1. 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..

2. 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

3. 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.

4. 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]

5. 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

6. Condition #1: A large ne,sep

7. 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

8. … 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

9. 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]

10. 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]

11. 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

12. 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

13. Condition #2: Plasma shape close to DN configuration

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

15. 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]

16. 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]

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

18. 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

19. 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

20. Physical interpretation

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

22. Strong plasma shaping extends the stability region
MISHKA
EPED-CH

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

24. Magnetic equilibrium including pedestal bootstrap current
CLISTE
CHEASE

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

26. 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

27. 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]

28. 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

29. 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

30. 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

31. 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)

32. Backup slides