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  1. H-mode characteristics close to L-H threshold powerITPA T&C and Pedestal meeting, October 09, PrincetonYves Martin1, M.Greenwald, A.Hubbard, J.Hughes, A.Loarte, A.Polevoi, F.Ryter, R.Sartori, et al.1Centre de Recherches en Physique des PlasmasAssociation Euratom - Confédération SuisseEcole Polytechnique Fédérale de Lausanne (EPFL)CH - 1015 Lausanne, Switzerland

  2. Outline • Introduction • PL/PThresh in present H-mode experiments • ELM type • Weak points • Discussion

  3. Introduction: H-mode accessibility in ITER (I) • ITER baseline scenario: • Stationary H-mode, Q~10 • Confinement factor H~1 • Bt=5.3T and ne=1x1020m-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)

  4. 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? • Bt=5.3T and ne=1x1020m-3 • Confinement factor H~1 • Additional power ≤70MW (110MW) • … in H/He, D, D-T

  5. ITER Scenarios • Phase I – H or He • Threshold power increased by ~100% or 30-50%, resp. • ~170MW, 110-130MW 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, dIp/dt, RMP, …, rotation / torque, …) • Phase II – D-T • Threshold power is lower: ~70MW (isotope mass effect) • Fusion power (a) can be added • In all phases, L-H transition could be obtained but little power available to access good confinement

  6. 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 PL/PT~1, what are the characteristics of these plasmas, and the differences with point 1? • In case of PL/PT~1 and H~1, what are the characteristics of these plasmas, and the differences with point 1&2?

  7. Good H-mode access - general • + Large ELMs • . Small ELMs From confinement DB: TS used in confinement studies Measure of tEand normalise by its value estimated with scaling Measure PL and normalise by its value estimated with the scaling, Pthresh, taken at the same time A lot of power is available in present day devices!

  8. JET • Confinement experiments done at PL/PT=1-3 • Type III ELMs at low powers above PT • ELM free, then type I ELMs phases obtained when power increased • Mixed phases (III->I) also observed 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

  9. ASDEX Upgrade • Confinement experiments done at PL/PT=1-4 • F.Ryter’s paper [JPCS 2008]: • PL/PT < 1.6 • Ion grad B drift towards X-pt

  10. ASDEX Upgrade • PL < 5MW because of PL/PT < 1.6 • PL/PT > 0.6 because of limit for H-L transition • ‘Empty’ zones because of operational constraints

  11. ASDEX Upgrade • Variation of H-factor as a function of PL/PT • Type III ELMs have lower confinement • No degradation of H-factor with decreasing PL/PT • TS with PL/PT<1.2 & H>1 have ne~8x1019m-3, bN~1.5

  12. DIII-D and Alcator C-Mod • Confinement studies are performed at • PL/PT ~ 1.5-2.5 • 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

  13. ELM types • Traditional ELM types: • Type III ELMs are found at powers close to PT, 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 PT. 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 ne*, high b , high q95 and high d • Type II: high density, high q95, high k and high d • EDA: high q95, high k and high d • QH: large gaps, • RMPs: more in the type I domain • Literature give little detail on PL/PT …

  14. 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?

  15. 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 PtI/PT density dependence • Quantify ne,crit for type I -> type III transitions • Change in scenario: • Enter H-mode at low density (0.5x1019m-3) • Stay in type III ELMs • Increase density • Transition to type I ELMs

  16. Discussion • Future plans • Explore PL/PT~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)

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