1. Primary experimental results of suppressing MHD instabilities in HT-7 by biased electrode Zhong Fangchuan, Luo Jiarong Shu Shuangbao College of Science, Donghua University Shanghai, 201620 HT-7 Data Meeting and Workshop, July 19-20

2.  Introduction  Experimental Setup  Results MHD instabilities Plasma rotation Profile  Conclusion Outline

3. Introduction MHD instabilities is one of the main obstacles to realize the tokamak advance operation with high parameters. MHD instabilities suppression and control is a basic and important topic of tokamak plasma research.

4. It has been proved that plasma rotation and shear flow is benefit for MHD stabilities. Locked mode and disruption Introduction

5. DeviceDIII-DJETJT-60UNSTX 1  2% 0 ． 6  1%  1%4  6% Minimum ration of plasma toroidal rotation to Alfven speed to keep the RWM stable in Tokamaks Introduction

6. How to maintain or increase the plasma rotation? NBI(most used one) RF Wave Biased Electrode Change E r  Vary V  、 V . Introduction

7. By using a biased electrode to add an external electrical field on plasma. Through E  B drift, the plasma edge rotation will be changed, and the MHD instabilities will be reduced. Introduction

8. Device name CoutryR/a(cm)B t (T)IP(kA)Ne(/m 3 )T e (eV) SINPIndia30/7.50.4510-30 1-3  10 19 ？ KT-5China32.5/8.50.510 1.0  10 19 20 CASTORCzech40/8.5115 1-2  10 19 ？ ISTTOKPortugal46/8.50.68 7  10 18 260 TCABRBrazil61/181.1110 1-4  10 19 600 TdeVCanada87/271.5210 ？？ Phased-TUSA93/260.7-1.070-100 1.0  10 19 400 CCTUSA150/400.350 2  10 18 150 T-10Russia150/332.5200- 300 1-4  10 19 ？ TEXTORGermany 175/462.35190 1-2  10 19 1000- 1500 Tokamaks which have conducted the biased electrode experiments

9. Device name Biased polarity Electrode size （ mm) Distance to limiter (mm) Bias voltage/cuurent SINPnegative  6  10 -20300V/100A KT-5positive  30  4 10150V/200A CASTORpositive  50 >0200V/40A ISTTOK,bipolar 3  125 0300V/400A TCABR,positive  20  5 -20600V/150A TdeV(multidivertor0300V/100A Phased-Tpositive  25  30 -30500V/300A CCTnegative??1000V/40A T-10positive 15  45  55 -20450V/80-200A TEXTORbipolar 130  35  15 -60900 V/100A

10. （ 1 ） Improve confinement （ L  H) Biased experiment in TEXTOR * R.R. WEYNANTS, et. al, NUCLEAR FUSION, Vo1.32, NOS (1992)

11. * E.Y. WANG*, Xin WANG, D.A. DIEBOLD,et al, NUCLEAR FUSION, Vol. 35. No.4 BIASED H MODE EXPERIMENTS IN PHAEDRUS-T Only Positive Biased trigger the L  H Same results in T-10 In N biased, Ion current is too small to trigger L  H ?

12. Biased experiment in ISSTK Negative biased improve the confinement. Positive biased degrade the confinement. * J A C Cabraly, et. Al, Plasma Phys. Control. Fusion 40 (1998) 1001–1019 Which is critical, Polarization ？ Current ？

13. (2)Change rotation Biased experiment in KT-5

14. Current injectionPlasma biasingHybrid biasing Biased experiment in Tdev

16. Rotation experiment in HBT-EP ( 2009 APS meeting)

17. (3)Effect on MHD I.C. Nascimento et. al, Nucl. Fusion 47 (2007) 1570–1576 Suppression and excitation of MHD activity at TCABR

18. Biased Electrode can (1 ） Improve confinement （ L  H) (2) Change the plasma rotation (3) Suppress or stimulate the MHD instabilities Introduction But the action mechanism is still not clear.

19. Retractable electrode Bias Power plasma Voltage  0 ～ 650V, Current 0 ～ 300A Experimental Setup

20. Three type of electrodes Movable limiter 150  50  40 Mushroom shape  60 Movable poloidal limiter 560  100  10

21. Results  Effect on MHD Typical time evolution of discharge with/without biased (Signals from top to bottom are : plasma current IP, line integrated density ne, SX-ray emision, H , Mirnov and biased current)

22. Effect on MHD

23. It is a time delay for the MHD is suppressed and burst out again after the biased voltage on and off

24. ~25ms Time delay for the effect on

25. ~60ms Time delay for the effect decay away after bias voltage is off

26. Suppression effect depend on biased current. Mushroom electrode

27. Suppression effect depend on biased current. Small limiter

28. Suppression effect depend on biased current. Large limiter

29. Electrode type surface area (cm2) Minimum action current(A) current density(A/cm2 ） mushroon28.27431140.49514913 small limiter150450.3 large limimter622.03482800.128610164 Minimum biased current density for the MHD suppression for different electrode MHD suppression effect is strongly depended on the biased current, there is a minimum biased current need for the action !!!

30. Results  Effect on rotation The effect on toroidal rotation M=ln(Is+/Is-)/K

31. Effect of biasing current on toroidal rotation

32. Toroidal rotation behavior in different radius

33. The phase difference from two floating potential signals measured by Langmuir probes nearby along the poloidal field line can be used to calculate the poloidal rotation speed *. Poloidal field line ZZ Suppose the cross phase of two probes signals is  (t), then: Where  t is wave travel time between two position, *Rev Sci. Instru. Vol 70(1), 874,1999 So V  (t) can be calculated Effect on poloidal rotation

34. Effect on ploidal rotation When biasing, the phase difference is decrease, imply the poloidal rotation is increased Shot 112252 Shot 112254

35. Time evolution of poloidal rotation with/without biasing

36. Poloidal rotation in different radius

37. Effect on profile

38. Time evolution of SX emission profile

39. Time evolution of normalzied SX emission profile

40. Conclusion Biasing electrode can suppress MHD instabilities effectively. Suppression effect is strong depended on the biased current Edge plasma rotation is increased greatly by biasing voltage Plasma profile is changed by biasing electrode. Further study is need.

41. Acknowledgements The supports and helps from ASIPP are great appreciated. Great thank Dr. Zhao Hailin and Dr. Kong Defeng from USTC for provide the Langmuir probe data. The research is supported by Ministry of Science and Technology with grant No. 2008CB717807 and 2009GB107006.

42. Thank you for your attention