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Primary experimental results of suppressing MHD instabilities in HT-7 by biased electrode

HT-7 Data Meeting and Workshop, July 19-20. 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. Outline. Introduction Experimental Setup Results

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Primary experimental results of suppressing MHD instabilities in HT-7 by biased electrode

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  1. HT-7 Data Meeting and Workshop, July 19-20 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

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

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

  5. Introduction Minimum ration of plasma toroidal rotation to Alfven speed to keep the RWM stable in Tokamaks

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

  7. Introduction 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.

  8. Tokamaks which have conducted the biased electrode experiments

  9. （1） Improve confinement（LH) Biased experiment in TEXTOR * R.R. WEYNANTS, et. al, NUCLEAR FUSION, Vo1.32, NOS (1992)

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

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

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

  13. Current injection Plasma biasing Hybrid biasing Biased experiment in Tdev

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

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

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

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

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

  19. 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)

  20. Effect on MHD

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

  22. ~25ms Time delay for the effect on

  23. ~60ms Time delay for the effect decay away after bias voltage is off

  24. Suppression effect depend on biased current . Mushroom electrode

  25. Suppression effect depend on biased current . Small limiter

  26. Suppression effect depend on biased current . Large limiter

  27. 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 !!!

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

  29. Effect of biasing current on toroidal rotation

  30. Toroidal rotation behavior in different radius

  31. Z Poloidal field line Effect on poloidal rotation 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*. Suppose the cross phase of two probes signals is (t), then: Where t is wave travel time between two position, So V(t) can be calculated *Rev Sci. Instru. Vol 70(1) , 874,1999

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

  33. Time evolution of poloidal rotation with/without biasing

  34. Poloidal rotation in different radius

  35. Effect on profile

  36. Time evolution of SX emission profile

  37. Time evolution of normalzied SX emission profile

  38. 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.

  39. 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.

  40. Thank you for your attention

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