1. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 1 Study on Power Threshold of the L-I-H Transition on the EAST Superconducting Tokamak L. Chen, 1 G.S. Xu, 1 H.Q. Wang, 1 S. Y. Ding, 1 N. Yan, 1,2 L. M. Shao, 1 S. C. Liu, 1 R. Chen, 1 W. Zhang, 1 L. Wang, 1 N. Zhao, 1 Y. L. Li, 1 Y. L. Liu, 1 J.C. Xu, 1 1 Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China 2 Association Euratom-RisÁ DTU, DK-4000 Roskilde, Denmark E-mail: cliang@ipp.ac.cn

2. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 2 Outline Introduction Power threshold of sharp L–H transition in EAST with C wall versus Mo/C wall L-H transition in EAST with Mo/C wall Summary and conclusion

3. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 3 Outline Introduction Power threshold of sharp L–H transition in EAST with C wall versus Mo/C wall L-H transition in EAST with Mo/C wall Summary and conclusion

4. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 4 R [m] 1.88 a [m] 0.44 B T [T] ~1.9 auxiliary heating LHW dominant (2010) LHW 2 MW and/or ICRF 1.5MW (2012) C wall in 2010 Mo/C wall in 2012  All carbon (C) wall in 2010, molybdenum (Mo) first wall and C divertor in 2012  reversed B T in 2010, normal or reversed B T in 2012  Auxiliary heating advanced  One cryopump at lower divertor  Intensive Lithium wall coating Brief introduction of EAST in 2010 and 2012

5. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 5  All the H-mode discharges have sharp (single-step) L-H transition, including some discharges with small dithering (~3% in Dα signal), in EAST with C wall.  Besides these sharp L-H transitions, I-phase displays dithering cycles with a much larger amplitude (~30% in Dα signal) has been observed in EAST with Mo/C wall.  In order to make comparison, all the H-mode discharges are classified as sharp L-H or dithering L-H transitions. The time history of (a) divertor D  emission and (b) its zoom-in plot of sharp L-H (left figure) and dithering L-H transition (right figure). EAST with C wall in 2010EAST with Mo/C wall in 2012 Behavior on Dα signal for L-H transition

6. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 6 Outline Introduction Power threshold of sharp L–H transition in EAST with C wall versus Mo/C wall L-H transition in EAST with Mo/C wall Summary and conclusions

7. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 7 Calculate the loss power (Ploss) through the separatrix and threshold power (P thr ) predicted by the international tokamak scaling[1]: [1] Martin Y.R., Takizuka T. and the ITPA CDBM H-mode Threshold Database Working Group 2008 J. Phys.: Conf. Ser. 123 012033

8. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 8 Power threshold of sharp L–H in EAST with C wall versus Mo/C wall EAST with C wall in 2010EAST with Mo/C wall in 2012  P loss is well consistent with P thr in 2010 with marginal input heating power.  P loss and P thr versus, shows that its lower boundary follows the scaling in 2012.

9. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 9 L–H power threshold in EAST with C wall versus Mo/C wall EAST with C wall in 2010EAST with Mo/C wall in 2012 There is no significant dependence of P loss versus different divertor configurations in EAST.

10. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 10 EAST with C wall in 2010EAST with Mo/C wall in 2012  P loss for sharp L-H transition which occurred during plasma current ramp-up is ~10% higher than that occurred during plasma current flat top in 2010.  There’s no clear dependence of P loss versus I p ramp in 2012. L–H power threshold in EAST with C wall versus Mo/C wall

11. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 11 EAST with C wall in 2010EAST with Mo/C wall in 2012  P loss increases with increasing plasma current (Ip) of sharp L-H transition in 2010.  P loss has no clear dependence on Ip during sharp L-H transition in 2012. L–H power threshold in EAST with C wall versus Mo/C wall

12. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 12 Outline Introduction Power threshold of sharp L–H transition in EAST with C wall versus Mo/C wall L-H transition in EAST with Mo/C wall Summary and conclusion

13. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 13 |dR sep |<0.01, DN or biased DN configuration dR sep <-0.01, LSN configuration dR sep >0.01,USN configuration Reversed B T : USN is favorable configuration EAST with Mo/C wall  Sharp L-H transitions are mostly obtained in LSN configuration, a few are obtained in DN and USN configurations.  In dithering L-H transitions with reversed B T, DN has the lowest power threshold. Power threshold for USN is higher than LSN. P loss versus dR sep

14. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 14  The dithering behavior in the D  emission signals varies significantly with different divertor configurations  The dithering cycles in the unfavorable configuration (normally LSN with reversed B T ) are much irregular in frequency and amplitude.  The dithering cycles in DN and biased DN configurations (|dR sep |<1 cm) are mostly regular large-amplitude oscillations, which typically exhibiting clear and sharp transition from the L-mode to the I-phase  The duration of I-phase in the favorable configuration (normally USN with reversed B T ) is usually very short in time, with small amplitude of dithers. The dithering behavior in the D  emission

15. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 15  P loss slightly decreases with increasing triangularity with DN configuration both in reversed B T (left figure) and normal B T (right figure).  The dotted lines are linear fitting of the data respectively. P loss versus triangularity δ

16. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 16 dithering L-H transition versus sharp L-H transition : what is role of the neutral particle density near the divertor region?  In the graph, the dithering L-H transition and sharp L-H transition have been somewhat separated by the maximum value of lower divertor Ha emission normalized by central line-averaged electron density. Ha max

17. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 17 Dithering L-H transition and sharp L-H transition obtained in one shot  Marginal heating with LHCD and ICRF  Similar density and plasma stored energy  Divertor configuration has changed  The divertor Da/Ha emission changed significantly -dithering L-H -sharp L-H

18. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 18 38905 2.7s dithering L-H 38905 4.5s sharp L-H Cryopump system Divertor corner  As calculated by EFIT code and divertor probe, the distance between the strike-point and the divertor corner has changed about 2 cm at the time of dithering L-H and sharp L-H transition. The strike-point is more far away from divertor corner in sharp L-H transition

19. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 19 The duration of I-phase relates to the ramp rate of power in excess of threshold power  Study of the duration of I-phase occurring at the transition reveals that it somewhat decreases with the ramp rate γ P of power in excess of L-I-H transition threshold power [2]. [2] Zohm H et al 1994 Phys. Rev. Lett. 72 222

20. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 20  Dedicated experiments in EAST with Mo/C wall have shown that: duration of dithering cycles decreases with γ P, which obtained by changing the auxiliary heating power at similar threshold power and energy confinement time.

21. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 21 Outline Introduction L–H power threshold in EAST with C wall versus Mo/C wall L–H transition in EAST with Mo/C wall Summary and conclusion

22. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 22  A statistical study of low-to-high confinement transition (L–H transition) in EAST over a broad range of operation is presented.  P loss for sharp L-H transition which occurred during plasma current ramp-up is ~10% higher than that for sharp L-H in EAST with C wall.  P loss slightly decrease with increasing triangularity with DN configuration, both in normal and reversed B T in EAST with Mo/C wall.  It exhibits lower power threshold for the ion drift away from the dominant X- point in EAST, in striking contrast to other tokamaks.  Study on the I-phase has revealed, a)the estimated divertor neutral particle density of dithering L-H transition has lower value than that of sharp L-H transition. b)the duration of I-phase decreases with the ramp rate γ P of power in excess of threshold power.

23. L. Chen US TTF meeting, 2014 April 22-25, San Antonio, Texas 23 Thank you for your attention!