The Challenges of Plasma-Surface Interactions in Magnetic Fusion

1. The Challenges of Plasma-Surface Interactions in Magnetic Fusion Dennis Whyte, MIT MIT ANS Seminar April 9, 2007

2. Worst Jobs in Science Popular Science Oct. 2003 x = Why don’t we have fusion reactors yet?

3. Materials impose one of our biggest challenges in developing fusion energy • Magnetic fusion produces athermonuclearvolumetric energy source, where all energy must be extracted through a single, surrounding material surface. • The nuclear fusionfuel cycle and the plasma physics of heat and particle exhaust simultaneously place severe demands on material surfaces. • The strongly coupled interaction between fusion plasmas and the material walls challenges our ability to control and predict plasma-surface interactions (PSI). • Significant worldwide research progress and effort continues on the PSI requirements of the ITER prototype fusion reactor.

4. 6Li + n --> T + 4He + 4.8 MeV + others “Real” fusion fuel cycle:6Li + D = 24He + 22.4 MeV 30 Million years of world energy demand in oceans Tbred / Tburned > 1 Deuterium-Tritium fusion represents a nearly inexhaustible energy source. Fuels: Deuterium: abundant in sea waterTritium: Half-life~12 years…must be produced?

5. D-T fusion requires a confined thermonuclear plasma • E > 10,000 eV  ionization separates electrons and ions. • Coulomb scattering >> fusion reaction  thermalized plasma T ~ 10,000 eV ~ 100,000,000 K • Far out of thermodynamic equilibrium in terrestrial environmentplasma exhausts power at eneed confinement.

6. B Fusion energy by magnetic confinement • Exploits Lorentz force • No confinement // to B. • Fusion energy balance: • 4/5 in neutron leaves  energy • 1/5 in He++ confined by B to heat & sustains plasma. • Every Joule of fusion and exhaust power must be extracted through a single surrounding surface.

7. The toroidal “tokamak” is the most prevalent magnetic confinement geometry • Self-closing helical magnetic fields. • Stabilizing toroidalfield produced by external coils. • Bf 1 / R • Plasma current provides confining poloidal field against -B drifts. R I

8. Triple Product Energy Gain Self- sustaining Size& Field $$$ PowerBalance Power Density Confinement Kink Stability Pfusion n2 T2 Q=5 “burning” Plasma pressure, confinement and stability set the requirements for a fusion reactor.

9. Translation of Fusion Strategy • Build it big, because bigger things hold their heat longer! • Make the magnetic field as big as possible to hold onto those particles! • Heat the heck out of it to get 100,000,000 K and light the fire!

10. et voila, ITER • Mission: • “Burning plasma” Q=10 • Reactor level fusion power • Pfusion ~ 400 MW for 500 s • Heating = 40 MW • 20% duty cycle • Size R~6 m +Field B~6 T = 5 Billion dollars • 1000 m3 plasma • 1000 m2 plasma-facing wall

11. Like a car, there are several things you have to do to keep the fusion “engine” running. Fusion Car Power removal Coolant in block, no melting Fuel control. Gas tank full, no flooding Helium ash exhaust Tailpipe Plasma purity No dirt in cylinders Magnetic topology, plasma physics & the fusion fuel cycle set unique requirements for wall materials in fusion

12. Magnetic topology • Magnetic field line directly terminated on a solid has no confinement due to sink action of wall. • This separatrix flux surface is the primary interface between thermonuclear plasma and outside world. • (SOL) Scrape-Off Layer outside separatrix is the “exhaust pipe” of a fusion plasma. • Divertor targets.

13. Divertor magnetic topologyis presumed for most fusion reactors • SOL “fed” by cross-field heat and particle losses from “core” • Advantage • Concentrates power and particle exhaust (q, ) in one location. • Disadvantages: • Concentrates power and particle exhaust in one location. • Wastes magnetic volume. Ploss ~ 100-400 MW Core plasma SOL q//  B // surface qtarget, target

14. Divertor magnetic topologyis used in ITER (and probably a reactor) • ITER Magnetic Field-Line Geometry in SOL: • Bpoloidal / B ~ 1/10 • SOL: L// ~ 100 m • 2  R ~ 50 m ~ 1m ITER Divertor Cross-section

15. Extraordinary plasma heat conduction along B primarily set SOL properties • // e- Heat conductivity: • ITER energy exhaust: PLoss ~ 150 MW

16. Field lines Distorted surface “proud” to the field line receives q// ~ 1 GW/m2 and is immediately melted/ablated. Distorted divertor surface Constant heat removal at divetor surfaces is daunting: #1 priority in edge “design” Field lines Conforming divertor surface 2 R Melts 10 cm of tungsten in ~20 seconds !

17. Dilute plasma (n~1020 m-3) extinguished by small particulate injection.ITER example:10 mm3 “drop” of W ~ Ne,plasma The core fusion plasma has little tolerance to impurities C • Plasma quasi-neutrality sets strict limits for impurities • Fuel dilution. • Radiation energy losses. Mo W

18. Heat exhaust, Tmelt, material stress and heat conductivity set armour thickness • Limits material choices to refractory metals (W, Mo) or graphite. q Coolant substrate coolant plasma d

19. castellations W & C bonding technology capable of exhausting ~ 25 MW/m2 ITER prototype divertor module ~ 1m

20. Plasma instability leads to large transient heating: Requires high Tmelt materials • MHD instability = Destruction of nested flux surfaces. • High Te flux surfaces connect to wall. • tMHD ~ 0.0001 seconds • Edge Localized Mode: ELM • Global instability: Disruption • All energy is “held up” near surface Kruger, APS04

21. Materials pushed past their thermal limits even in present fusion devices. • Mo tile “limiter” positioned outside hot core plasma in MIT tokamak (C-Mod) • Plasma heat exhaust and “non-thermal” electron populations increase past Tmelt ~ 2900 K in < 2 seconds exposure. • Reactor must run 24/7.

22. Movie Clips of Alcator C-Mod

23. Understanding competition between “density-driven” radiation and “T-driven” conduction leads to safe shutdown technique Uniform radiation Prad n2 (1/T)a Localized heatconduction Q// k// T5/2 Solution: “Force” high density by massive impurity injection

24. Competition between “density-driven” radiation and “T-driven” conduction critical to benign energy dissipation

25. Prad ~ 3 TW ~ electricity output of US Bemelt ~ 100 kG Even ideal radiative energy dissipation can cause material melting in ITER. Whyte 2004 PSI

26. Global breeding ratio in Blanket Maximum allowed core helium fraction Measured helium (de)enrichment from core to divertor Allowed rate of Tritium Deposition in wall Particle control #2 priority: Fusion reactors do not burn their fuel efficiently, forcing very large recycling of the tritium fuel.

27. World tritium inventory impacts ability to “start” a fusion energy economy J. Schmidt, IEA 2005 • A 1000 MWe will burn / produce ~ 0.5 kg ~ 1 pound of Tritium per day. • The time window for non-T-breeding burners (e.g. ITER, CTF) is short. • Tritium Breeding Ratio of starting reactors must be > 1 or whole system will grind to halt from lack of fuel.

28. Developing neutron tolerant materials will probably be last problem solved for fusion • Uniform material bombardment by 14 MeV neutrons • ~ 1m thick blanket to thermalize, shield neutrons & breed Tritium • Displacements per atom in wall ~10-20 per year for 1 GW • Leads to serious thermal degradation of materials. • Internal p, He production by nuclear reactions

29. Developing neutron tolerant materials will probably be last problem solved for fusion • Uniform material bombardment by 14 MeV neutrons • ~ 1m thick blanket to thermalize, shield neutrons & breed Tritium • Displacements per atom in wall ~10-20 per year for 1 GWth • Leads to serious thermal degradation of materials. • Internal p, He production by nuclear reactions  not an issue in fission reactors due E • Solution will require dedicated experimental & modeling, probably exploiting self-annealing at high material temperatures

30. IFMIF Neutron irradiation tests of wall materials are needed for fusion reactors • International Fusion Materials Irradiation Facility will probably be part of broader fusion program. • Deuteron beam (0.25 MA, ~40 MeV) on flowing lithium target produces fast neutron spectrum • But only produces 0.5 L volume that will receive reactor-like neutron damage (~20 dpa/year)!

31. Plasma ions can rapidly sputter target material away. The material targets must also resist erosion caused by massive energetic particle throughput in wall • Vsheath ~ 5 x Te ~ 100-500 eV

32. D/T saturated deposits Material migration is set by sputtering / recycling asymmetry.

33. ne increasing Alcator C-Mod Divertor “detachment”: Critical to easing power exhaust and sputtering

34. DIII-D:Mapof divertor Erosion / deposition Detachment solved erosion? Whyte, IAEA 2000

35. DIII-D:Mapof divertor Erosion / deposition Detachment solved erosion? Yes!Yes, but another problem appeared Tritium trapped in plasma deposited films from other wall locations Rate ~ 1 in 10 fuelled T lost But requirement: < 1 in 1000 ! Solution: > 1000 K walls to deplete H/D/T ? Whyte, IAEA 2000

36. Turbulent cross-field particle transport Erosion sources outside divertor  Long-range transport to divertor S. Zweben, J. Terry, C-Mod A. Mclean, et al. DIII-D

37. C-Mod now shows a strong link between ballooning transport, rotation, long-range SOL transport, T retention and H-mode! Ballooning transport ALCATOR C-Mod, M.I.T.LaBombard, Greenwald APS 04 SOL flow Core Rotation

38. MAGNUM-PSI High power, low Te H plasmas Besides groundbreaking research on tokamaks like C-Mod, laboratory experiments are coming online to enhance our understanding of PSI DIONISOS Dynamics of PSI MIT FOM, Netherlands

39. MAGNUM-PSI High power, low Te H plasmas Besides groundbreaking research on tokamaks like C-Mod, laboratory experiments are coming online to enhance our understanding of PSI DIONISOS Dynamics of PSI MIT FOM, Netherlands

40. Worst Most Exciting Job in Science x Whyte Fusion Wall equation = A Grand Scientific Challenge for Ours & Future Generations

41. Size& Field $$$ The suppression of edge MHD ELMs is one of the most criticalresearch issues for ITER & beyond Eich 2004 PSI

42. Net erosion & deposition arises from ~1-10% local flux imbalance Local Impurity release via sputtering at PFC surface > 90% impurity re-deposited at surface Impurity ionization & transport near surface SOL transport Release of impurities to SOL Edge plasma modification by impurity Global Core plasma modification from impurities Net redeposition or erosion: Deposition rate – erosion rate

43. Divertors concentrate particle flux and recycling, making practical He and H pumping exhaust possible particle control • This is not a trivial feat; fuel particle inventory in a fusion device (ITER) is dominated by the wall. Nwall = f Awall ~ 5•1021 m-2 103 m2 ~ 5• 1024 m Nplasma = n Vplasma ~ 1020 m-3 103 m3 ~ 1023 m

44. Ploss SOL Core plasma q//  Bf G// surface qtarget, Gtarget ionization Gion,e GH Parallel heat conduction sustains particle ionization / recycling loop in divertor. • Consider simple pressure and particle conservation /w Bohm sheath criterion (vtarget = cs) • Two-point model (Stangeby) • Upstream: SOL • Target: Divertor

45. Competition between “density-driven” radiation and “T-driven” conduction critical to benign energy dissipation