The Role of the Boundary Plasma in Defining the Viability of Magnetic Fusion Energy

1. The Role of the Boundary Plasma in Defining the Viability of Magnetic Fusion Energy Dennis Whyte Plasma Science & Fusion Center, MIT, Cambridge USA Director, Plasma Surface Interaction Science Center (psisc.org) APS-DPP Providence, October 30, 2012

2. Overview • Boundary and PMI (Plasma-Materials Interaction) is too large and multi-disciplinary to cover any single topic in detail • Views expressed are my own • For efficiency the “standard” path of MFE is mostly assumed & assessed: tokamaks, solid-walls, etc. producing base-load electricity Takeaway messages • The boundary issues of power density, pulse duration & high material T are the “right” problems to work on because they are directly and critically involved in making MFE a practical and attractive energy source • The boundary issues involve interconnected and complex physical processes – they will not be solved by “technology” alone

3. Acknowledging the support of numerous colleagues in boundary & PMI research • B. Lipschultz, P. Stangeby, B. LaBombard, J. Terry, G. Wright, T. Leonard, A. Hubbard, C. Wong, R. Doerner, B. Wirth, D. Buchenauer, G. Matthews, S. Krasheninnikov, R. Goldston, J. Hughes, R. Sullivan, D. Rudakov, R. Maingi, P. Snyder, P. Coad, M. Mayer, G. de Temmerman, R. Neu, G. van Rooij, S. Harrison, M. Reinke, E. Marmar, M. Greenwald, S. Allen, M. Fenstermacher, C. Lasnier, J. Boedo, J.P. Allain, J. Brooks, A. Mahdavi, B. Terreault, B. Gregory..Students: Z. Hartwig, H. Barnard, R. Ochoukov, G. Olynyk, B. Sorbom, D. Brunnerand the late Phil West (GA) who was largely responsible for my start in this field

4. Fusion Boundary Challenge: Every Joule of energy is extracted through a distant surface with small area to volume ratio Fission Fusion

5. Boundary challenge from physics viewpoint

6. Boundary challenge from physics viewpoint:The plasma-facing surface is an “extreme interface”

7. Boundary challenge from physics viewpoint:and physics does not stop at the material!

8. Boundary challenge from physics viewpoint:and physics does not stop at the material! Progression of talk

9. Boundary challenge from power plant viewpoint: ITER example Pfusion = 500 MW Sarea = 700 m2 Cost ~ 20 G$ Twall ~ 450 K fon~0.1 (duty factor)

10. Boundary challenge from power plant viewpoint: ITER example Pfusion = 500 MW Sarea = 700 m2 Cost ~ 20 G$ Twall ~ 450 K fon~0.1 (duty factor)

11. Boundary challenge from power plant viewpoint: ITER example Pfusion = 500 MW Sarea = 700 m2 Cost ~ 20 G$ Twall ~ 450 K fon~0.1 (duty factor) Pf x 5  2500 MW

12. Boundary challenge from power plant viewpoint: ITER example Pfusion = 500 MW Sarea = 700 m2 Cost ~ 20 G$ Twall ~ 450 K fon~0.1 (duty factor) Pf x 5  2500 MW ηth x 2  0.5

13. Boundary challenge from power plant viewpoint: ITER example Pfusion = 500 MW Sarea = 700 m2 Cost ~ 20 G$ Twall ~ 450 K fon~0.1 (duty factor) Pf x 5  2500 MW ηth x 2  0.5 fon x 10  1

14. Boundary challenge from power plant viewpoint:Viability directly linked to boundary challenges ITER example Pfusion = 500 MW Sarea = 700 m2 Cost ~ 20 G$ Twall ~ 450 K fon~0.1 (duty factor) Pf x 5  2500 MW ηth x 2  0.5 fon x 10  1 Pf / S ~ 4 MW m-2 Twall > 1000 K > yr-long fluence

15. Robust arguments hold regardless of configurationT > 1000K, Pf / S ~ 3-4 MW/m2 , > 30,000,000 s http://aries.ucsd.edu/ARIES/DOCS/bib.shtml

16. Boundary “Chasm”to FNSF/Reactors:But these are the right problems to work on for MFE viability

17. Global power exhaust: Pheat/S ~ 1 MW m-2 a severe challenge due to local limit ~ 5-10 MW m-2 of actively cooled Plasma Facing Components

18. Global power exhaust: Pheat/S ~ 1 MW m-2 a severe challenge due to local limit ~ 5-10 MW m-2 of actively cooled Plasma Facing Components

19. Global power exhaust: Pheat/S ~ 1 MW m-2 a severe challenge due to local limit ~ 5-10 MW m-2 of actively cooled Plasma Facing Components Excellent control of power dissipation will be required

20. Global power exhaust: Pheat/S ~ 1 MW m-2 a severe challenge due to local limit ~ 5-10 MW m-2 of actively cooled Plasma Facing Components Sdivertor/ Sblanket~ 10% Regardless of Configuration ARIES-CS ARIES-ST ARIES-AT http://aries.ucsd.edu/ARIES/DOCS/bib.shtml

21. Plasma power exhaust is a complex, dealing with many facets of plasma and confinement physicsOngoing efforts required for confidence in extrapolation

22. Plasma power exhaust is a complex, dealing with many facets of plasma and confinement physicsOngoing efforts required for confidence in extrapolation Issues • Pedestal stability • Neo-classical drifts • Turbulent cross-field transport • // to B (Spitzer, pressure balance) • Atomic physics  dissipation • Sheath physics M. Makowski NO4 Wed. AM

23. Plasma power exhaust is a complex, dealing with many facets of plasma and confinement physicsOngoing efforts required for confidence in extrapolation Issues T. Eich IAEA 2012 • Pedestal stability • Neo-classical drifts • Turbulent cross-field transport • // to B (Spitzer, pressure balance) • Atomic physics  dissipation • Sheath physics R. Goldston TP8.00034 Thur AM

24. Plasma power exhaust is a complex, dealing with many facets of plasma and confinement physicsOngoing efforts required for confidence in extrapolation Issues • Pedestal stability • Neo-classical drifts • Turbulent cross-field transport • // to B (Spitzer, pressure balance) • Atomic physics  dissipation • Sheath physics J. Terry, S. Zweben, G. McKee

25. Plasma power exhaust is a complex, dealing with many facets of plasma and confinement physicsOngoing efforts required for confidence in extrapolation Issues • Pedestal stability • Neo-classical drifts • Turbulent cross-field transport • // to B (Spitzer, pressure balance) • Atomic physics  dissipation • Sheath physics Whyte PSI 2012

26. Plasma power exhaust is a complex, dealing with many facets of plasma and confinement physicsOngoing efforts required for confidence in extrapolation Issues • Pedestal stability • Neo-classical drifts • Turbulent cross-field transport • // to B (Spitzer, pressure balance) • Atomic physics  dissipation • Sheath physics Fenstermacher Phys. Plasma 4 (1997)

27. Plasma power exhaust is a complex, dealing with many facets of plasma and confinement physicsOngoing efforts required for confidence in extrapolation Issues • Pedestal stability • Neo-classical drifts • Turbulent cross-field transport • // to B (Spitzer, pressure balance) • Atomic physics  dissipation • Sheath physics D. Brunner APS 2010

28. Plasma and PFC geometry critical in power exhaust  Tradeoffs between flux expansion & tile alignment Vertical Target In C-Mod Snowflake in NSTX • Magnetic topology altered to increase flux expansion at target + promote radiative dissipation • Vertical target • Snowflake • Super-X • But such flux-expansion = 1 degree incident B angle • Must never get leading edges! B. Lipschultz APS2012 V. Soukhanovskii PSI 2012

29. The available operation window that satisfies heat exhaust and core requirements will be smaller in reactorsExcellent use/control of radiated species required DIII-D: Hybrid H-mode + Ar radiative divertor • Divertor solutions require high density: Tdiv ~ 1/ nSOL2 &Prad ~ n2 Petrie IAEA FEC 2006

30. The available operation window that satisfies heat exhaust and core requirements will be smaller in reactorsExcellent use/control of radiated species required Hughes NF 51 (2011) DIII-D: Hybrid H-mode + Ar radiative divertor • Divertor solutions require high density: Tdiv ~ 1/ nSOL2 &Prad ~ n2 • But reactor SOL has important tradeoffs • Greenwald ne limit • Current drive efficiency ICD/P~ 1 / ne • Pedestal/separatrix pressure? • Confinement (Pped) vs. core Prad Pouter-div / Pheat Alcator C-Mod: Impurity seeding reducing Pdiv at P/S ≤ 0.72 MWm-2 @ H98 ~ 1 Petrie IAEA FEC 2006

31. A reactor requires high plasma pressure & energy density which will damage materials if transiently released

32. A reactor requires high plasma pressure & energy density which will damage materials if transiently releasedReactor transient challenge 3x ITER TungstenBeforeexposure ARIES-ST ARIES-AT Ideal W Melt limit ARIES-RS Transient energy density (Wth/S/s1/2) After5 “large” ELMs ~30 MJ/m2/s1/2 Realistic W Transient limit ITER 30 mm Klimov PSI 2008

33. Critical development for MFE reactors: Regimes with adequate pedestal but which are not regulated by large instability I-mode on C-Mod /w RF only QH-mode on DIII-D /w τexternal~0 K. Burrell IAEA 2012 A, HubbardIAEA 2012

34. Critical development for MFE reactors: Regimes with adequate pedestal but which are not regulated by large instability I-mode on C-Mod /w RF only QH-mode on DIII-D /w τexternal~0 + external methods such as pacing & RMP coils  But all choices have design consequences in reactor

35. Recent example of JET changing to metal (ITER-like) wall re-emphasizes critical role of PFC choice in global performance & scenarios • Motivated by tritium retention concerns in ITER for carbon • X10 reduction found with metals • But also modifications to H-mode threshold and to pedestal! R. Neu GI-2 Tues AM

36. Recent example of JET changing to metal (ITER-like) wall re-emphasizes critical role of PFC choice in global performance & scenarios • Motivated by tritium retention concerns in ITER for carbon • X10 reduction found with metals • But also modifications to H-mode threshold and to pedestal! Neu XXX

37. Recent example of JET changing to metal (ITER-like) wall re-emphasizes critical role of PFC choice in global performance & scenarios • Motivated by tritium retention concerns in ITER for carbon • X10 reduction found with metals • But also modifications to H-mode threshold and to pedestal! Core, boundary and PFC are inextricably linked in a successful reactor design Neu XXX

38. Critical MFE reactordesign goals for PFCs • Continuous > 1 year (31,000,000 s) operation without need for replacement. • High resistance to erosion < mm /year by plasma ions and CX neutrals • Low tritium storage: < 1 kg in PFCs (safety and tritium economy) • Minimize free matter (dust, particulates, films) for safety + plasma

39. Power density + duration • Divertor PFC must be thin ~ 2-5 mm for heat conduction • q=10 MW/m2 ~ 100 K/mm for W

40. Power density + duration • Divertor PFC must be thin ~ 2-5 mm for heat conduction • q=10 MW/m2 ~ 100 K/mm for W • Tdiv< 10 eV required for heat exhaust  but still ~ 10 MW/m2 • Yearly fluence of D-T-He ions through sheath: 4x1031 ions / m2 • Areal density of 1 mm solid:6x1025 W atoms / m2

41. Power density + duration:Divertor erosion resistance for > year-long viability poses a severe challenge  PFC choice of tungsten • Divertor PFC must be thin ~ 2-5 mm for heat conduction • q=10 MW/m2 ~ 100 K/mm for W • Tdiv< 10 eV required for heat exhaust  but still ~ 10 MW/m2 • Yearly fluence of D-T-He ions through sheath: 5x1031 ions / m2 • Areal density of 1 mm solid:6x1025 atoms / m2 • Conclusion: net loss “yield” of divertor atoms < 10-6 atom/ion • Sputtering must be turned off.

42. Power density + duration:Divertor erosion resistance for > year-long viability poses a severe challenge  PFC choice of tungsten Tungsten yield In ASDEX-U divertor • Divertor PFC must be thin ~ 2-5 mm for heat conduction • q=10 MW/m2 ~ 100 K/mm for W • Tdiv< 10 eV required for heat exhaust  but still ~ 10 MW/m2 • Yearly fluence of D-T-He ions through sheath: 5x1031 ions / m2 • Areal density of 1 mm solid:6x1025 atoms / m2 • Conclusion: net loss “yield” of divertor atoms < 10-6 atom/ion • Sputtering must be turned off. Achieving Tdiv<10 eV necessary For both erosion & heat flux control Krieger JNM 266-269 (1999)

43. Campaign integrated divertor tungsten erosion in present devices ASDEX-Upgrade Alcator C-Mod W film On C Solid W 1020W m-2 Barnard JNM 415 (2011) Mayer Phys. Scr. T138 (2009)

44. Even with tungsten’s superior erosion resistance, demonstrated divertor erosion rate must be decreased further • Campaign integrated using wide variety of divertor plasmas • But reactor must have ndiv~1021 m-3 Tdiv~10 eV continuously. • Erosion / deposition from complex interplay of erosion + transport • Calls for in-situ erosion measurement 1M Mayer et al., Phys. Scr. T128 (2007) 106 2H. Barnard et al., J. Nucl. Mater. 415 (2011) S301

45. In-situ diagnostics must be developed to provide instantaneous erosion rates in our present confinement devicesAGNOSTIC on C-Mod • RFQ linear accelerator injects 0.9 MeV D+ beam into vacuum vessel • Tokamak B files provide steering • D+ induces high-Q nuclear reactions in PFC surfaces producing ~MeV neutrons and gammas • Advanced in-vessel neutron and gamma spectroscopy + GEANT4 model Hartwig, et al, Rev. Sci. Instrum 2010

46. In-situ diagnostics must be developed to provide instantaneous erosion rates in our present confinement devicesFirst in-situ surface measurements Summer 2012 Boron Gamma spectra resulting from D-induced nuclear reactions at two poloidal locations e+ annihilation Oxygen

47. Charge-exchange neutral erosion of first-wall material  long-range migration  how to deal with tonnes of “debris”? • Diffuse, energetic CX neutrals impinge on blanket surface. • Likely migration result: // transport to divertor Behrisch JNM 313-316 (2003)

48. The plasma-surface interface is perturbing & complex e.g. the divertor surface is reconstituted ~100 times per second Wirth, Whyte, et al MRS 2011

49. The plasma-material interface never comes to full equilibrium because the fusion environment perpetually evolves the surface and material properties

50. And even with a constant plasma, erosion/deposition changes material temperatures  Evolving, exponentially sensitive boundary response Arrheniusrates