L-H power threshold and ELM control techniques: experiments on MAST and JET Carlos Hidalgo EURATOM-CIEMAT Acknowledgments to: A. Kirk (MAST) European Fusion Physics Workshop (December 2008): Y. Andrew, L. Horton, E. Nardon, W. Suttrop ITPA meeting, April (2009)

L-H power threshold physics and ITER plasma scenarios Large uncertainties are still present in the empirical description of the L-H power transition with impact in the overall research programme of next step magnetic confinement devices (e.g. ITER). This fact is reflecting the lack of basic understanding of the physics of sheared flows and edge transport barriers. ITPA meeting, April (2009)

Pin > 1.5Pth for H98 = 1.0, Type-I ELMS Pin = Pth is insufficient for H98 = 1 access R Sartori et al., PPCF 46 (2004) 723 – 750. Values of total Pin for Type-III and Type-I ELMy phases are plotted along with measured Pth for L-H transition Pin > 1.5Pth for H98 = 1.0, Type-I ELMS In JET Type-III ELMy H-modes H98 is lower than for Type-I ELMy H-modes over the entire density range. Y. Andrew et al., EFPW, December 2008. Pin>1.5Pth is required for H-mode plasma H98=1 access on JET ITPA meeting, April (2009)

Type I ELMs must be drastically reduced in ITER A. Loarte et al., Type-I ELMs are a factor in the order of 10 too large in ITER baseline scenario Development of mechanisms allowing controlling ELM size is a prime target for ITER: Vertical kicks Pellet pace-making Resonant Magnetic Perturbations ITPA meeting, April (2009)

Alternatives to the Type-I ELMy H-mode? Many regimes observed in different machines Feasibility at ITER parameters and without loosing confinement? Alternatives to Type-I ELMy H-mode: No clear solution but some promising paths (e.g. QH) that need to be explored further E. Nardon, EFPW, December 2008 ITPA meeting, April (2009)

Interplay between L-H power threshold and ELMs control techniques: an open issue for ITER (Pin≈ 1.5 x Pth) ITPA meeting, April (2009)

ELM mitigation with magnetic perturbations Edge stochastic magnetic field edge pressure gradient kept below threshold Ex vessel coils (error field correction coils) in JET In-vessel coils in DIII-D and ITER Advantage: large toroidal mode number spectrum of the perturbation . Advantage: External Coils, more relevant for reactor application. Limitation: Internal coils, subject to neutron radiation => reactor relevance? Limitation: low toroidal mode number spectrum of the perturbation ITPA meeting, April (2009)

ELM supression in MAST MAST has new ELM control coils (in-vessel, n=3) Vacuum modelling predicts σChirikov larger than for DIII-D I-coils No ELM suppression so far but experiments are at the beginning. On MAST there is already observation of the delay of the L-H transition time if the n=1 coils are applied before the L-H transition. E. Nardon, European Fusion Physics Workshop, Cork, December 2008 ITPA meeting, April (2009)

n=1 experiments / MAST Pin ≈ 2.5 x Pth Increasing error field B L-H transition effected by size of Error field –need to ramp field only after the transition A.Kirk et al., (MAST) ITPA meeting, April (2009)

n=3 experiments / MAST L-H transition not effected by size of Elm coil –natural jitter in start of L-H makes conclusions difficult A.Kirk et al., (MAST) ITPA meeting, April (2009)

ELM mitigation with external magnetic perturbation field #69564 Ip = 1.5 MA; Bt = 1.78 T; q95 ~ 4.0; dU ~ 0.45 Coil current kAt Density Temperature keV Confinement normalised to H-mode scaling D emission ELM frequency increased form 30Hz to 120Hz and ELM energy loss reduced from 7% to below noise level (~2%). Reduction in ELM peak heat fluxes and carbon erosion Electron density decreases (pump out) Electron and ion temperatures increase (core and edge) Reduction in the thermal energy confinement but no change compared to H-mode scaling Wide range in q95 (4.8 – 3.0) with n = 1, 2 N up to ~2.9 (no locked mode excited by n=1 field) Low collisionality: n*e~ 0.09 Y. Liang PRL, 98, 265004 (2007) ITPA meeting, April (2009)

Conclusions and actions On MAST there is already observation of the delay of the L-H transition time if the n=1 coils are applied before the L-H transition (Pin = 2.5 x Pth) but not with n = 3.. Experiments are planned in JET (2009 experimental campaign). Actions: L-H power threshold with ergodic divertor / resonant magnetic perturbations (RMP) ( e.g. EU: JET / MAST / TEXTOR / AUG-2010,..): Influence of RMP (different n) on L-H power threshold (Pin ≈ 1.5 x Pth) Power minimum at a certain density and Bt dependence (role of RMP) L-H power threshold physics: Tokamak vs Stellarators e.g. Why is it so easy (in terms of heating power) to trigger the L-H transition in low shear / low q stellarators?; Role of magnetic shear / q ? Impact of q on zonal flows / GAMs? ITPA meeting, April (2009)