C release  increased radiation  “power detachment” Jsat(Acm-2) M. Wischmeier, Phd Thesis (EPFL: TH3176 (2005))M. Wischmeier et al.,ECA 29C P-5.013 (2005)M. Wischmeier, R. A. Pitts, in preparation for Nucl. Fusion R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  11. Turbulent transport • Expts. on TCV some of the first to identify profile broadening with increased plasma density • Fast RCP under midplane • ne, Te and fluctuation driven flux • Large database in ohmic plasmas Broad profiles at high density  increased main chamber wall interactionWhy does this happen? R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  12. Intermittency • In the far SOL, density fluctuations more bursty and rare • Almost all the radial transport in these regions carried by the blobs • Convect plasma quickly to the wall regions • Competes equally with parallel transport • Consistency with known statistical distributions discovered on TCV J. P. Graves, J. Horacek, R. A. Pitts, Plasma Phys. Control. Fusion 47 (2005) L1 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  13. Modelling the turbulence • 2-dimensional fluid turbulence simulations – ESEL code (Risø) • Centred on outer midplane • ne, Te and vorticity evolution • Collective motions driven by non-uniform B-field • Linear SOL damping terms driven by SOL transport • Model parameters set by a high density TCV case • Sample turbulent fields over long time series by an array of trial probes O. E. Garcia, J. Horacek, R. A. Pitts, et al., Plasma Phys. Control. Fus. 48 (2006) L1 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  14. Encouraging agreement with expt. Conditionally averaged density • Code matches turbulent statistics • Relative fluctuation level • Higher moments of PDF (Skewness, Flatness) • Detailed “structure” of blobs – sharp front and trailing wake O. E. Garcia, R. A. Pitts, J. Horacek et al., PSI 2006, Heifei& Plasma Phys. Control. Fus. 48 (2006) L1 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  15. Implications for main wall fluxes PDF of turbulent flux at wall radius • Good agreement between expt. & simulation provides extremely strong evidence for interchange motions as the origin of anomalous SOL  transport • Flux-gradient paradigm: G = Deffn/r not adequate to describe TCV data. • Convection: G = nVeff does better across region of broad SOL profile • TCV results show that the scaling of wall flux with density seen elsewhere is due to turbulent interchange motions J. Horacek, O. E. Garcia, R. A. Pitts et al., IAEA EXP4/21 (2006) R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  16. ErxB, pxB EqxB Ballooning Pfirsch-Schlüter Bj Divertor sink BxB BxB REV Bj SOL Flows • Determine transport of impurities from source to destination in a tokamak – material migration – T-retention Poloidal Bj Parallel FWD Bj R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  17. Studying flows on TCV #26092 #27585 #27582 #27588 • Use configurational flexibility of TCV to study flows in simplest possible diverted, ohmic plasmas • Emphasis on direction of Bj, configuration and density (|Bj| = 1.43 T) • Bj and Ip always reversed together to preserve helicity Mach probe BxB BxB R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  18. FWD-B/REV-B, Ip = 260 kA, density scan • Strong field direction and density dependence near outer midplane • Flows always co-current • Direction consistent with Pfirsch-Schlüter flow • Slight, field independent negative offset wall R. A. Pitts et al., PSI 2006 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  19. = 4.2 x 1019m-3 Ballooning drive? • Change of M|| with location above and below plasma midplane is consistent with a ballooning drive for the field independent flow offset • Not unambiguous owing to presence of lower divertor sink – new results this week(!!) show that is a real “ballooning” drive BxB +10 cm 0 cm -10 cm OUTER divertor R. A. Pitts et al., PSI 2006 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  20. 1.7 1.7 2.5 2.5 4.2 4.2 6.3 6.3 7.3 7.3 INNER divertor INNER divertor FWD Bj INNER divertor INNER divertor OUTER divertor OUTER divertor OUTER divertor OUTER divertor (1019m-3) (1019m-3) REV Bj REV Bj Field dependent component 260 kA density scan in FWD/REV-B: Choose radial band in the main SOL:8 < r-rsep < 12 mm Take mean exptl. M|| and plot versus density Compare with predicted Pfirsch-Schlüter flow  R. A. Pitts et al., PSI 2006 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  21. Interchange driven flows • Assume radial transport by interchange motions in outer midplane vicinity – estimate parallel flow due to transients • Time averaged Mach No. due to transport driven flow: M|| ~ 0.5fp>ap • fp>ap= t(p > ap)/Dt • Duration of time series, Dt • t(p > ap) time over which p exceeds p by factor a • Reasonable agreement with experiment • TCV results show that parallel flows can be explained by combination of classical (drift driven) and transport (turbulence driven) components R. A. Pitts et al., PSI 2006W. Fundamenski, R. A. Pitts et al., accepted for publication in Nuclear Fusion R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  22. JET R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  23. Retarding field energy analyser RFA • Designed and built at CRPP as part of enhancement project (JW0-ED-3.7) for reciprocating probe head upgrades • Previous attemps to make such a device function had always failed on JET • Provides radial profile of SOL Ti • Almost never measured, especially in large tokamaks • Can also yield plasma potential and local Mach flow JET DOC-L Discharges R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  24. RFA Principle Vs -180 V 0 V -150 V • Usual application is to sweep ion retarding grid to generate I-V characteristic and extract Ti, Vsheath agreement with experiment • Negative slit bias allows simultaneous extraction of parallel ion flux  Mach flows can be measured with a bi-directional device • Long cable lengths (> 100 m on JET!) and small signals (mA) prevent fast grid sweeping, but ELMs can be measured – see later R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  25. Slit plate Collector 40 mm Grids March 2003 The JET RFA • Complex design – probes on JET subject to much greater constraints than elsewhere • Bi-directional – 2 RFA cavities looking along Bj • All boron-nitride design, like all JET probes • 30 mm wide entrance slits • 2 mm grid separation • Theoretical ion transmission ~0.2 JET DOC-L Discharges • They don’t always last long either • Probe drive accident on last day of operation in Campaign C14 After March 2004 R. A. Pitts et al., Rev. Sci. Instr. 74 (2003) 4644 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  26. Excellent flow data from top LFS • Probe samples at the near zero point for Pfirsch-Schlüter flow – and yet, large flows – not understood nor reproduced by edge codes (SOLPS5, EDGE2D) • BB strong parallel flow towards inner divertor at RCP • BB flow stagnates at RCP • Mean flow offset towards inner divertor – consistent with transport driven flow as in TCV – verified also with ESEL on JET S. K. Erents, R. A. Pitts et al., PPCF. 46 (2004) 1757 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  27. SOL flow confirmed by Ti data • Large change in ion-side/electron side Ti ratio with field reversal • BB: Ti,i-side/Ti,e-side > 1 • BB : Ti,i-side/Ti,e-side ~ 1 • Due to the strong perturbing effect of the probe itself • Ions depleted on the “downstream” side  strong electric fields develop  ion f(v||) modified • JET RFA provided first ever demonstration of this theoretically expected effect – quantitative agreement with theory for FWD Bj R. A. Pitts et al., ECA Vol. 27A, P-2.84R. A. Pitts et al., J. Nucl. Mater. 337-339 (2005) 146 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  28. Edge Localised modes Da t = 19.05 s, ELM-free t = 19.06 s, Type I ELM JET #62218 H-mode  Edge MHD instabilities  periodic bursts of particles and energy into the SOL. Type I ELMing H-mode is currently the baseline ITER scenario Time (s) R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  29. Far SOL ELMs on the RFA #63214 • Delicate edge probes can never be used close to separatrix in high power discharges • But measurements in the far SOL just as important (determine wall interaction) • Use RFA to detect the ELM transient near the limiter radius • Use constant grid bias and catch ELM convected ions able to surmount the potental barrier (~400 V) • Only a few measurements in H-mode Hydrogen plasmas dsep at probe (mm) Wdia(kJ) Ha (outer) /1015 jslit (Acm-2) Vslit (V) Icoll (mA) Vgrid1 (V) Vgrid2 (V) Time (s) R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  30. Individual ELMs dsep ~134 mm dsep ~86 mm dsep ~74 mm dsep ~73 mm Ha Wdia (kJ) jslit (Acm-2) Icoll (mA) R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  31. Far SOL ELM ion energies r - rsep ~ 80 mm at the probe • Clear filaments in each ELM • Net apparent flow to inboard side ELM enters SOL mainly on outboard side • Multiple filaments and clear trend for lower energies in successive filaments suggest picture of ELM as a train of toroidally rotating, feld aligned structures Current of ions with energy > 400 eV R. A. Pitts et al., Nucl. Fusion 46 (2006) 82 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  32. Modelling the ELM transient Input • Filament origin location • Te, Ti, ne at ELM origin • ELM radial speed, vELM • Parallel connect. length, L|| New transient model of ELM parallel losses • Solves dynamical particle and energy two-fluid equations in the ELM filament frame subject to parallel losses determined by sheath boundary conditions Output • Te, Ti, ne in the ELM filament at any radial distance • Normalisation parameter: tn,0 = L||/cs • Characteristic parallel loss time evaluated at the initial conditions of the transient Compute ion collector current with simple model of RFA function  compare with expt. W. Fundamenski & R. A. Pitts PPCF 48 (2006) 109 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  33. Model consistent with RFA data • Good agreement with i-side fluxes • Assume ELM starts anywhere from pedestal top to separatrix but with “mid-pedestal” Ti, Te, n • Semi-adiabatic broadening • vrELM = 600 ms-1 (from previous JET scaling) • Predicts Ti,RFA/Ti,ped = 0.30.5 • Te,RFA/Te,ped = 0.130.25 • ne,RFA/ne,ped = 0.30.4 • Filament cools faster than it dilutes, electrons cool more rapidly than ions W. Fundamenski & R. A. Pitts PPCF 48 (2006) 109 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  34. ELMs can interact strongly with main chamber surfaces • New KL8 visible (Photron) camera at JET picks up ELM induced main wall interactions during recent dedicated ELM ablation session (1 MJ ELMs!) Images courtesy of CIEMAT team and C. Silva R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

  35. Model prediction for ITER ELM starts mid-pedestal • Model implies significant ELM wall erosion in ITER and beyond, even for high Z-wall ~1100 eV D+ W: 0.5% yield(1% for T+) ~600 eV ~260 eV D+ W threshold(209 eV D+, 136 eV T+) Confirmed experimentally on JET (RFA) Ion impact energy = 3Te + 2Ti R. A. Pitts et al., PPCF 47 (2005) B303W. Fundamenski & R. A. Pitts PSI 2006 R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

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