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Edge Localised Modes control by resonant magnetic perturbations

Edge Localised Modes control by resonant magnetic perturbations. E. Nardon EURATOM / UKAEA Fusion Association. Many thanks to M. B é coulet, P. Cahyna, T.E. Evans, Kirk, R. Hawryluk, S. Saarelma, S. Sabbagh, O. Schmitz and all the community of ELM controllers. Outline. Introduction

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Edge Localised Modes control by resonant magnetic perturbations

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  1. Edge Localised Modes control by resonant magnetic perturbations E. Nardon EURATOM / UKAEA Fusion Association • Many thanks to M. Bécoulet, P. Cahyna, T.E. Evans, • Kirk, R. Hawryluk, S. Saarelma, S. Sabbagh, • O. Schmitz and all the community of ELM controllers E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  2. Outline • Introduction • Experimental results • Modelling • A proposed global picture • Status of the ITER coils • Upcoming experiments • Summary and outlook E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  3. Introduction • ITER ELMs are estimated to release ~20MJ… • …but PFCs can only tolerate ~1MJ! • Resonant Magnetic Perturbations (RMPs) have been shown to mitigate the ELMs on different machines • ELM control coils under study for ITER • Other ELM control techniques exist (ELM pace making…) • Original idea: stochastize the edge to degrade dTe/dr in the transport barrier in order to keep the plasma stable • The reality does not entirely conform to this picture • To date,ELM control by RMPs is not fully understood E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  4. Outline • Introduction • Experimental results • Modelling • A proposed global picture • Status of the ITER coils • Upcoming experiments • Summary and outlook E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  5. ELM suppression on DIII-D I-coils 4kAt time (ms) Evans ’06 I-coils: 6+6 coils, n=3, ~4kAt • ELM suppression for tens of τE • Works only in a narrow resonant q95 window • Drop in density: « pump-out » • dTe/dr|ETB increases rather than drops! ne Te E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  6. EFCCs: n = 1 or 2, ~20kAt ELM mitigation on JET Liang ’07 n=1 experiment Pump-out (~DIII-D) No drop in dTe/dr|ETB (~DIII-D) fELM , ΔWELM  (but not enough for ITER) E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  7. Different effects observed on NSTX n = 2 / 3 / (2&3) • Destabilisation of Type I ELMs in otherwise ELM-free plasmas • Decrease in fELM in ELMing plasmas • No density pump-out, possibly due to absence of pumping E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  8. MAST EFCCs have a possible effect on ELMs EFCCs: n = 1 or 2; ~12kAt • n=1 perturbations • Affected L-H transition • Often caused H-L back transition • n=2 worked better • Possible effect on ELMs • Increase in Type IV ELM frequency in low collisionality discharges (Difficulties to obtain a good ref. discharge with Type I ELMs) n=2 experiment fELM EFCC current E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  9. Other machines • JFT-2M: triggering of ELMs in ELM-free periods Shoji ’90 • COMPASS: triggering of ELMs in ELM-free plasmas / increase in ELM frequency in ELMing plasmas Fielding ’01 • TEXTOR: reduction in ELM size with the DEDUnterberg ’08 • In summary • Complex phenomenology • We will focus on the DIII-D and JET results E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  10. Outline • Introduction • Experimental results • Modelling • A proposed global picture • Status of the ITER coils • Upcoming experiments • Summary and outlook E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  11. Magnetic modelling: vacuum approximation Br ERGOS modelling (Nardon ’07) for DIII-D I-coils E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  12. ELM mitigation/suppression is correlated with stochastisation in vacuum field modelling (1/2) RMP ELM-free • DIII-D (Fenstermacher ’08): Width of stochastic layer (ΔChir>1) = good ordering parameter for ELM size ELM size ΔChir>1 • Critical width for ELM suppression = ~3 pedestal widths E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  13. ELM mitigation/suppression is correlated with stochastisation in vacuum field modelling (2/2) • JET (Liang ’07): the ELM mitigation threshold depends on q95 • Consistent with a dependence on σChirikov(Nardon ’08) • (ΔChir>1 at JET mitigation) < (ΔChir>1 at DIII-D suppression) • Remark: ELM mitigation also seen at DIII-D • before ELM suppression E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  14. Evidence for stochastisation: strike point splitting IR and Dα view DIII-D results(Schmitz ’08) swall [cm] • Vacuum modelling of field lines • impacts on divertor targets φ [degrees] • Consistent structure seen on Dα • Less marked on heat flux E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  15. Ideal MHD stability analysis • In DIII-D, RMPs maintain the profiles in the stable region for peeling-ballooning modes • The peak value of dp/dr is not affected much • The region most affected by the RMPs is actually inside the pedestal 0kA (ELMing) 4kA (ELMing) 6.3kA (ELM free) Snyder ’07 Evans ’08 E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  16. Ideal MHD stability analysis • In JET, the plasma remains unstable but moves towards the peeling boundary (Saarelma ’08) • Mode structure less extended radially, consistent with smaller ELM size Before EFCCs pulse During EFCCs pulse (jedge,max+jsep)/2 (MA.m-2)   Toroidal mode number of most unstable mode E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  17. Density transport modelling: pump-out? • Pump-out = increased transport + pumping • Pumping • Active pump • Changes in recycling conditions (strike point splitting) • Increased transport • // transport driven by dp/dr(Tokar ’07) • Stronger transport than // one related to complex role of Er • (Tokar ’08) • Neoclassical prediction: • Stochastic field  positive Er holding back the electrons  Ion neoclassical flux • RMPs are also observed to modify turbulent spectra(Moyer ’08) See poster 30 by M. Tokar See poster 37 by A. Kramer-Flecken E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  18. Heat transport : why does dTe/dr|ETB survive? E3D simulations Experimental profiles • Parallel conduction in vacuum field  large transport… Joseph ’07 • …unless one uses a strong flux limiter: • Tokar ’07, Bécoulet ’05 •  With screened RMPs? See poster 30 by M. Tokar Numerical factor E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  19. Magnetic modelling: plasma response • RMPs screening by rotation: cylindrical modelling • Fluid (Fitzpatrick theory) Bécoulet ’08 • KineticHeyn ’08 •  Weak screening at the very edge •  Strong screening in the core • 3D MHD equilibrium: realistic geometry • IPECPark ’07 •  Plasma response can be non negligible • even without rotation • « All » • JOREK (3D non-linear MHD) Nardon ’07 • A large resistivity is needed in these simulations  Screening predictions only qualitative E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  20. Magnetic modelling: plasma response vExB and v*e cancel each other  expect reconnection • Key physics of RMPs screening by rotation Induction equation: Origin of screening: Typical vExB and v*e H-mode profiles vA vExB and v*e add up  expect little reconnection Small vExB and v*e and large η  expect reconnection v*e vExB+v*e vExB E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  21. Outline • Introduction • Experimental results • Modelling • A proposed global picture • Status of the ITER coils • Upcoming experiments • Summary and outlook E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  22. A proposed global picture Vacuum field Stochastic Pedestal r/a 1 0 Instead of one large stochastic layer… E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  23. A proposed global picture With plasma response Stoch. 1 Stoch. 2 Flux surfaces Pedestal r/a 1 0 Two stochastic layers isolated by good flux surfaces E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  24. A proposed global picture With plasma response Stoch. 1 Stoch. 2 Flux surfaces Pedestal r/a 1 0 Two stochastic layers isolated by good flux surfaces • First stochastic layer at the very edge • Easily created • Island overlap does not require large RMPs because of strong magnetic shear (in vacuum approximation) • Vacuum approximation is OK • Large η • Small rotation • Responsible for • Strike point splitting • Density pump-out • Remark: in DIII-D, pump-out is observed in a larger q95 window than for ELM suppression in DIII-D • ELM mitigation in JET (and DIII-D) E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  25. A proposed global picture With plasma response Stoch. 1 Stoch. 2 Flux surfaces Pedestal r/a 1 0 Two stochastic layers isolated by good flux surfaces • Good flux surfaces in the middle of the pedestal • Due to strong rotational screening of RMPs • Small η • Strong vExB+v*e • Helps maintain dTe/dr • Cuts // connection between core plasma and escaping field lines •  Strike point splitting seen more clearly on recycling than on heat flux E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  26. A proposed global picture With plasma response Stoch. 1 Stoch. 2 Flux surfaces Pedestal r/a 1 0 Two stochastic layers isolated by good flux surfaces • Second stochastic layer from top of pedestal • Although η is small, reconnection occurs because v*e compensates vExB • Stochasticity is more difficult to obtain • Magnetic shear is smaller than at the very edge •  Islands overlap requires larger RMPs • Possibly not reached at JET • Responsible for ELM suppression on DIII-D • Consistent with correlation ELM suppression ↔ stochasticity • Consistent with the fact that RMPs affect the pressure gradient profile mainly inside the pedestal E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  27. Outline • Introduction • Experimental results • Modelling • A proposed global picture • Status of the ITER coils • Upcoming experiments • Summary and outlook E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  28. Status of coils design on ITER • Main physics requirements: • σChirikov>1 for ψN1/2>0.92 for all scenarios • Minimise braking due to Neoclassical Toroidal Viscosity (NTV) • NTV arises from broken axisymmetry • (Shaing) • NTV produces a global braking • NTV is strong at low collisionality • Concern for ITER • NTV theory needs to be compared • to experiments • Only NSTX so far(Zhu ’06) • Current concept: • 3 rows of 9 coils between blanket and VV • n=4 • Coupled with VS coils • Spectral flexibility E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  29. Outline • Introduction • Experimental results • Modelling • A proposed global picture • Status of the ITER coils • Upcoming experiments • Summary and outlook E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  30. Upcoming experiments: a very active program COMPASS (Cahyna ’08) • Ongoing experiments on already involved machines • More machines getting involved / new coils systems • COMPASS restarting in IPP Prague • ELM coils to be implemented on ASDEX-U • Studies for possible new ELM coils on DIII-D, • JET, NSTX • MAST has new ELM control coils • First results in L-mode • First H-mode experiments next week E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  31. Summary and outlook • ELM control coils could be an essential element in ITER • ELM control is accompanied by density pump-out, but dTe/dr survives • Is it possible to compensate the pump-out? • Compatibility with pellet fuelling? • Unified criterion / recipe for ELM control? • Requires to understand the physics • Stochasticity seems to play a key role • Linear MHD stability analysis suggests that ELM control results from changes in profiles • RMP-induced transport is a difficult topic • Plasma magnetic response being investigated • Likely to be a screening in the middle of the pedestal • Suggestion: two stochastic layers isolated by good flux surfaces • Plasma braking due to NTV in ITER is an open question • Active upcoming experimental program including new contributors E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  32. Back-up slides E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  33. Screening currents do not change magnetic footprint envelope q=5 q=5 JET modelling (EFCCs n=1) Vacuum field Nardon ’08 With helical currents put by hand on q=5 to screen the local RMPs E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  34. ELM mitigation/suppression is correlated with stochastisation in vacuum field modelling Pump-out does not require to be in the resonant q95 window • DIII-D (Fenstermacher ’08): the width of the resonant q95 window depends on the “RMP mix” I: 4kAt C: 9.6kAt I: 3kAt C: 9.6kAt I-coils: n=3 C-coils: n=1 I: 3kAt C: 4.8kAt E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  35. References Experimental results COMPASS: S.J. Fielding et al., 28th EPS conference (Funchal, 2001), ECA vol. 25A, p. 1825 JFT-2M: T. Shoji et al., 17th EPS conference (Amsterdam, 1990), ECA vol. 14B, p. 1452 TEXTOR: B. Unterberg et al., submitted to J. Nucl. Mater. DIII-D: T.E. Evans et al., Nucl. Fusion 48(2008) 024002 Vacuum modelling and design of ITER coils E. Nardon et al., J. Nucl. Mater. 363-365 (2007) 1071 M. Bécoulet et al., Nucl. Fusion 48 (2008) 024003 M. Schaffer et al., Nucl. Fusion 48 (2008) 024004 E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  36. Evidence for stochastisation DIII-D: M.E. Fenstermacher et al., Phys. Plasmas 15, 056122 (2008) JET: E. Nardon et al., submitted to J. Nucl. Mater. Strike point splitting at DIII-D: O. Schmitz et al., submitted to Plasma Phys. Control. Fusion MHD stability analysis DIII-D: P.B. Snyder et al., Nucl. Fusion 47 (2007) 961 JET: S. Saarelma et al., submitted to Plasma Phys. Control. Fusion Plasma response E. Nardon et al., Phys. Plasmas 14 (2007) 092501 M. Bécoulet et al., Nucl. Fusion 48 (2008) 024003 M.F. Heyn et al., Nucl. Fusion 48 (2008) 024005 E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

  37. Transport modelling: M.Z. Tokar et al., Phys. Rev. Lett. 98 (2007) 095001 M. Bécoulet et al., Nucl. Fusion 45 (2005) 1284 I. Joseph et al., Nucl. Fusion 48 (2007) 045009 M.Z. Tokar et al., Phys. Plasmas 15 (2008) 072515 NTV: W. Zhu et al., Phys. Rev. Lett. 96 (2006) 225002 A. Cole et al., Phys. Plasmas 15 (2008) 056102 and refs. therein E. Nardon, 13th EU-US TTF workshop, Copenhagen, September 2008

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