H-mode characteristics close to L-H threshold power ITPA T&C and Pedestal meeting, October 09, Princeton Yves Martin 1, M.Greenwald, A.Hubbard, J.Hughes, A.Loarte, A.Polevoi, F.Ryter, R.Sartori, et al. 1 Centre de Recherches en Physique des Plasmas Association Euratom - Confédération Suisse Ecole Polytechnique Fédérale de Lausanne (EPFL) CH Lausanne, Switzerland

Outline Introduction P L /P Thresh in present H-mode experiments ELM type Weak points Discussion Yves Martin, ITPA Meetings, October

Introduction: H-mode accessibility in ITER (I) ITER baseline scenario: Stationary H-mode, Q~10 Confinement factor H~1 B t =5.3T and n e =1x10 20 m -3 ITER exploitation phases: H or He D D-T Baseline scenario to be tested in non or low activating phases H-mode access in all phases ITER available power: ~45MW -> ~70MW (~110MW later) Yves Martin, ITPA Meetings, October

H-mode accessibility in ITER (II) L-H transition threshold power: With empirical scaling based on magnetic field, plasma density and size, L-H threshold power is estimated:  ~85MW in nominal magnetic field and density, D plasma  ~50MW at ½ density Large uncertainty in threshold power H-mode with Good Confinement - Experience in present tokamak Good (H~1), ‘steady state’, confinement: Type I ELMs Type I ELMs obtained at powers exceeding ‘significantly’ the threshold power Question: Can all these conditions be realised simultaneously in ITER? B t =5.3T and n e =1x10 20 m -3 Confinement factor H~1 Additional power ≤70MW (110MW) … in H/He, D, D-T Yves Martin, ITPA Meetings, October

ITER Scenarios Phase I – H or He Threshold power increased by ~100% or 30-50%, resp.  ~170MW, MW in nominal plasmas  ~45MW at ½ magnetic field and ½ density (He) Phase II – D Threshold power with large uncertainty  ~85MW (45-160MW)  Several parameters known to play a role (X-point height, plasma shape, dI p /dt, RMP, …, rotation / torque, …) Phase II – D-T Threshold power is lower: ~70MW (isotope mass effect) Fusion power (  ) can be added In all phases, L-H transition could be obtained but little power available to access good confinement Yves Martin, ITPA Meetings, October

Question Background Scaling based on pre-transition data, in L-mode Extrapolated to ITER, predictions give power to enter the H- mode Expected H-mode regime: type III ELMs More power required to access H~1 regime, type I ELMs Question What is the power ratio in the most common H-mode regime in your device, what are the plasma parameters, heating scheme and H-mode characteristics? In case of P L /P T ~1, what are the characteristics of these plasmas, and the differences with point 1? In case of P L /P T ~1 and H~1, what are the characteristics of these plasmas, and the differences with point 1&2? Yves Martin, ITPA Meetings, October

Good H-mode access - general + Large ELMs. Small ELMs Yves Martin, ITPA Meetings, October From confinement DB: TS used in confinement studies Measure of  E and normalise by its value estimated with scaling Measure PL and normalise by its value estimated with the scaling, P thresh, taken at the same time A lot of power is available in present day devices!

JET Confinement experiments done at P L /P T =1-3 Type III ELMs at low powers above P T ELM free, then type I ELMs phases obtained when power increased Mixed phases (III->I) also observed Yves Martin, ITPA Meetings, October Proportionality between threshold powers for L-H transitions and ELM type transitions (up to factor 2) ELM type transition occurs at reduced power when triangularity is increased (30% reduction) Increase of plasma density leads to transition from type I to type III R.Sartori, PPCF, 2004

ASDEX Upgrade Confinement experiments done at P L /P T =1-4 F.Ryter’s paper [JPCS 2008]: P L /P T < 1.6 Ion grad B drift towards X-pt Yves Martin, ITPA Meetings, October

ASDEX Upgrade P L < 5MW because of P L /P T < 1.6 P L /P T > 0.6 because of limit for H-L transition ‘Empty’ zones because of operational constraints Yves Martin, ITPA Meetings, October

ASDEX Upgrade Variation of H-factor as a function of P L /P T Type III ELMs have lower confinement No degradation of H-factor with decreasing P L /P T TS with P L /P T 1 have n e ~8x10 19 m -3,  N ~1.5 Yves Martin, ITPA Meetings, October

DIII-D and Alcator C-Mod Confinement studies are performed at P L /P T ~ In Alcator C-Mod, confinement studies are done for plasmas close the threshold power. This is due to the natural increase in density which follow the L-H transition Dedicated experiments are currently performed Yves Martin, ITPA Meetings, October

ELM types Traditional ELM types: Type III ELMs are found at powers close to P T, but confinement low, H~1 cannot be reached Type I ELMs have better confinement, H~1 is regularly obtained, but input power must be significantly increased above P T. Intermediate ELM free phase has good confinement but is not stationary Other H-mode regimes (grassy ELMs, type II ELMs, EDA, QH, no ELM with RMP, …) ? Reduced operational domain => Compatibility with ITER ?  Grassy: low e *, high , high q 95 and high   Type II: high density, high q 95, high  and high   EDA: high q 95, high  and high   QH: large gaps,  RMPs: more in the type I domain Literature give little detail on P L /P T … Yves Martin, ITPA Meetings, October

Weak points Little experience of tokamak operation close to threshold power (except Alcator C-Mod) Recipes to obtain good confinement at low power not clearly identified Characteristics of such plasmas Influence of the gas puffing Recipes for gas puffing not always clearly described in the literature Impact of the gas puffing on the confinement Behaviour at high density back transition type I -> type III; H-L transition (hysteresis) Operational domain at high density  Access to type I ELMs? Yves Martin, ITPA Meetings, October

Discussion Comments from devices Pedestal: link between power, confinement, plasma parameters Increase in power induces increase in total stored energy Pedestal stored energy is proportional to the total stored energy => Pedestal top pressure increases with the power Requirements for RMP coil system to access better confinement Polevoi’s comments: Quantify P tI /P T density dependence Quantify n e,crit for type I -> type III transitions Change in scenario:  Enter H-mode at low density (0.5x10 19 m -3 )  Stay in type III ELMs  Increase density  Transition to type I ELMs Yves Martin, ITPA Meetings, October

Discussion Future plans Explore P L /P T ~1 operational domain  Existing discharges  New dedicated experiments Determine requirements (plasma, heating scheme, scenario, …) to obtain H~1 Test ITER scenarios (TC-2 JEX; link with hysteresis) Yves Martin, ITPA Meetings, October