Recent Progress in MHD studies on HL-2A tokamak and Future plans

1. Recent Progress in MHD studies on HL-2A tokamak and Future plans Yi Liu, Y.B.Dong, W. Deng, J.Zhou, X.T.Ding Southwestern Institute of Physics, Chengdu 610041, China e-mail contact of main author:yiliu@swip.ac.cn

2. Outline Experimental results on MHD activities • Identification and analysis of magnetic structures • Sawtooth features during ECRH (humpback, hill,..) • Control of sawtooth period (Stabilization and destabilization) • Exiting of the e-fishbone during ECRH • Disruption pridiction and mitigation Future plan in MHD studies on HL-2A 1

3. The mission of HL-2A include: Improve the hardware Realize good plasma performance Explore physics issues HL-2A tokamak Main parameters of HL-2A • Major radius: 1.65 m • Minor radius: 0.40 m • Toroidal field: 2.8T • Plasma current: 480 kA • Magnetic flux: 5.0 Vs • Discharge duration:1.5 sec. • Plasma density: 6.0 x 1019 m-3 • Electron temperature:5.0k eV • Ion temperature: 600 eV ECRH system on HL-2A 2

4. HL-2A装置放电控制及参数进展 Up to now, the stable and reproducible discharges with diver configuration have been obtained, using the reliable feedback control. Discharge progress Te=4930 eV The maximum temperature is 5keV • 1.6MW ECRH heating ECRH • SMB injection(non-local effect, ITB formation) • Study on GEM Zonal Flow • Study on MHD activities (e-fishbone, central relaxations, disruptions)

5. Mirnov coils 16 channel fast ECE Soft X-ray camera Hard X-ray camera The soft x ray camera on HL-2A NO.3 NO.1 NO.5 NO.2 NO.4 q=1 surface Techniques developed for MHD studies Fast & Reliable Algorithms to construct images for SX, HX ,Bolometer,HAlpha,… • Analytic algorithm (Series-expansion method) • Constrained-optimization Method(Pixel based) • HybridMethod • Hopfield Neural Network m=1 mode structure 3

6. Improvement of HybridMethod FEEDBACK TECHNIQUE Since the Radon transform is linear With Feedback(Y.Liu,Peterson) Fitting accuracy Without Feedback(Y. Ohno) Kyoto University 4

7. reconstruction with no use of gm dataf gm Improvement to Regularizationmethod Tikhonov /Maximum entropy with GCVoptimization Equation GENERALIZED CROSS VALIDATION FOR OPTIMIZING g Tikhonov /Maximum regularization P( f ) is a Penalty Function (Roughness of Image) Final Prediction Error Prediction of fm by 5

8. R HFS LFS (a) (b) Z (m) Z (m) R (m) R (m) Identification and analysis of magnetic structures • 28 Mirnov coils for MHD instability studies (m≤17, n≤4) • Analysis of magnetic structures by modeling IP=200~480kA, Bφ=2.0~2.8T Simulation the magnetic island Current filaments m=3 island 18 pick-up coils 6

9. (a) (b) B(T)*10-4 Z (m) (d) (c) a b B(T)*10-4 Z (m) c d Analysis of magnetic structures The least-square fitting to the poloidal Mirnov data to determine the parameters of perturbation currents, j0, and ∆Φ Reconstruction of magnetic island Comparisons between inversion and measurement data, Shot03792. The structure of m=2 magnetic island 7

10. MHD activities observed on HL-2A IP Te0 or SXR Auxiliary Heating Start-up Low sawtooth Fishbone m=2 Mirnov Large sawtooth, Small disruption Disruption Plasma MHD Equilibrium(EFIT) A wide variety of MHD instabilities has been observed on the HL-2A tokamak. Other topics including: • Feature of steady m=1/n=1 phenomena triggered by MBI • MHD mode activity and Sawtooth behaviour during ECH • Features of sawtooth and m=1/n=1 mode after laser blow-off 8

11. m=1 mode and m=2 mode in a dischargeafter MBI and PI on the HL-2A . Snake oscillation (m=1) m = 1 m = 2 9

12. Resistive Internal kink mode(m=1/n=1), instability responsible for sawteeth. MISHKA: Ideal MHD Code radial displacement Simulation of MHD mode on HL-2A The ideal MHD code MISHKA is capable to perform a computational normal-mode analysis for routine ideal MHD stability analysis of HL-2A discharge. • Internal and external kink stability • Extension to non-ideal MHD including • Drift • Neoclassical • Kinetic effects 10

13. CASTOR: Resistive MHD Code With resistivity h, the equations for the perturbed quantities r, u, T and b : lr = -Ñ(rou), lrou= -Ñ(roT + Tor) + (Ñ × Bo ) × b + (Ñ × b) × Bo， lroT = -rou ÑTo- (g -1)roToÑ u+ (g -1)[2hÑ × Bo Ñ × b]， lb = Ñ × (u × Bo- hoÑ× b). The m=2/ n=1 double tearing mode at the time of the reconnection The CASTOR plasmas code computes the entire spectrum of normal-modes in resistive MHD for general tokamak configurations. 11

14. Feature of steady m=1/n=1 Phenomena Triggered by MBI Stair-shaped density increments Multi-pulse MBI experiment • Effective refuelling • Strong influence on MHD properties The plasma density increases after MBI, while a temperature drop was observed in the central ECE channel. Arrangement for MBI in the HL-2A • Gas pressure: 1 M Pa to 3 M Pa • Pulse duration of Jet : 2 ms • Pulse interval: 50 ms to 100 ms • Observation of snake • Trigger of ITB during ECRH 12

15. m=1 oscillation 0.8 0.2 Center channel 0.32 Outer channel 340 360 Isx (a.u.) 0.20 MBI 0 1 1 1 0 0.1200 0.2400 0.2800 0.3600 0.4200 0.4800 0.5600 0.6000 0.7000 0.7200 0.8400 0.8400 0.9800 0.9600 1.120 1.080 1.260 1.200 1.400 1.440 1.560 1.820 1.680 1.960 1.800 2.100 1.920 2.240 2.040 2.380 2.160 - 1 2.520 2.280 2.660 - 2.800 1 1 1 - 0 0.1400 1 0.2800 r/a 0.4200 0.5600 0.7000 0.8400 0.9800 r/a 1.120 1.260 1.680 1.400 1.540 1.680 1.820 1.960 0 2.100 Isx (a.u.) 0.1300 2.240 2.380 0.2600 2.520 0.3900 2.660 0.5200 First sawtooth crash 2.800 0.6500 - - - 1 1 1 0.7800 0.9100 1.040 1.170 1.300 1.430 1.560 1.690 T (m s) 1.820 1.950 2.080 m=1 oscillation 2.210 2.340 2.470 2.600 Crash phase Feature of steady m=1/n=1 Phenomena Triggered by MBI Snake-like events surviving a crash A large, persistent m/n=1/1 perturbation with a rotating frequency of 4kHz has newly been observed in the core region after injection of MBI . This usually happens in the decay phase after a stair-shaped density increment. The oscillations grew rapidly at the beginning, then saturated for a long time. It can survive the subsequent sawtooth crash. Steady-state m/n=1/1 perturbation with a feature of hot core displacement. It indicates the jet may penetrate into the core region of the plasma, and cause the formation of a persistent m/n=1/1 oscillation jut as pellets 13

16. Possible explanation: Feature of steady m=1/n=1 Phenomena Triggered by MBI SVD analysis of signals: (a) time evolution of principal components; (b) spatial eigenfunctions. The central q(r) profile may be nonmonotonic with qmin above but close to 1 because of the increased resistivity due to the sudden drop of temperature over the core region. Such a q profile may lead to a nonlinear saturated ideal m=1 displacement as: the is the critical value 14

17. ITB formed between q=1 and q=2 rational surface high field side • Ha and radiation losses drop（Enhanced energy confinement） q=2 Showing formation of an ITB • Happened in low density range （ne~1-2x1013cm-3） • Located between q=1 and q=2 surface • Central temperature increase of 200ev A steep Te profile in the plasma core is sustained for about 40ms 15

18. ITB Triggered by MBI ITB formation during ECRH ITB events after SMBI (PECRH = 800 kW, TSMBI = 810 ms) Improvement of global confinement ITB formation during ECRH and OH. ( Bt = 2.31 T, ne = 1.0×1019 m-3, Ip = 250 kA,.) 16

19. Features of sawtooth and m=1/n=1 mode after Laser blow-off Impurities have been injected into ohmic hydrogen discharges by laser blow-off for transport studies. The influences of sawtooth activity on impurity transport are studied. The system of laser blow-off Parameters: • 30ns pulse length • 1053nm wavelength • 10J energy • 0.6-1.5x1013cm-3 background density • Titanium and aluminum Time evolution of the plasma current, loop voltage and electron density, the Al line brightness from VUV，the soft X-ray and bolometer signals. 17

20. Features of sawtooth after Laser blow-off The impurity ion flux is strongly affected by the sawtooth activity exhibiting discontinuities at sawtooth crashes. Inverted sawtooth A jump is clearly seen during the inverted sawtooth crash within 200us indicating a rapid inwards flux of impurity particles during the crash. The impurity transport is greatly enhanced during sawtooth crash: • up to 10-15 times (inflow phase) • in the range of 2-3(decay phase) 18

21. ECRH 75GHz, 0.8 MW gyrotron power ~300ms pulse length Upgrade : 2.0 MW, 0.5s Lower Hybrid Current Drive 2.4GHz, 1MW klystron power 1.7 ≤ n// ≤ 2.3 2005 upgrade: 2MW, 500ms NBI -1MW(2007-2008) HL-2A Heating and Current Drive ECRH LHCD NBI 19

22. normal (b) off-axis ECH (PECH=340kW) (a) on-axis ECH ( PECH=450kW); larger smaller giant double saturated Sawtooth behaviour during ECH The auxiliary heating power is up to 2MW in the ECRH experiment of HL-2A tokamak. • on-axis ECH (with a more peaked profile) • off-axis ECH (with flat profile, no “hot ears”) A sawtooth tends to saturate or decrease in its ramp phase, and the sawtooth shape is usually changed, leading to formation of a saturated sawtooth, a compound sawtooth, a humpback or a hill. Parameters of ECRH experiment 20

23. Non-standard sawtooth during ECRH • Double Sawtooth m=1 island SVD • Compound(saturated+double)Sawtooth m=1island SVD 21

24. Possible explanation: Central plasma relaxation oscillation during ECRH hills humpback The Te growth in both case corresponds to some temporary improvement of the global confinement Heating effect during crash or reconnection 22

25. Reduced core transport after off-axis ECRH switch-off(1) Delayed Te decrease Off-axis ECRH Steep SX profile Te increase Similar results observed on T-10 and TEXTOR(Nucl.Fusion 44,2004) 23

26. Possible explanation: Reduced transport after ECRH switch-off(2) Nearly unchanged Te0 Larger sawtooth Steep SX profile ne increase slightly Off-axis ECRH switch-off leads to current density redistribution and transiently low shear, causing a local confinement improvement. 24

27. Instability control by RF injection(1) Stabilization of sawtooth activity–giant sawtooth The most significant response to a change in the heating location is the rapid change in sawtooth shape and period as the heating location crosses the inversion surface. Large sawtooth A large increase in sawtooth period occurs when changing the resonance location by just 1 cm(>1.3MW). 25

28. Instability control by RF injection(2) A normal sawtooth becomes saturated with strong m=1 mode or becomes a compound sawtooth . Destabilization of sawtooth with ECRH On-axis ECRH shortens the sawtooth period The current diffusion: The effect of localized ECRH on the local conductivity profile gives to variation in the current penetration time in the core The critical shear criterion: On-axis ECRH leading to an increased Such effect of ECH are qualitatively validated by the HL-2A discharges. 26

29. Destabilization of sawtooth with ECRH 热岛结构 By changing the heating location(on-axis or off-axis), the control of sawtooth period can be realized. 冷岛结构 27

30. Exiting of the m=1/n=1 kink mode during ECRH • Electron fishbone excited by the barely trapped electrons The experimental results of E-fishbone onHL-1M(Ding X.T,Liu Yi,Nuclear Fusion Vol.42,No5(2002)491) • Electron fishbone excited by the circulating electrons The experimental results of E-fishbone onHL-2A(Z.T.Wang,Nuclear Fusion Vol.42,No5(2002)491) 28

31. 20 15 10 f（kHz） 5 705 710 715 720 725 730 700 wavelet spectrum 0.5 0.4 Fast particle driven instabilities: Fishbones BT=2.36,T, PECRH = 650kW (800-1100ms), Ne = 2.2 x 1013 cm –13 Crash Fishbone like instabilities population of trapped fast particles drives central kink mode resonance between banana precession drift and mode rotation central kink mode ejects fast particles  nonlinear cycles 29

32. c a Soft x ray array a b E-fishbone q =1 Tearing mode b d c d Soft x ray array Two kinds of mode: Tearing mode, E-fishbone mode Before ECRH During ECRH SHOT 6061: BT=2.45T (off-axis heating) PECRH =150kW (800-1200ms) Ne = 2.9 x 1013 cm –13 I sx （a.u.） Shot 6061 Ion diamagnetic drift 30

33. E-fishbone disappeared when power >900kW Low field side deposition PECRH =650kW m=1/n=1 “fishbone like instaility” exited with • Electron density < 3x1013cm-3 • Power of ECRH < 900kW • Both high field side and low field side deposition High field side deposition PECRH=280kW 31

34. 30 – 60 keV 10 –30 keV Destabilizing effect of suprathermal electrons Increase of hard X ray during ECRH • Electron fishbone excited by the barely trapped electrons(Procession reversal ) (Ding X.T,Liu Yi,Nuclear Fusion Vol.42,No5(2002)491) • Electron fishbone excited by the circulating electrons Z.T.Wang,Nuclear Fusion47 Vertical Energy of the Superthermal Trapped Electron versus Bounce angle Normalized precession velocity of circulating electrons 32

35. MHD instabilities can also be beneficial MHD instabilities as a trigger of internal transport barriers • Snake as a trigger of stationary large pressure gradient near q=1 surface • Fishbone as a trigger of periodical reduction of transport coefficient • Internal transport barriers formed near a rational surface 33

36. Snake as a trigger of RI-Mode RI-Mode L-Mode Stationary large pressure gradient near q=1 surface (RI-Mode): confinement enhancement 34

37. ITB triggering by E-fishbone E-fishbone An electron transport barrier has been probably formed at the position just outside the q = 1 surface A slight increase in core energy constant is observed, while the off-axis temperature decreases at the same time (hence increase),leading to the conclusion that the transport coefficients must be reduced. Possible explanation: a redistribution of the resonant fast particles 35

38. Disruption Studies Disruption: density-,low-q limits, locked mode at low density • Warning signals • MHD mode • Mixture of mode stable state disruption Features of disruption on HL-2A 热岛结构 冷岛结构 dIP/dt during current quench fast disruption in 4-6ms 36

39. (b) m = 1 kink-like radiation 395 (a) 395 Plasma current r, mm 475 t, ms -382 472 r, mm Disruption -382 Shot:03039 Shot:03037 b, a.u. Mirnov Shot:03038 Shot:03054 0.5 470 475 t, ms 1/qa Disruption Greenwald limit 0.0 0.0 4.0 neR/BT Density limit disruption Discharge trajectories, w/o disruption discharges on Hugill diagram. A radiative collapse Disruption free discharges Evolutions of plasma radiation at disruption. Healing of plasma modes with additional heating (ECRH,NBI) is planned 37

40. 300 IP, kA 0 1.0 Pimp, a.u. CIII, 97.7nm CIII, 464.7nm 0.0 800 5.0 ne, m-3 t≤500ms ×1019 500ms~560ms Te(r), eV 0.0 570ms~590ms 5.0 PR, a.u. r =3cm r =-20cm r =-38cm 0.0 800 0 Te(t), eV Evolution 12 0 -41 r, cm 0 340 r, mm Last sawtooth -330 440 485 t, ms Evolution of Parameters during a density limit disruption. Temporal evolution of the electron temperature profile Te(r) during disruption. The plasma current, impurity intensities, electron density, plasma radiations, profiles of electron temperature, and the soft X ray emissions. 38

41. Prediction and mitigation of disruptions • A neural network has been developed to predict the occurrence of disruptions • Mitigation of disruptions by fast helium gas puffs/MBI injection 42

42. Artificial neural networks (ANN method ) ANN has a complex structure, including many neurons and layers, and Signals transmitted to the neural network through the input layer. By calculating, the output of Layer 1 has been got, and then, the output of Layer 1 became the input of Layer 2. This process can be repeated until a final output has been got. ANN are trained to forecast the plasma disruptions in HL-2A tokamak. 39

43. ANN results database • 13 experimental diagnostic signals: Mirnov probes, bolometer, H-alpha line intensity, VUV SXR, line integrated density The optimized network architecture is obtained. In the future, we want to use more (Mirnov signals, soft X ray,ect.) and then, we can predict short discharges with ANN. And at last, we are trying to predict disruption on-line. alarm 35ms in advance Disruption 40

44. Mitigation of disruptions A relatively large amount of helium gas puffing A relatively large amount of helium gas puffing 41

45. Proposals – Stability area At present, the heating power is not enough to study NTMs • this will change when we get NBI – then, studies can be done Work on ECCD (de)-stabilisation of sawteeth will be straightforward • maybe together with q-profile diagnostics (stabilisation mechanism) How different is disruption on HL-2A from the rest of the world Work on ECCD stabilisation/mitigation of disruptions could be interesting • can be done with present system • either heat MARFE or stabilise tearing mode Work on correlation between MHD activities and ITB emergence (monotonic q-profiles or non-monotonic q-profiles, ITBs linked with q=2 or q=3 radius) Continue work on E-fishbone, its characteristic and its influence on the formation of ITB (with q-profile measurement or calculation, reflectometry for trbulence) 42

46. High Power Auxiliary Heating • Profile control Ip ( r ), P ( r ) • NTM Control • ITB Control NBI 1MW ECRH/CD 2MW LHCD 1MW MSE CXRS ECE imaging To realize the physics objective of the HL-2A tokamak, the high power auxiliary heating systems will be established. Re-establish H-mode in HL-2A Study on ELM mode Study ITBs – also helps to get good performance • electron cyclotron heating on the current ramp • electron ITB formation with MBI and ECRH • electron ITB should be possible with central ctr-ECCD More ambitious: study fluctuations and transport Study on correlation between MHD activities and ITB emergence 43

47. NTM control is a key issue for achieving steady state high βN ECCD system 1-2MW Modulatedand non-modulated CD Steerable mirror with wide steering range ECCD Steering mirror Control of NTM by ECCD Schematic of Current driving on resonant surface 44

48. Interaction between energetic electrons and waves Charged fusion products transfer energy to electrons firstly, so the energetic electrons must be confined long enough without appreciable degradation due to collectivemodes and plasma turbulence. Diagnostics To investigate the 2-D velocity distribution of the energetic electrons: • Hard x ray imaging • Analysis of the ECE non-thermal spectra To observe the direction of the wave excited by the energetic electrons • ECE imaging • Soft x ray arrays in toroidal direction 45

49. Future plans Future plan in MHD studies on HL-2A • ELM and its control(Plasma shaping & ECRH) • MHD activity and formation of the ITB • MHD activity and formation of the ITB • Control of NTM mode • Control of sawtooth period • Control of sawtooth period • Development of ELM/RWM/EF control coils • Exiting of the e-fishbone during ECRH • Disruption mitigationand healing 46

50. Thank you for your attention! 47