1. ASIPP HT-7 belt limiter Houyang Guo, Sizhen Zhu and Jiangang Li Investigation of EAST Divertor Asymmetry in Plasma Detachment & Target Power Loading Using B2/Eirene Seminar presented at AS-IPP, Hefei, 10/10/2006

2. Acknowledgements HT-7 belt limiter Jiangang Li, Baonian Wan, Yuanxi Wan, Sizhen Zhu for support of this work. Youzhen He, Weiping Huang, Qing Li, Ying Zhang for facilitating my visit. And all of you!

3. Introduction HT-7 belt limiter A major concern for EAST and future high- powered steady-state machines such as ITER is the power handling capability of divertor target plates. Localized injection of highly radiating impurities such as Ne may provide as a means of reducing power fluxes to the divertor targets and actively controlling inner/outer divertor asymmetry in power loading.

4. Outline HT-7 belt limiter EAST divertor geometry and major modeling parameters for B2/Eirene – SOLPS4.0. Basic performance of EAST divertor in terms of target power loading and impurity screening for both single-null and double-null configurations. Active control of plasma detachment and target power loading using neon puffing. Summary and conclusions.

5. Unique Features of EAST HT-7 belt limiter High power 1 st phase: 10 MW 2 nd phase: 20 MW (with NB) And Long pulse 1000 s, sustained by LHCD

6. EAST was built to allow both single null and double null divertor operations HT-7 belt limiter

7. Basic Divertor Functions HT-7 belt limiter Exhaust power and particles (including helium ash in a reactor) Provide sufficient screening for impurities to minimize core contamination

8. A major concern for EAST and ITER is divertor power handling capability - Provided by Dr. Damao Yao

9. Modeling of Divertor Performance Using B2/Eirene – SOLPS 4.0 B2/Eirene code package – SOLPS 4.0  A multi-fluid code B2 for electrons and ions, and  A Monte-Carlo code Eirene for neutrals Major control parameters  Total heat fluxes from confined core: P s  4 MW (equally split between the i and e channels) P s,out  3P s,inn for double dull configuration n s = 0.5  3.5  10 19 m -3  ITER-like cross-field transport: D  = 0.3 m 2 s -1  i   e  1.0 m 2 s -1

10. Impurity Sources (1) Intrinsic Carbon  Physical sputtering  Chemical sputtering Y ch = 2% (2) Active neon puffing  To reduce target power loading  Control divertor inner/outer asymmetry

11. As expected, CDN reduces peaked heat fluxes to both targets. Z s is also reduced for CDN. However, plasma detaches at inner target occurs at a much lower density for CDN, thus resulting in strong divertor asymmetry. Comparison between single null (SN) & double null (CDN) configurations

12. Plasma is fully detached from inner target, with heat flux to the target dramatically reduced. Heat flux at the outer strike point is substantially reduced - a key feature of vertical divertor, but remains high elsewhere.  Localized gas puffing from outer divertor may accelerate detachment, further reducing heat flux to the outer divertor. CDN operation leads to stronger divertor asymmetry n s = 1.5  10 19 m -3

13. Consequences of Divertor Asymmetry HT-7 belt limiter  Full plasma detachment at inner target may lead to confinement degradation due to excessive neutral influxes around X-point to the core.  Further, most of the outer divertor plasma still remains attached, with substantial power flux going to the outer target, which is undesirable for long pulse operation.

14. Ne is introduced from outer lower divertor. Ne is well restricted to the vicinity of lower divertor due to strong divertor screening for Ne. However, significant radiation from neon is also present in the inner divertor, presumably due to leakage of Ne through private region or around X-point. Localized neon puffing is used as a means to reduce heat flux to outer target Ne

15. It is remarkable that neon puffing does not appear to affect much the edge impurity content, suggesting very strong divertor screening for neon under modeled conditions. Nevertheless, Ne puffing dramatically reduces heat fluxes to the outer target

16. To further reduce heat flux to the outer target and divertor asymmetry:  Optimizing Ne puffing location to maximize Ne ionization inside outer divertor.  Inducing SOL flow by mid- plane fueling & pumping.  A physical septum may help preventing direct Ne leakage from outboard into inboard. Work in progress: Reduce neon leakage into inner divertor

17. Summary & Conclusions HT-7 belt limiter  A major concern for EAST and future high-powered steady-state machines such as ITER is the power handling capability of divertor target plates.  As expected, double null operation distributes output power more widely, reducing peak target power loading.  However, double null leads to early detachment at inner target, resulting in strong divertor asymmetry.  Ne puffing from outer divertor dramatically reduces peak heat flux to the target, without affecting much Zeff.  To further reduce outer target power loading and divertor asymmetry, Ne leakage into inner divertor must be minimized.