1. Study of the pedestal dynamics and stability during the ELM cycle A. Burckhart Advisor: Dr. E. Wolfrum Academic advisor: Prof. Dr. H. Zohm MPI für Plasmaphysik, EURATOM Association A. Burckhart, PhD network, September 30 th, 2010

2. 2/30 Motivation: why study ELMs? Overview on peeling ballooning theory and ELMs Evolution of T e, n e and p e during the ELM cycle Stability calculations Conclusions Overview

3. A. Burckhart, PhD network, September 30 th, 20103/30 Motivation: why study ELMs? Overview on peeling ballooning theory and ELMs Evolution of T e, n e and p e during the ELM cycle Stability calculations Conclusions Overview

4. A. Burckhart, PhD network, September 30 th, 20104/30 Motivation Highest performance is found in H-mode plasmas featuring type-I ELMs  Forseen operation scenario for ITER Edge localized modes (ELMs) are instabilities of the plasma edge, leading to a periodic deterioration of the pedestal profiles They cause massive power loads on the divertor, up to 10% of the confined energy in less than 1ms They cause high energy and particle losses They “clean” the plasma by expelling impurities They are not yet fully understood Investigating the dynamic behavior of pedestal profiles can lead to a better understanding of ELMs, and maybe ultimately to their control Stability calculations are usually performed for one time point directly before the ELM crash, what about the temporal evolution of the stability?

5. A. Burckhart, PhD network, September 30 th, 20105/30 Motivation: why study ELMs? Overview on peeling ballooning theory and ELMs Evolution of T e, n e and p e during the ELM cycle Stability calculations Conclusions Overview

6. A. Burckhart, PhD network, September 30 th, 20106/30 Plasma stability We consider a plama equilibrium with the total potential energy W, and calculate the effect of an arbitrary small displacement ξ If δW > 0, the system is stable, with δW < 0 it is unstable ξ ξ stable (oszillation) unstable δW < 0δW > 0

7. A. Burckhart, PhD network, September 30 th, 20107/30 Instability: The change of potential energy is negative Stability code: Find  that minimises  W Field line bending Compression of magnetic field lines Plasma compression Pressure driven modes Current driven modes Stabilising: Destabilising: (vacuum field line bending, always stabilising) Edge stability [Saarelma, JET 13.09.2010]

8. A. Burckhart, PhD network, September 30 th, 20108/30 Current driven instabilities that are localized radially at the plasma edge and poloidally near the X- point. Usually low-n n=7 Mode localized near the x-points. [Saarelma, JET 13.09.2010] Peeling modes

9. A. Burckhart, PhD network, September 30 th, 20109/30 Driven by the pressure gradient. localized on the low field side. Radially more extended than peeling modes. Usually high-n. n=20 Mode localized at the bad curvature region [Saarelma, JET 13.09.2010] Edge ballooning modes

10. A. Burckhart, PhD network, September 30 th, 201010/30 Driven by both the current and the pressure gradient. Intermediate-n Mode structure has features from both modes. Peeling component Ballooning component [Saarelma, JET 13.09.2010] ELMs are thought to be combined peeling-ballooning modes

11. A. Burckhart, PhD network, September 30 th, 201011/30 ELMs expel particles from the plasma [T. Lunt] Glow: mostly H-alpha, cold and dense Filaments expelled, travelling along field lines Highest intensity lasts less than 1ms

12. A. Burckhart, PhD network, September 30 th, 201012/30 T e and n e pedestals relax due to ELMs Ipolsola: currents in the divertor, used as ELM indicator T e relaxes quickly during ELM crash, then slowly recovers n e also relaxes, but evolves around „pivot point“: n e inside drops, n e in the SOL increases

13. A. Burckhart, PhD network, September 30 th, 201013/30 Motivation: why study ELMs? Overview on peeling ballooning theory and ELMs Evolution of T e, n e and p e during the ELM cycle Stability calculations Conclusions Overview

14. A. Burckhart, PhD network, September 30 th, 201014/30 T e recovery shows several distinct phases during ELM  T e is small initial recovery of  T e  T e recovery stalls  T e recovery continues  T e exhibits large fluctuations Max(  T e ) mapped relative to t=0

15. A. Burckhart, PhD network, September 30 th, 201015/30 Only one short recovery phase ~ 2-3ms n e recovers differently than T e Overshoot at the end of the recovery Last phase is stationary, but large scale fluctuations T e pedestal takes around 7ms to fully recover, n e pedestal only 4ms

16. A. Burckhart, PhD network, September 30 th, 201016/30 Interplay between n e and T e buildup In the phase where  T e recovery stalls,  n e rapidly recovers In the following phase where  T e recovers rapidly,  n e stays constant This is observed in all analyzed discharges, at all gas fueling levels Reminiscent of critical value of  e =L n /L T above which  T e is clamped: possibly ETGs are responsible for pause in the recovery of the T e profile

17. A. Burckhart, PhD network, September 30 th, 201017/30 New laser triggering method will allow for a higher density of TS data points Next steps: Improve characterization of edge T e profile recovery using TS data, which does not have the shine through problem (no accurate ECE data outside ρ pol =0.995 )  long discharges necessary Thomson scattering diagnostic: Six 20Hz Nd-Yag lasers  One profile every 8.3ms Or: burst mode, fire all lasers in a given time interval Next campaign: ELM detected by XVR Delay A + n*x optical signal triggers laser Result: All data points from the TS system lie in time interval of the ELM cycle that we want to analyze TS data point

18. A. Burckhart, PhD network, September 30 th, 201018/30 Next steps concerning pedestal dynamics Test reproducibility of interplay between T e and n e in the recovery of the pedestal profiles on other devices, starting with JET Very good Thomson scattering profiles available, but only 20Hz  long discharges and coherent data selection necessary Fast ECE data available, but low resolution in the pedestal region, and shine through effect often more pronounced than on AUG Reflectometry measurements, Li-beam and DCN also available but not suited for calculating gradients and/or temporal resolution not sufficient No method that combines the data from different diagnostics to one joint profile (c.f. integrated data analysis at AUG) Start with analyzing TS and ECE data of „old“ discharges Then analyze dependencies of the pedestal dynamics on collisionality and on fueling level

19. A. Burckhart, PhD network, September 30 th, 201019/30 AUG Length of the last, quasi stationary phase, is dependent on fueling level Length of initial recovery phase barely changes with fueling Next Step: trying to find a solid dependence of the different phases in the ELM cycle on the fueling level (AUG and JET) „Steady state“ phase dependent on fueling level JET Fueling [E. Giovannozzi, EPS 2009]

20. A. Burckhart, PhD network, September 30 th, 201020/30 Reaching the  p e limit does not necessarily lead to an ELM Clearly, a limit to max(  p e ) exists, but reaching it does not automatically trigger an ELM Some ELMs seem to be triggered by reaching max(  p e ) (consistent with JETTO simulation [J. Lönnroth, PPCF 2004]) Others can sit at the pressure limit for several ms before ELM happens (seen before on AUG [T. Kass, NF 1998] and DIII-D, [R. Groebner, NF 2009]) ‚fast‘ ELM frequency‚slow‘ ELM frequency

21. A. Burckhart, PhD network, September 30 th, 201021/30 Current diffusion cannot explain delayed ELM peeling boundary ballooning boundary [J. Connor, Phys.Plasmas 1998] ELM thought to be driven by combination of pressure and current gradient Pressure gradient limited by ballooning mode Edge current gradient limited by peeling (=very edge localized kink) mode Bootstrap current roughly proportional to  p e, but delayed by about 1ms because of current diffusion  cannot explain 6-7ms observed between reaching final  p e and occurrence of the next ELM

22. A. Burckhart, PhD network, September 30 th, 201022/30 Pressure further inside continues to increase Pressure further inside might influence peeling-ballooning stability Next step: stability calculations Pressure further inside might have an effect on ballooning stability Pedestal slope and pedestal top stay constant ‚fast‘ ELM frequency ‚slow‘ ELM frequency

23. A. Burckhart, PhD network, September 30 th, 201023/30 Motivation: why study ELMs? Overview on peeling ballooning theory and ELMs Evolution of T e, n e and p e during the ELM cycle Stability calculations Conclusions Overview

24. A. Burckhart, PhD network, September 30 th, 201024/30 ELM-coherent averaged profiles and magnetic data CLISTE equilibrium (with kinetic profiles) ILSA stability calculations Stability calculations performed in several steps HELENA High resolution equilibrium

25. A. Burckhart, PhD network, September 30 th, 201025/30 Experimental data is ELM synchronized n e (10 19 m -3 ) T e (eV) [M. Dunne] ELM-coherent averaged profiles and magnetic data CLISTE equilibrium (with kinetic profiles) ILSA stability calculations HELENA High resolution equilibrium

26. A. Burckhart, PhD network, September 30 th, 201026/30 Grad-Shafranov equation is solved using CLISTE Inputs for CLISTE – Pressure data (10%) – Sauter Formula (1%) – MAC currents (<1%) – Magnetics (~90%) – cBoot (1%) – [SXR] – [CES/CEZ] Current density bootstrap Pressure ELM-coherent averaged profiles and magnetic data CLISTE equilibrium (with kinetic profiles) ILSA stability calculations HELENA High resolution equilibrium Output

27. A. Burckhart, PhD network, September 30 th, 201027/30 Different  p - j combinations are tested for stability [C. Maggi, C. Konz] Destabilising: edge current density pressure gradient Reminder: ELM-coherent averaged profiles and magnetic data CLISTE equilibrium (with kinetic profiles) ILSA stability calculations HELENA High resolution equilibrium Code changes pedestal profiles to vary  p and j, while keeping β and I p constant

28. A. Burckhart, PhD network, September 30 th, 201028/30 First calculations would suggest a pedestal far away from stability limit, but: CLISTE output had too low gradients Need to run CLISTE and ILSA again, carefully comparing CLISTE results with experimental data First stability calculations performed, but need to be repeated [C. Konz]

29. A. Burckhart, PhD network, September 30 th, 201029/30 Motivation: why study ELMs? Overview on peeling ballooning theory and ELMs Evolution of T e, n e and p e during the ELM cycle Stability calculations Conclusions Overview

30. A. Burckhart, PhD network, September 30 th, 201030/30 Conclusions and to-do’s Study of ELM cycle with high temporal and spatial resolution sheds new light on both transport and stability of the pedestal Interplay between recovery of T e and n e observed on all analyzed AUG discharges, still to be studied on JET Better characterization of T e recovery might be possible with TS (no ECE shine through) Maximum of  p e and j b can be reached well before ELM onset While peeling ballooning model is consistent with limits to pedestal pressure, there is still a physics ingredient missing to explain the ELM trigger - Influence from pressure further inside to be assessed - Temporal evolution of the stability to be characterized, comparison between stability of slow and fast ELM-cycles