Recent work on the control of MHD instabilities at ASDEX Upgrade

1. Recent work on the control of MHD instabilities at ASDEX Upgrade S. Günter, J. Hobirk, P. Lang, P. Merkel, A. Mück, G. Pereverzev, ASDEX Upgrade Team Max-Planck-Institut für Plasmaphysik Garching, Germany • Sawtooth control by ECCD • ELM control by plasma shaping and pellets • Current profile control by off-axis NBI? • RWM physics on ASDEX Upgrade? • NTM control, see next talk

2. Sawtooth behaviour depends on NBI sources one beam only

3. Sawtooth behaviour for different NBI sources one beam only • Off-axis heating only, leads to density peaking • j’ decreased (increased off-axis BS current) • diagmagnetic stabilization (* increased)

4. 20000 (q=1)=0.2 (q=1)=0.2 (q=1)=0.1 f [kHz] 15000 12000 10000 Sawteeth/ fishbones two q=1 surfaces 2.0 3.0 4.0 5.0 6.0 t [s] Sawtooth behaviour for different NBI sources two off-axis beams • no sawteeth, but continous (1,1) activity • two q=1 surfaces in the plasma • (off-axis NBI-CD)

5. Sawtooth tailoring by co- ECCD Experiments with slow Bt-ramp, 0.8 MW co-ECCD and 5.1 MW NBI

6. Influencing (1,1) mode activity by co-ECCD • co-ECCD at pol = 0.4 • no sawteeth, only fishbones • FB amplitude also decreases • (SXR amplitude reduced by • factor of 3)

7. Sawtooth tailoring by ctr-ECCD

8. Destabilisation of (1,1) activity by on-axis ctr-ECCD • For ctr-ECCD deposition close to plasma center (here pol = 0.1) •  reversed q-profile •  destabilization of (1,1) mode • No sawteeth or fishbones, but continous (1,1) activity

9. NTM control by sawtooth mitigation (off-axis-ECCD) • Co –ECCD: • no sawteeth as expected • Reduced fishbone amplitude • NTM triggered after ECCD (by ST) • Counter-ECCD: • NTM triggered by FB during ECCD

10. Outer divertor power density Inner divertor ELM mitigation: type II ELMs

11. Consider two discharges with different plasma shape #15863 #15865 Type I Type II

12. 12 2.5 ] -3 10 15863 m 2.0 15865 19 8 1.5 6 Electron temperature [keV] Electron density [10 1.0 4 15863 0.5 2 15865 0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 r r poloidal poloidal but with similaredge temperature and density profiles

13. n=8 peeling mode Influence of closeness to Double Null • Ballooning Stability unchanged • Low-n modes become more stable (broad mode structure) • Stability of medium-n modes unchanged, • but eigenfunctions more localised at plasma edge

14. Operational regime for type II ELMs • closeness to DN/high  • q95> 3.5 • high n/nGW

15. Why do we need high density/high q95? Hypothesis: type II ELMs only if low-n modes are stable JET, ELM precursors low n modes only for high density (Perez, Koslowski et al., IAEA 2002)

16. jBS since n  jBS since *  Influence of edge collisionality Theory: low mode number MHD activity destabilised by current gradient

17. Is type II ELM regime accessible for ITER? • Higher density increases edge collisionality •  BS current density reduced •  reduced drive for low-n modes If not n/nGW, but collisionality counts, type II ELMs would not occur in ITER  Other means for ELM control?

18. ELM mitigation: by pellets Control of ELM frequency possible (each pellet triggers an ELM)

19. Control of ELM frequency by pellets • small pellets (2 … 3x1019 D atoms, not strong fuelling) • Confinement degradation ~ f-0.16 • (less than for frequency change by, e.g., heating power, density puff) ~ f-0.6

20. Mitigation of ELM size possible • same plasma parameters • natural ELM frequency 52 Hz

21. Mitigation of ELM size possible

22. Energy loss per pellet triggered ELM as for type I ELMs at same frequency

23. Current profile control by off-axis NBI? • Redirected NBI box provides off axis deposition of 93 keV ions: • NB driven current clearly seen by reduced OH flux consumption • current profile changes seem much smaller than expected

24. Two off-axis beams, an example (#14513) Plasma current pol dia li S3+S5 S6+S7 NBI gas Raus ne,0

25. One off-axis beam (Te change compensated by ICRH) #18091 Plasma current dia li pol ICRH NBI S6 S3 gas ne,0 Raus

26. on- off-axis beams one-beam discharge Change in li for one-beam case in agreement with ASTRA code experiment ASTRA two-beam discharge Very small change in li, much smaller than predicted (ASTRA li shifted up) Strong change in li only for one-beam discharge ASTRA experiment

27. q-profile for two-beam discharge (q=1 surface) 20000 (q=1)=0.2 (q=1)=0.2 (q=1)=0.1 f [kHz] 15000 12000 10000 Sawteeth/ fishbones two q=1 surfaces 2.0 3.0 4.0 5.0 6.0 t [s] But: in the plasma centre (tor < 0.15) q-profile changes as two q=1 surfaces at tor < 0.10 and tor = 0.2 observed ASTRA predicts observable change of q-profile, but no change measured (MSE, q=1 radius)

28. Current profile modifications due to one off-axis beam Change in radius of q=1 surface is significant and agrees with ASTRA predictions

29. Current profile modifications due to one off-axis beam Current profile modifications mainly caused by off-axis beam (ASTRA)

30. Comparison to MSE measurements ASTRA predictions

31. Non-stiff electron temperature profiles for one-beam discharge one beam (without additional heating) two beams (#14513)

32. Non-stiff ion temperature profile for one beam case Ion temperature during off-axis NBI modeled by MMM95, agreement with measured pressures

33. To explain unchanged current profile one needs a particle pinch! Anomalous particle pinches are well-known in theory (density peaking) Simple picture: strong turbulence of background plasma redistributes particles while maintaining the two adiabatic invariants  and with the density follows from const.

34. Does theory predict such a particle pinch? Need: full non-linear turbulence simulation with marker particles, in progress (B. Scott) So far: quasi-linear GS2-calculations (G. Tardini, A. Peeters) G. Tardini First results: particle pinch exists, but too small To be done: realistic density profile of fast particles, parameter scan

35. low triang. Simulations for realistic wall structures (as planned for AUG) Wall structures only relevant on low field side (ballooning mode structure) high triang. Plasma separatrix + 3 cm in midplane Realistic model for AUG wall structures

36. 3D MHD code with 3d wall structures • 3d MHD code CAS3D extended for • 3d ideally conducting walls • MHD eigenfunctions fully self-consistent • Benchmark with 2d MHD code CASTOR successful

37. 0.08 0.06 0.04 0.02 0.0 A closed wall <ß> = 4.5 % 1.2 1.4 1.6 1.8 2.0 rw/rpl Simulation results for realistic wall structures Without wall: ßmarg = 1 % Efficiency of realistic wall compared to closed wall

38. Wall resistivity causes mode growth on wall time (RWMs) Further plans: - Resistive 3d walls (already started) - Feedback system (active coils to stabilise RWM)

39. Summary • Sawtooth mitigation by localized ECCD demonstrated • Seed island control allows to control NTM onset • type II-ELMs achieved by plasma shaping compatible with required plasma parameters: N, q95, H, n/nGW • open question: does collisionality count? (BS current) • ELM mitigation by pellets demonstrated • smaller pellets at higher frequency needed • off-axis NBI current for current profile control only for non-stiff ion temperature profiles? • RWM physics: 3D ideal MHD code with 3D ideally conducting wall structures, finite wall resistivity being implemented

40. Second stable regime low density Ideal ballooning limit: ne = 9 1019 m-3 ne = 1.1 1020 m-3 Experiment Influence of edge density (BS current): ballooning modes • Higher density increases edge collisionality •  BS current density reduced •  Increased magnetic shear prevents access to second • stable regime

41. Non-stiff ion temperature profiles for one-off-axis beam one beam discharge electron temperature and density constant, but diamagnetic pressure decreases  hint to non-stiff ion temperature profiles diamagnetic pressure nearly constant, pol increases for off axis beams (fast increases, mainly ||) two beam discharge

42. 20000 20000 f [kHz] 15000 10000 5000 12000 10000 2000 on-axis beams on-axis beams off-axis beams off-axis beams 2.0 3.0 4.0 5.0 6.0 1000 t [s] 2.0 3.0 4.0 5.0 t [s] Non-stiff rotation profiles? (mode frequency also dependent on diamagnetic drift) Strong reduction in (1,1) mode frequency for one-beam discharge two beams (#14513) one beam (#18091)

43. Good match of electron temperature profiles ... … by additional central ICRH for the one-beam discharge to adjust Inductive current profiles

44. Two-beam discharges: so far non-symmetric beam deposition Q5, Q6, Q7, Q8 A. Stäbler

45. Future two-beam experiments: try to match symmetric deposition (closeness to DN) Z = 0 z = 9.5 cm Q5, Q6, Q7, Q8 A. Stäbler

46. tor Re P  jant B cos  ~  1/ ~ CASTOR with antenna: calculate torque Torque on the plasma due to external error fields: Maximum torque

47. An example: Interaction of NTMs with perturbation fields No simultaneous large NTMs of different helicities observed in experiments

48. An example: Interaction of NTMs with perturbation fields • Analytic theory: • for NTMs stabilising effect of additional helical field can be proven for small values of || • effect vanishes for ||  Is there an effect remaining for realistic values of || ? If so: new stabilisation method for NTMs can be propsed: stabilisation by external helical perturbation fields Many other problems, but: so far no non-linear MHD code can deal with realistic ||

49. Proposal for a solution in non-aligned coordinate system In the following, for simplicity (not in the code): Cartesian coordinates with one perturbation field component Heat conduction equation for different Fourier components of temperature: … … To close the equations one should not truncate the Fourier series in T, but in q  heat flux along perturbed magnetic field line remains finite (nearly vanishing temperature gradients)

50. Fourier decomposition for perturbation In the following, for simplicity (not in the code): Cartesian coordinates with one perturbation field component Heat conduction equation for different Fourier components of temperature: To lowest order (for explanation): include only terms up to first order in q  T2 adjusts itself such that q||1 becomes small