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Edge pedestal physics and its implications for ITER Y.Kamada 1, A.W.Leonard 2, G.Bateman 3, M.Becoulet 4, C.S.Chang 5, T.Eich 6, T.E.Evans 2, R.J.Groebner 2, P.N.Guzdar 7, L.D.Horton 6, A.Hubbard 8, J.W. Hughes 8, K.Ida 9, G.Janeschitz 10, K.Kamiya 1, A.Kirk 11, A.H.Kritz 3, A.Loarte 12, J.S.Lonnroth 21, C.F.Maggi 6, R.Maingi 13, H.Meyer 11, V.Mukhovatov 14, T.Onjun 15, M.Osipenko 16, T.H.Osborne 2, N.Oyama 1, G.W.Pacher 17, H.D.Pacher 18, A.Y.Pankin 3, V.Parail 11, A.R.Polevoi 14, T.Rognlien 19, G.Saibene 12, R.Sartori 12, M.Shimada 14, P.B.Snyder 2, M.Sugihara 14, W.Suttrop 6, H.Urano 1, M.R.Wade 2, H.R.Wilson 20, X.Q.Xu 19, M.Yoshida 1, and the ITPA Pedestal & Edge Physics Topical Group 1 Japan Atomic Energy Agency, 2 General Atomics, 3 Lehigh Univ., 4 Association Euratom-CEA, 5 New York Univ., 6 Association Euratom-IPP, 7 Univ. Maryland, 8 MIT Science and Fusion Center, 9 NIFS, 10 FZK-PL- Fusion, 11 Association Euratom-UKAEA, 12 EFDA-CSU, 13 Oak Ridge National Laboratory, 14 ITER International Team, 15 Thammasart Univ., 16 Kurchatov Institute, 17 Hydro-Quebec, 18 INRS, 19 Lawrence Livermore National Laboratory, 20 Univ. of York, 21 Association EURATOM-Tekes, Remarkable progress has been achieved by integrating experimental results obtained in single- and inter-machine experiments (C-Mod, AUG, DIII-D, JET, JFT-2M, JT-60U, MAST and NSTX) with theoretical progress.

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core confinement as boundary condition -limit through p(r)&j(r) heat/particle pulse to Div. Edge Pedestal determines Edge Pedestal : Key area determining integrated performance of ITER Present experiments indicate ELM heat flux could be a problem for ITER. ITER plasma performance is determined by the pedestal height. How the Pedestal Structure is determined? How the type I ELM cycle evolves? => How control?

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Outline 1) Identification of the processes determining the pedestal structure 2) Understanding of the type I ELM cycle 3) Development & evaluation of small / no ELM regimes 4) Type I ELM mitigation techniques 5) Development of integrated prediction codes. 6) Summary ITPA-PEP

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Parameter Linkage Determined ITPA-PEP

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Pedestal height determines Q in ITER Reason of uncertainty is large scatter in T ped [V. Mukhovatov, PPCF 45 (2003) A235] ITPA-PEP ITER fusion gain predicted by theory based transport models depends strongly on T ped (p ped ). Critical edge grad-p based on the P-B model : the limit of T ped increases nearly proportional to ped /a; T ped ~5keV at ped /a =0.03 P. Snyder, PPCF 45 (2003) 1671

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ITPA-PEP Temperature Width: determined by the magnetic structure and non-dimensional parameters. ( Multi-machine comparison exp.) Density width: DIII-D&JET: neutral penetration explains. AUG&C-mod: width ~constant. Temperature Width: plasma transport Density width: plasma & neutral transport Fenstaermacher (NF2005) C-Mod: Neutral transport by 1D kinetic model: High n e (C-mod) : largely self-screening to D 0. Low n e (DIIID) : Longer D 0 penetration lengths, then pedestal narrows slightly at higher n e Hughes (EXP3-9)

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Plasma rotation & ripple affect pedestal height ITPA-PEP JET & JT-60U comparison with matched 'absolute’ parameters: pedestal pressure: JET > JT60U => possible reason = TF ripple. Ferritic steel tile installation in JT-60U: Both co-directed shift of Vt & reduction of filed ripple improves pedestal. (wider pedestal width & higher pedestal pressure) Thermal ion transport enhanced by ripple has been proposed (Parail THP8-5) Saibene (NFsubmitted) Urano EX5-1

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Type I ELM Trigger & Crash identified ITPA-PEP Kirk (PRL ) ELM filament motion : radial+poloidal+toroidal MAST Type I ELM Trigger: The P-B model has been confirmed on a number of tokamaks. ELM crash dynamics: 2-3D structure : poloidal asymmetry of erosion inside the separatrix and helical filament structure expanding into the SOL. Type I = P-B critical JET, AUG, C-Mod, JT-60U, MAST, NSTX Saarelma (POCF2005)etc. AUG: Strong local perturbations of n e & T e in the near SOL Horton (NF2005)

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ELM Energy Release : dependence clarified ITPA-PEP W ELM /W ped : increases with decreasing *, (multi-machine) ~ % at the expected * in ITER. tends to increase with triangularity(AUG), with increasing co-directed rotation(JT-60U). * -dependence of the efflux is large for conductive loss and small for the convective loss (JET, DIII-D) Loarte (PPCF 2003), Kamiya (PPCF 2006) Energy release at an ELM is carried partly by the filaments : <20%. Main loss channel has not been identified. One possibility is that the filaments tear the closed flux surfaces allowing parallel transport.

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ELM crash & Transport recovers quickly Er Well Flattened at ELM Wade (PoP 2005) Inter ELM transport close to neoclassical Urano(PRL 2005) ITPA-PEP Structure of the edge Er shear is suddenly broken by the ELM crash (DIII-D) After an ELM crash, recovery of the pedestal rotation profile takes place faster than recovery of the pedestal pressure (DIII-D & JT-60U). Then the edge pressure recovers in the time scale of the inter-ELM transport inter-ELM ~ close to neoclassical (JT60U), still anormalous remains(AUG) Yoshida(PPCF 2006) Vt(r) recovers quickly r/a~0.94 D burst t (ms)

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Peeling-Ballooning Model explaines ELMs Successfully Type I ELM onset: The P-B model has been confirmed in many tokamaks A non-linear analytic theory, valid early in the evolution of a ballooning mode, predicts that filamentary structures should grow explosively. A number of codes support this general result. Sufficient edge current density is required to cause the filaments to be ejected outwards towards the wall (otherwise they erupt inwards, towards the plasma core) (H. Wilson TH4-1Rb) Nonlinear explosive evolution of the filaments reproduced numerically by using the 3D electromagnetic two-fluid code BOUT (Snyder, PoP 2005) ITPA-PEP

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Small/no ELM regimes; accessibility identified, reproduced by inter-machine comparison ITPA-PEP W ELM /W ped <5%. All small/no ELMregimes reproduced in multiple devices Except for Grassy and type V, the edge fluctuations enhance particle transport, and the pedestal pressure is below the type I ELM limit. Oyama (PPCF 2006)

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Small/no ELM Regimes need to be extended to ITER regime ITPA-PEP Only Grassy ELM and QH-mode achieved at close to ITER. Better understandings of the effects of , plasma shape and driving mechanisms of the edge fluctuations are needed. Oyama (PPCF, submitted) Plasma rotation seems to be important. DIII-D etc.: CTR rotation produces QH mode. JT-60U: grassy ELM freq. increases linearly up to 1400 Hz with CTR rotation. Even at no-rotation: ~400 Hz Oyama (PPCF 2006)

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ELM control with pellet pace making Natural ELM frequency in ITER f ELM W ELM = 0.4 P loss P loss 80MW, W ELM = 20 MJ, f ELM = 1.6 Hz critical value for sublimation of CFC: T W / S / 0.5 40 MJ m -2 s -0.5 f ELM p 1.6 (2-3) 3-5Hz ITPA-PEP Issue: confinement degradation; 10~15% reduction when f ELM is increased by a factor of 2-3 xf ELM, (AUG) AUG: f ELM = f pellet, W ELM decreases with f ELM. (Lang NF 2004) (Polevoi NF 2005)

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ELM control with Resonant Magnetic Perturbation Becoulet IT P1-29 For ITER Required ergodization for ELM suppression can be realized with In- vessel coil (20kA), Ex-vessel coil (≤150kA) or external coils (400kA). Effect of generated island in core and impact on engineering design need further study. ITPA-PEP DIII-D : elimination of Type I ELMs at ITER's * by applying external field. RMP increases particle transport. Issuers: compatibility of operation at high n e Evans (PRL 2004)

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Progress of Integrated Modeling ITPA-PEP The modeling capability for integrating core transport, pedestal (NC + PB), SOL and divertor regions has achieved remarkable progress. LEHIGH-JETTO, ICPS, JETTO, TOPICS-IB… Accurate simulations underway: kinetic effects, finite gyroradius effects TEMPEST: particle distribution functions are represented as continuous functions in 4D / 5D with full toroidal geometry. (Xu TH P6-23) XGC: the turbulence suppression after the H-mode transition can be sustained by neoclassical sheared flow alone. (Chang TH P6-14) LEHIGH -JETTO ITER baseline ELMing Q=16.6 with T ped =4.9keV Onjun (PoP2005) TOPICS-IB * dependence of W ELM is caused by bootstrap j edge which changes the eigenfunction of the unstable modes and the parallel heat conduction in the SOL decreasing with *. Hayashi (TH4-2)

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Summary 1) The complex parameter linkages in pedestal have been identified. ped is determined by both plasma processes and neutral transport. largest issue = prediction of the pedestal width in ITER 2) Type-I ELM onset: explained successfully by the P-B modes. Evolution of the type I ELM cycle (crash and recovery): revealed Explosive evolution: predicted theoretically and reproduced numerically. Issue: Change of surface current across an ELM ELM Energy Loss mechanisms 3) Small and no-ELM regimes: reproduced in multiple devices, and accessibility to these regimes has been identified and categorized. Issue: extend to ITER regime 4) Rotation plays important roles in determining pedestal structure and ELMs. Issue: rotation controlability 5) Modeling capability integrating the core, pedestal and SOL regions has achieved remarkable progress. Issue: Pedestal width, and j edge (t) across an ELM crash 6) Based on these results, the pedestal height required for ITER has been evaluated, the ELMing ITER base line scenario has been simulated, type I ELM mitigation methods have been evaluated for ITER. Issue: Confinement degradation & island formation ITPA-PEP

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