Study on Power Threshold of the L-I-H Transition on the EAST Superconducting Tokamak

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,1N. Yan,1,2 L. M. Shao,1 S. C. Liu,1 R. Chen,1 W. Zhang,1 L. Wang,1N. Zhao,1 Y. L. Li,1 Y. L. Liu,1 J.C. Xu,1 1Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China 2Association Euratom-RisÁ DTU, DK-4000 Roskilde, Denmark E-mail: cliang@ipp.ac.cn

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. 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. Brief introduction of EAST in 2010 and 2012 C wall in 2010 Mo/Cwall in 2012 • All carbon (C) wall in 2010, molybdenum (Mo) first wall and C divertor in 2012 • reversed BT in 2010, normal or reversed BT in 2012 • Auxiliary heating advanced • One cryopump at lower divertor • Intensive Lithium wall coating

5. Behavior on Dα signal for L-H transition EAST with C wall in 2010 EAST with Mo/C wall in 2012 • All the H-mode discharges have sharp (single-step) L-H transition, including some discharges with small dithering (~3% in Dαsignal), 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 plotof sharp L-H (left figure) and dithering L-H transition (right figure).

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. Calculate the loss power (Ploss) through the separatrixand threshold power (Pthr) 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. Power threshold of sharp L–H in EAST with C wall versus Mo/C wall EAST with C wall in 2010 EAST with Mo/C wall in 2012 • Ploss is well consistent with Pthr in 2010 with marginal input heating power. • Plossand Pthr versus <ne> , shows that its lower boundary follows the scaling in • 2012.

9. L–H power threshold in EAST with C wall versus Mo/C wall EAST with C wall in 2010 EAST with Mo/C wall in 2012 There is no significant dependence of Ploss versus different divertor configurations in EAST.

10. L–H power threshold in EAST with C wall versus Mo/C wall EAST with C wall in 2010 EAST with Mo/C wall in 2012 • Plossfor 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 Ploss versus Ip ramp in 2012.

11. L–H power threshold in EAST with C wall versus Mo/C wall EAST with C wall in 2010 EAST with Mo/C wall in 2012 • Ploss increases with increasing plasma current (Ip) of sharp L-H transition in 2010. • Ploss has no clear dependence on Ip during sharp L-H transition in 2012.

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. Ploss versus dRsep |dRsep|<0.01 , DN or biased DN configuration dRsep<-0.01 , LSN configuration dRsep>0.01 ,USN configuration Reversed BT: 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 • BT, DN has the lowest power threshold. Power • threshold for USN is higher than LSN.

14. The dithering behavior in the Demission • The dithering behavior in the D emission signals varies significantly with different divertor configurations • The dithering cycles in the unfavorable configuration (normally LSN with reversedBT) are much irregular in frequency and amplitude. • The dithering cycles in DN and biased DN configurations (|dRsep|<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 reversedBT) is usually very short in time, with small amplitude of dithers.

15. Ploss versus triangularity δ • Ploss slightly decreases with increasing triangularity with DN configuration both in • reversed BT (left figure) and normal BT (right figure). • The dotted lines are linear fitting of the data respectively.

16. dithering L-H transition versus sharp L-H transition :what is role of the neutral particle density near the divertorregion? • In the graph, the dithering L-H transition and • sharp L-H transition have been somewhat separated • by themaximum value of lower divertor Ha emission • normalized by central line-averaged electron density. Hamax

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. The strike-point is more far away from divertor corner in sharp L-H transition 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 thestrike-point • and the divertor corner has changed about 2 cm at the time of dithering L-H and sharp • L-H transition.

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 γP of power in excess of L-I-H transition threshold power [2]. [2] Zohm H et al 1994 Phys. Rev. Lett. 72 222

20. Dedicated experiments in EAST with Mo/C • wall have shown that: duration of dithering • cycles decreases with γP, which obtained by • changing the auxiliary heating power at similar • threshold power and energy confinement time.

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. A statisticalstudy of low-to-high confinement transition (L–H transition) in EAST over a broad range of operation is presented. • Ploss 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. • Ploss slightly decrease with increasing triangularity with DN configuration, both in normal and reversed BT 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, • the estimated divertor neutral particle density of dithering L-H transition has lower value than that of sharp L-H transition. • the durationof I-phase decreases with the ramp rate γP of power in excess of threshold power.

23. Thank you for your attention!