Edge pedestal physics and its implications for ITER

1. Edge pedestalphysics and its implications for ITER Y.Kamada1, A.W.Leonard2, G.Bateman3, M.Becoulet4, C.S.Chang5, T.Eich6, T.E.Evans2, R.J.Groebner2, P.N.Guzdar7, L.D.Horton6, A.Hubbard8, J.W. Hughes8, K.Ida9, G.Janeschitz10, K.Kamiya1, A.Kirk11, A.H.Kritz3, A.Loarte12, J.S.Lonnroth21, C.F.Maggi6, R.Maingi13, H.Meyer11, V.Mukhovatov14, T.Onjun15, M.Osipenko16, T.H.Osborne2, N.Oyama1, G.W.Pacher17, H.D.Pacher18, A.Y.Pankin3, V.Parail11, A.R.Polevoi14, T.Rognlien19, G.Saibene12, R.Sartori12, M.Shimada14, P.B.Snyder2, M.Sugihara14, W.Suttrop6, H.Urano1, M.R.Wade2, H.R.Wilson20, X.Q.Xu19, M.Yoshida1, 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, 9NIFS, 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, 17Hydro-Quebec, 18INRS, 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.

2. Edge Pedestal : Key area determining integrated performance of ITER Edge Pedestal determines core confinement as boundary condition b-limit through p(r)&j(r) heat/particle pulse to Div. • 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?

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

4. Parameter Linkage Determined ITPA-PEP

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

6. C-Mod: Neutral transport by 1D kinetic model: High ne (C-mod) : largely self-screening to D0. Low ne (DIIID) :Longer D0 penetration lengths, then pedestal narrows slightly at higher ne Hughes (EXP3-9) Temperature Width: plasma transport Density width: plasma & neutral transport ITPA-PEP Density width: DIII-D&JET: neutral penetration explains. AUG&C-mod: width ~constant. Temperature Width: determined by the magnetic structure and non-dimensional parameters. ( Multi-machine comparison exp.) Fenstaermacher (NF2005)

7. 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) Urano EX5-1 Saibene (NFsubmitted) Thermal ion transport enhanced by ripple has been proposed (Parail THP8-5)

8. AUG: Strong local perturbations of ne & Te in the near SOL ELM filament motion : radial+poloidal+toroidal MAST Kirk (PRL 2004. 2006) Horton (NF2005) ITPA-PEP Type I ELM Trigger & Crash identified 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.

9. ITPA-PEP ELM Energy Release : dependence clarified • WELM/Wped : • increases with decreasing *, (multi-machine) • ~ 15- 20% 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) 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. Loarte (PPCF 2003), Kamiya (PPCF 2006)

10. D burst r/a~0.94 r/a~0.94 Inter ELM transport close to neoclassical -10 0 10 Vt(r) recovers quickly Dt (ms) Yoshida(PPCF 2006) Urano(PRL 2005) ELM crash & Transport recovers quickly 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) Er Well Flattened at ELM Wade (PoP 2005)

11. Peeling-Ballooning Model explaines ELMs Successfully ITPA-PEP Nonlinear explosive evolution of the filaments reproduced numerically by using the 3D electromagnetic two-fluid code BOUT (Snyder, PoP 2005) 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)

12. Small/no ELM regimes; accessibility identified , reproduced by inter-machine comparison ITPA-PEP • DWELM/Wped <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)

13. Small/no ELM Regimes need to be extended to ITER regime ITPA-PEP Only Grassy ELM and QH-mode achieved at n* close to ITER. Better understandings of the effects of n*, plasma shape and driving mechanisms of the edge fluctuations are needed. 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, submitted) Oyama (PPCF 2006)

14. ITPA-PEP AUG: fELM = fpellet, DWELM decreases with fELM. (Lang NF 2004) ELM control with pellet pace making  Natural ELM frequency in ITER fELMWELM = 0.4 Ploss Ploss80MW, WELM = 20 MJ,  fELM= 1.6 Hz • critical value for sublimation of CFC: TW / S / 0.5 40 MJ m-2s-0.5 fELMp1.6(2-3) 3-5Hz (Polevoi NF 2005) Issue: confinement degradation; 10~15% reduction when fELM is increased by a factor of 2-3 xfELM, (AUG)

15. ELM control with Resonant Magnetic Perturbation ITPA-PEP DIII-D : elimination of Type I ELMs at ITER's n* by applying external field. RMP increases particle transport. Issuers: compatibility of operation at high ne Evans (PRL 2004) 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. Becoulet IT P1-29

16. n* dependence of DWELM is caused by bootstrap jedge which changes the eigenfunction of the unstable modes and the parallel heat conduction in the SOL decreasing with n* . TOPICS-IB Hayashi (TH4-2) 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) ITER baseline ELMing Q=16.6 with Tped=4.9keV LEHIGH -JETTO Onjun (PoP2005)

17. Summary ITPA-PEP 1) The complex parameter linkages in pedestal have been identified. Dped 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 jedge(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