Plasma shape and fueling dependence on small ELM regimes in TCV and AUG B. Labit1 T. Eich2 G. Harrer3, M. Bernert2, H. De Oliveira1, M. Dunne2, L. Frassinetti4, P. Hennequin5, R. Maurizio1, A. Merle1, H. Meyer6, P. Molina1, S. Saarelma6, U. Sheikh1, J. Stober2, E. Wolfrum2, the TCV Team, the AUG team and the EUROfusion MST1 Team 1 Ecole Polytechnique Fédérale de Lausanne (EPFL), Swiss Plasma Center (SPC), Switzerland 2 IPP Garching, Germany, 3UT Wien, Austria, 4KTH Royal Institute of Technology, Stockholm SE, 5LPP, Ecole Polytechnique, France 6CCFE, Abingdon UK It is my pleasure to report on plasma shape and fueling dependence on small ELM regimes in TCV and AUG. The results presented in this paper are coming from joint experiments on both devices in the framework of the EUROfusion MST1 work package..

ITER Baseline Scenario and Type-I ELMs Recent progress at AUG towards low collisionality at q95=3.6 Progress on parameter space for Type-I ELM suppression with RMP [T. Putterich, EX/P8-4] [W. Suttrop, EX/7-3] [E. Viezzer, NF, 2018] As a starting point, I would like to remind you the parameters of the ITER Baseline Scenario. ITER will operate at poloidal beta of 0.9, Greenwald fraction of 0.8, a low collisionality 0.1 and in terms of plasma shape: a triangularity of 0.4 with q95 equals 3. If you want to know more about the recent results on the ITER Base Line Scenario studies at AUG, in particular towards low collisionality for the alternative scenario at q95 equals 3.6 please give a look at Thomas Putterich’s poster on Friday. We also know that Type-I ELMs are to be avoided in ITER. And on Friday, Wolfgang Suttrop will give a talk on the recent progress done at AUG on Type-I ELM suppression wiith resonant magneitc pertubations

Type-II and grassy ELM parameters far from ITER BLS ones Recent progress at AUG towards low collisionality at q95=3.6 Progress on parameter space for Type-I ELM suppression with RMP Small ELM regimes become attractive ... Type-II ELMs Grassy ELMs ... Although parameter space for these ELM regimes quite far from ITER BLS Can we extend these parameters towards ITER BLS values? [T. Putterich, EX/P8-4] [W. Suttrop, EX/7-3] [E. Viezzer, NF, 2018] Since you want to get rid of Type-I ELMs, small ELM regimes, such as Type-II and grassy ELMs, are attractive, as soon as they can give you the confinement you want.

Accessibility to small ELM regimes in AUG and TCV Small ELM regimes and plasma fueling Small ELM regimes and plasma shape Physical interpretation [E. Viezzer, NF, 2018]

Take-home message Experimental observations: Small ELM regimes achieved if two simultaneous conditions are fulfilled: 1) a large plasma density at the separatrix ne,sep 2) close to Double-Null configuration Interpretation: 1) Type-I ELMs stabilized by reduced Dped when ne,sep is large 2) Ballooning modes unstable since magnetic shear reduced close to DN shape

Condition #1: A large ne,sep

Close to DN, small ELMs with strong gas fueling… AUG Dsep = 7 mm ~ 5 lq [G. F. Harrer et al, NF, 2018] [H. Meyer, FEC 2016] small Dsep: distance of the secondary X-pt to the separatrix at the midplane

… but Type-I ELMs restored with pellet fueling AUG Dsep = 7 mm ~ 5 lq [G. F. Harrer et al, NF, 2018] [H. Meyer, FEC 2016] small Type-I Dsep: distance of the secondary X-pt to the separatrix at the midplane

A large ne,sep, flattening the pressure profile, is a necessary condition for small ELMs AUG ne,sep= 4x1019 m-3 Gas fueling Lp = 12 mm ne,sep= 2x1019 m-3 Pellet fueling Lp = 9 mm small Type-I [G. F. Harrer et al, NF, 2018] [H. Meyer, FEC 2016]

Type-I ELMs when far from DN with little fueling TCV Dsep = 20 mm ~ 7 lq Type-I Type-I ELMs scenario: q95=4.5, d=0.4, k=1.5, 1 MW NBH N2 seeding decreases Pe,ped conversely to AUG and JET-ILW results, nevertheless consistent with P-B model [L. Frassinetti, EX/P8-22]

Mixed ELMs regime with strong gas fueling TCV Dsep = 20 mm ~ 7 lq Type-I small Type-I ELM frequency decreases and small ELMs appear in between

ne,sep increases by a factor 2, pe,ped unchanged, pedestal shrinks TCV ne,sep= 0.8x1019 m-3 Gas fueling Dsep = 20 mm ~ 7 lq small ne,sep= 1.6x1019 m-3 Gas fueling Dsep = 20 mm ~ 7 lq Type-I

Condition #2: Plasma shape close to DN configuration

Small ELMs with the usual recipe AUG Dsep = 7 mm ~ 5 lq small [G. F. Harrer et al, NF, 2018]

Relaxing closeness to DN partly restores Type-I ELMs AUG Dsep = 14 mm ~ 10 lq small Type-I [G. F. Harrer et al, NF, 2018]

Two ELM regimes with same pedestal profiles: large ne,sep not a sufficient condition AUG ne,sep= 4x1019 m-3 Gas fueling Dsep = 7 mm ~5 lq small mixed ne,sep= 4x1019 m-3 Gas fueling Dsep = 14 mm ~ 10 lq small Type-I [G. F. Harrer et al, NF, 2018]

Type-I ELM with the usual recipe TCV Dsep = 20 mm ~ 7 lq

Type-I ELMs fully stabilized when top triangularity is increased TCV Dsep = 6 mm ~ 2 lq Caveat: top triangularity and closeness to DN strongly coupled

Pedestal profiles almost unchanged TCV ne,sep= 0.9x1019 m-3 Gas fueling Dsep = 20 mm ~ 7 lq ne,sep= 0.8x1019 m-3 Gas fueling Dsep = 6 mm ~ 2 lq small Type-I

Physical interpretation

Type-I pedestal close to P-B stability boundary MISHKA EPED-CH

Strong plasma shaping extends the stability region MISHKA EPED-CH

Small ELM regimes are also close to the P-B limit MISHKA EPED-CH

Magnetic equilibrium including pedestal bootstrap current CLISTE CHEASE

Close to DN, magnetic shear is reduced at the separatrix CLISTE CHEASE

Physical interpretation: Type-I stabilized Type-I (global modes) and small ELMs (local ballooning modes) are able to co-exist With strong gas fueling, pedestal width D is reduced because ne,sep is large and/or pedestal shifted Type-I ELMs (global modes) are more stable since the pedestal width is reduced Type-I Small

Physical interpretation: ballooning modes more unstable Ballooning modes are driven by pressure gradient and stabilized by magnetic shear Close to DN, this stabilizing effect is reduced because the magnetic shear is reduced Consistent with turbulent transport close to the separatrix increasing with small ELMs magnetic shear Type-I Small [A. Kirk et al, JPCS, 2008] [P. Hennequin et al, EPS, 2017]

Conclusions and outlook To access small ELMs regimes at AUG and TCV, a large density at the separatrix (ne,sep/ne,ped0.4) is needed together with a large averaged triangularity (d>0.4) and/or close to DN Simulations from non linear MHD codes or turbulence codes are required together with detailed turbulence measurements across the separatrix

Conclusions and outlook Implications for ITER If closeness to DN (Dsep <5lq) is a stringent condition, an extrapolation to ITER might be difficult since Dsep,ITER~4 cm >> lq, ITER

Conclusions and outlook Implications for ITER If closeness to DN (Dsep <5lq) is a stringent condition, an extrapolation to ITER might be difficult since Dsep,ITER~4 cm >> lq, ITER Matching simultaneously the pedestal density (fGW,ped>0.6) and the low collisionality n*ped might be difficult. Both parameters are strongly coupled through their dependence on the plasma current

Conclusions and outlook Implications for ITER If closeness to DN (Dsep <5lq) is a stringent condition, an extrapolation to ITER might be difficult since Dsep,ITER~4 cm >> lq, ITER Matching simultaneously the pedestal density (fGW,ped>0.6) and the low collisionality n*ped might be difficult. Both parameters are strongly coupled through their dependence on the plasma current ITER will operate at Psep / PL-H |ITER <1.4. The accessibility to small ELM regimes needs to be assessed under this condition (Psep / PL-H |AUG ~ 2 & Psep / PL-H |TCV ~ 1.8)

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