Recent Progress on Long Pulse Divertor Operation in EAST

1. Recent Progress on Long Pulse Divertor Operation in EAST H.Y. Guo, J. Li, G.-N. Luo Z.W. Wu, X. Gao, S. Zhu and the EAST Team 19th PSI ConferenceMay 24–28, 2010, San Diego, CA

2. Content of Talk • EAST Superconducting Tokamak& Recent Upgrade on Plasma Wall Interface • Long PulseDivertor Plasma Behavior • Comparison Between Single Null & Double Null Divertor Configurations • Divertor Asymmetry & Effect of BT Reversal • B2/EIRENE Modeling & Implications for High Power Long Pulse Operation

3. Content of Talk • EAST Superconducting Tokamak& Recent Upgrade on Plasma Wall Interface • Long PulseDivertor Plasma Behavior • Comparison Between Single Null & Double Null Divertor Configurations • Divertor Asymmetry & Effect of BT Reversal • B2/EIRENE Modeling & Implications for High Power Long Pulse Operation

4. EAST Was Built to Address High Power, Long Pulse Operation Issues for ITER • t = 1000 s • R = 1.9 m, a = 0.5 m • Ip = 1 MA • BT = 3.5 T • t = 400 s • R = 6.2 m, a = 2 m • Ip=15 MA • BT=5.3 T EAST ITER

5. Divertor Configurations: SN & DN EAST ITER SN DN

6. Divertor Configurations: SN & DN EAST • Adopted ITER-like vertical target structure with tightly fitted side baffles and a central dome to minimize neutral. • Reduce peak power loading on the divertor target plates, most of the particles incident on the vertical target are re-emitted towards and ionized near the separatrix below the X-point. • Facilitate partial detachment near the strike points: minimize peak power loading, also ensure adequate particle exhaust. D0 ITER

7. EAST Initial SS First Wall Was Upgraded to Full Graphite Wall w/ Active Cooling SiC coating reduces erosion near strike points by suppressing chemical sputtering; also reduces C redeposition elsewhere by enhancing C reflection (QianXu, Thu PM, P3-69) • SiC coated, doped graphite GBST1308 (1%B4C, 2.5%Si, 7.5%Ti) tiles are used to cover the divertor panels and vessels. • Graphite tiles are bolted to the water-cooled plates and vessel wall to limit temperature rise at tiles to 900 C at 2 MW/m2.

8. A New Internal Divertor Cryopump Was Installed to Provide Active Divertor Pumping • Pumping speed: • 75 m3/s for D2 • 107 m3/s for H2

9. Content of Talk • EAST Superconducting Tokamak& Recent PWI Upgrade • Long PulseDivertor Plasma Behavior • Comparison Between Single Null & Double Null Divertor Configurations • Divertor Asymmetry & Effect of BT Reversal • B2/EIRENE Modeling & Implications for High Power Long Pulse Operation

10. Steady Divertor Discharges Have Been Achieved with Pulse Duration > 60 s

11. Steady Divertor Discharges Have Been Achieved with Pulse Duration > 60 s

12. Demonstrated Three Distinct Divertor Plasma Regimes with Density Ramp-up Plasma detachment reduces peak particle & heat fluxes, as well as associated material damage, essential for steady-state operations. Sheath-Limited • Ion saturation current Is (ion flux) increases with density ne. Conduction-Limited • Is further increase until roll over. Detached • Particle flux starts to decrease as ne increases.

13. Demonstrated Three Distinct Divertor Plasma Regimes with Density Ramp-up Plasma detachment reduces peak particle & heat fluxes, as well as associated material damage, essential for steady-state operations. Sheath-Limited • Ion saturation current Is (ion flux) increases with density ne. Conduction-Limited • Is further increase until roll over. Detached • Particle flux starts to decrease as ne increases.

14. Radiation Appears to Move toward X-point during Detachment Visible Light Dɑ Intensities

15. Achieved Partial Detachment w/ Te < 2 eV Near Sepatratrix, As Expected for V Target Detachment occurs near separatrix, with far SOL still attached to reduce peak heat flux and ensure adequate pumping Te < 2 eV at detachment

16. Active Control of Peak Heat Flux Using Localized Gas Puffing Reduce particle and heat fluxes near outer strike point with D2 gas puffing from outer divertor

17. Content of Talk • EAST Superconducting Tokamak& Recent PWI Upgrade • Long Pulse Divertor Plasma Behavior • Comparison Between Single Null & Double Null Divertor Configurations • Divertor Asymmetry & Effect of BT Reversal • B2/EIRENE Modeling & Implications for High Power Long Pulse Operation

18. Divertor Inn-Out Asymmetry for DN • Particle and heat fluxes to the outer target are much higher than inner target. • Strong fluctuations are present in the outer divertor, presumably due to enhanced turbulence on the outboard side with bad curvature, but decreases at higher densities

19. Comparison between DN and SN • Particle fluxes to both divertor targets are higher for SN divertor, presumably due to reduced target surface area. • But, divertor asymmetry is reduced for SN divertor, possibly due to fast parallel transport along SOL via the top. • Fluctuations are significantly reduced at inner target

20. Content of Talk • EAST Superconducting Tokamak& Recent PWI Upgrade • Long Pulse Divertor Plasma Behavior • Comparison Between Single Null & Double Null Divertor Configurations • Effect of BT Reversal • B2/EIRENE Modeling & Implications for High Power Long Pulse Operation

21. Influence of Classical Drifts on Divertor Asymmetry Reversing toroidal field direction affects both drifts. Affects top/down asymmetry Affects inner/outer asymmetry

22. Effect of BT Reversal – DN Top/Down Asymmetry Bottom Bottom • Normal BT(clockwise, viewing from top): more fluxes in lower divertor, in the ion B × B Direction. • Reverse BT: Reduce, even reverse top/down asymmetry.

23. Effect of BT Reversal – SN Inner/Outer Asymmetry Reversed BT for SN operations led to early divertor detachment and stronger core radiation, reducing Ohmic density limit. • Revering BTled to strong particle fluxes on outer divertor target, most likely driven by Er ×BTdrift via private region. • This resulted in early detachment in both outer and inner divertors, accompanied by large core radiations seen by SX, ultimately affecting density limit.

24. Effect of BT Reversal – SN Inner/Outer Asymmetry Reversed BT for SN operations led to early divertor detachment and stronger core radiation, reducing Ohmic density limit. • Revering BTled to strong particle fluxes on outer divertor target, most likely driven by Er ×BTdrift via private region. • This resulted in early detachment in both outer and inner divertors, accompanied by large core radiations seen by SX, ultimately affecting density limit.

25. Content of Talk • EAST Superconducting Tokamak& Recent PWI Upgrade • Long Pulse Divertor Plasma Behavior • Comparison Between Single Null & Double Null Divertor Configurations • Effect of BT Reversal • B2/EIRENE Modeling & Implications for High Power Long Pulse Operation

26. Comparison with Initial SOLPS-B2/EIRENE modeling Density decays much slower in outboard SOL, possibly arising form enhanced radial transport by turbulence • SN – Normal BT, Ps = 0.25 MW with Pi = Pe, ns = 51018 m-3 ~ ½ <ne> • D = 0.5 m2/s, i = e = 1 m2/s • Carbon: Phys. + Chem @ 0.5eV, w/ Ych = 2%

27. Predictions for High Powered Dischargeson EAST w/ Ps ~ 4 MW (D = 0.3 m2/s) EAST SS power handling capacity is presently limited to 2 MW/m2 additional impurity seeding is needed to promote detachment at lower density, ns ~ 1019 m-3, for good confinement & LHCD efficiency • DN exhibits strong in-out divertor asymmetry in target power loading. • SN in-out peak heat fluxes are similar (without taking into account of drifts and turbulence- induced transport). • Detachment and hence significant reduction in peak target head load occurs at much higher separatrix density, ns ~ 1019 m-3.

28. Summary & Conclusions • EAST has demonstrated stable, long pulse divertor operation up to ~ 65 s, entirely driven by LHCD, and • Carried out first systematic assessment of divertor performance, e.g., detachment, SN/DN comparison, divertor asymmetry and effect of field reversal etc., in Ohmic and L-mode conditions. • Modeling by SOLPS-B2/EIRENE indicates that additional impurity seeding is needed to control peak target heat load for high power long pulse operation on EAST.

29. Other EAST Divertor Presentations • P1-33, Mon-PM: Y.P. Chen, “Modelling Studies of SOL-Divertor Plasmas in EAST Tokamak with High Heating and Current Driving Power” • P2-67, Tue-PM: Q. S. Hu, “Particle Exhaust and Recycling Control by Active Divertor Pumping in EAST” • O-24, Thu-AM: Z.W. Wu, “First Study of EAST Divertor by Impurity Puffing” • P3-11, Thu-PM: W. Gao, “Effect of Localized Gas Puffing on Divertor Plasma Behavior in EAST”