1. The effect of ELM pacing by vertical kicks on the access to stationary H-mode with H98~1 on JET
E. de la Luna
24th IAEA Fusion Energy Conference, 8-13 October, San Diego, USA

2. Contributors
E. de la Luna1,2, R. Sartori3, G. Saibene3, P. Lomas4, F. Koechl5, F. Maviglia6, S. Saarelma4, V. Parail4, R. Albanese6, R. Ambrosino6, M. N. A. Beurskens4, E. Delabie7, D. Dodt8, J. Flanagan4, C. Giroud4, A. Loarte9, C. Maggi8, M. Mattei6, F. Rimini and JET-EFDA contributors*
1) EFDA-CSU, Culham Science Centre, Abingdon, OX14 3DB, UK
7) Dutch Institute for Fundamental Energy Research, Association EURATOM-FOM,, Nieuwegein, The Netherlands
2) Laboratorio Nacional de Fusion, Asociacion EURATOM-CIEMAT, Madrid, Spain
8) MPI für Plasmaphysik, EURATOM Association, D Garching, Germany
3) Fusion for Energy, Joint Undertaking, Barcelona, Spain
9)TER Organization, Route de Vinon sur Verdon, St. Paul Lez Durance, France
4) EURATOM/CCFE Fusion Association, Culham Science Centre, OX14 3DB, U.K.
5)Association EURATOM-ÖAW/ATI, Atominstitut, TU Wien, 1020 Vienna, Austria
* See the Appendix of F. Romanelli et al, 24th IAEA Fusion Energy Conference, San Diego, 2012, paper OV/1-3
6) Assoziacione EURATOM-ENEA sulla Fusione, Consorzio CREATE, Napoli, Italy

3. Motivation JET experience on operation at low power margin over Pth:
ITER will operate at powers close to the H-mode threshold power scaling  Pnet ~1.3xPth(Martin08) to access H-mode with H~1
JET experience on operation at low power margin over Pth:
H-mode plasma has a transient behaviour (cyclic transition to Type III ELMs or L-mode)
 H-mode with H~1 in stationary conditions is achieved above a critical input power (Ploss ~aPth, with a~1.3-2, CFC-wall) [Sips- ITR/P1-11]
H~1
PIN increases

4. Outline of the talk
Motivation: can ELM pacing methods help to sustain H-mode with H98~1 at heating powers marginally above Pth?
How do kicks work in JET?
Modelling of kick cycle. Mechanism responsible for the ELM triggering
Use of vertical kicks (ELM pacing) at powers marginally above Pth in JET with CFC wall
Plasma performance at powers marginally above Pth in JET with Be/W wall
Summary + outlook for ITER

5. Vertical kicks in JET Minimum kick size is required to trigger ELMs
Before kick
After kick DV
Kick:
Plasma moves downwards, plasma volume decreases
Minimum kick size is required to trigger ELMs
In JET (and AUG) ELMs are triggered when the plasma is moving down (towards lower X-point)

6. Kick cycle modelling
Pre-kick equilibrium solved with CREATE-NL (includes eddy current, magnetics only)
Transport code (JINTRAC) where the plasma profiles are evolved (0.5 ms steps)
Plasma boundary
dyext/dt
Work in progress to perform closed-coupled simulations

7. Kick: ELM triggering mechanism
Simulation shows that current driven instabilities are enhanced by increase in Jz (and lower shear).
j increases at y~1 (dytot/dt>0)
j reduces at the edge when plasma moves down
Current perturbation propagating inwards

8. Use of vertical kicks (ELM pacing) at powers marginally above Pth in JET with CFC wall

9. ELM pacing (kicks) at Ploss~Pth
Ploss ~ 1.2 x Pth
dav ~0.4, q95=3
Vertical kicks can reliably trigger ELMs at input powers marginally above Pth
Prad,bulk and W/t are reduced with the appearance of the triggered ELMs (10-20 Hz)
Ploss = Pabs- W/t
Pnet = Ploss-Prad,bulk
The increase in fELM with kicks is sufficient to control particle and impurity density  stationary H-mode with H98~1

10. ELM pacing (kicks) at Ploss~Pth
Ploss ~ 1.2 x Pth
dav ~0.4, q95=3
Pre-ELM profiles
The increase in fELM with kicks is sufficient to control particle and impurity density  stationary H-mode with H98~1

11. Pnet vs. Ploss in the access to H~1
Power scan (no gas fuelling) q95=3, dav=0.4
Minimum Ploss/Pth to access stationary H-mode conditions with H~1 is reduced with kicks
H~1 (Type I ELMs, stationary)
Ploss/Pth≥1.8
Low ne
Type III ELMs
Transient
H-mode
LH

12. Pnet vs. Ploss in the access to H~1
Power scan (no gas fuelling) q95=3, dav=0.4
Minimum Ploss/Pth to access stationary H-mode conditions with H~1 is reduced with kicks
H~1 (Type I ELMs,stationary)
Ploss/Pth≥1.8
Low ne
Type III ELMs
H~1 (kicks )
Ploss/Pth≥1.2
LH

13. Edge current and MHD stability
Stability analysis (MISHKA-1):
With kicks: From marginally stable (no ELMs) to low n peeling unstable when additional edge current, as predicted by simulations, is included
amax
With additional jf (simulations)
amax

14. Edge current and vertical kicks
Ploss/Pth =1.1
Minimum kick size required for successful ELM triggering at Ploss~Pth
Increasing velocity of plasma movement improves the probability of triggering an ELM (larger induced current)
Kick size scan
Increase in gas puffing  reduction in edge current (larger collisionality)
For fixed kick size, the ELM triggering probability increases with increasing edge Te (lower resistivity, larger bootstrap current). Encouraging in view of ITER (high Te,ped)
Successful ELM triggering

15. Plasma performance at powers marginally above Pth in JET with Be/W wall

16. fELM and core W accumulation in ILW
PIN/Pth ~1.4
ELM frequency control essential for JET operation with Be/W wall to prevent core W accumulation (same as AUG) [Joffrin- EX/1-1]
Gas fuelling typically used to keep ELM frequency high and reach stationary conditions (H98<0.9)
At low gas fuelling (and low power)
cyclic transitions to L-mode at constant PIN are typically observed (similar to CFC-wall).

17. ELM frequency control in JET-ILW
Kicks exp. in JET with Be/W have just started
Kicks: ELM control without affecting any other plasma parameter (no added impurities or density)
Kicks (increase in fELM) can help in reducing W core accumulation in gas fuelled H-mode plasmas in JET with Be/W wall
Te,ped lower with the Be/W wall  kick size larger than in previous exp.
Be/W (gas fuelled) : Te,ped < 0.6 keV
CFC (unfuelled): Te,ped ~ 1 keV
[Beurskens-EX-P7-20]

18. Conclusions
Vertical kicks can reduce the power requirements for achieving H-modes with H~1 (with type I ELMs) in JET (in CFC-wall so far)
Inter-ELM dynamics and radiation to be included in the evaluation of the minimum power required to access stationary H-modes with H98~1(Pnet vs Ploss)
Large variation in PH~1 in JET remains to be explained (Ploss~(1.3-2)x Pth)
Experimental and modelling results suggest that the mechanism by which vertical kicks trigger ELMs is related with the increase in edge current and its effect on edge stability.
Encouraging in view of ITER: The induced current in ITER (lower edge resistivity) would be relatively large compared to that at JET for the same perturbation (vz)
Use of vertical kicks may provide a potential route towards minimizing the W impurity build-up during the early phase of the H- mode in ITER (no penalty on confinement).
Vertical kicks using the existing coils are an option in ITER at Ip <10 MA (current ramp up/down) [Loarte- ITR/1-2]
H~1