The Future of Boundary Plasma and Material Science

The Future of Boundary Plasma and Material Science Dennis Whyte Plasma Science & Fusion Center, MIT, Cambridge USA Director, Plasma Surface Interaction Science Center (psisc.org) APS Sherwood Meeting of Fusion Theory Atlanta, April 2012 1

Outline Defining the Challenge for Fusion Energy Boundaries The Multiscale Science of Plasma-Material Interactions Ø Processes, measurement and exposure Developing a dimensionless parameter wind-tunnel for fusion PMI Critical needs for boundary plasma understanding & prediction. 2

Outline Defining the Challenge for Fusion Energy Boundaries COMMENTS The Multiscale Science of Plasma-Material Interactions - Ø Boundary/PMI Processes, measurement science and exposure is too broad to be inclusive of every topic of interest Developing a dimensionless parameter wind-tunnel for fusion PMI - The following comments reflect my personal views on critical paths forward in both experiment, theory and computation Critical needs for boundary plasma understanding & prediction. 3

Demo constants: T > 1000K, P heat /S ~ 1 MW/m 2 for 30,000,000 seconds. ITER falls far short ITER ARIES-AT ARIES-CS ARIES-ST Duration (s) 400 3x10 7 3x10 7 3x10 7 Ambient T (K) 400 1300 1000 900 R (m) 6.2 5.2 7.8 3.2 R/a 3.1 4.0 4.6 1.6 P fusion / S (MW/m 2 ) ~1 4.3 2.6-5.4 4.9 P/S (MW/m 2 ) 0.21 0.85 0.7-1.1 0.99 P/A div (MW/m 2 ) 2.4 10 >20 20 A divertor / S ~ 5-10% 5

Boundary/PMI Science Gap to FNSF/ Reactors is More like a 3-D Chasm 6

Boundary/PMI Science Gap to FNSF/ Reactors is More like a 3-D Chasm Why these axes? 7

The PMI Science Challenge & Fusion Viability are inextricably linked Fusion Viability 1. Average neutron power loading ~ 4 MW/m 2 PSI Challenge 1. Global average exhaust power P/S ~ 1 MW/m 2 8

PFCs must be thin (~5 mm) to satisfy heat exhaust but thick to resist erosion & material removal & Continually maintain conformability to B field Steady-state 10 MW/m 2 heat exhaust pushes high- T He gas cooling to limits, no allowance for transients. Small Transient heat loading limits lifetime of even best materials While loss of conforming surface to B greatly accelerates loss of PFC viability & severe plasma effects. 9

The PMI Science Challenge & Fusion Viability are inextricably linked Fusion Viability 1. Average neutron power loading ~ 4 MW/m 2 2. Continuous 24/7 power production. PSI Challenge 1. Global average exhaust power P/S ~ 1 MW/m 2 2. Global energy throughput > 30 TJ/m 2 delivered by plasma 10

Erosion limits are set by complex PMI interplay & total energy throughput: Extrapolation from present devices to FNSF/reactors at least x10,000 Tungsten main-wall: ~1-10 tons of erosion from charge-exchange neutrals 300 s 2,000 s 4,300 s 9,000 s 22,000 s The wall surface never truly equilibrates because erosion cannot be turned off at all surfaces. 11

Material limits set by complex PMI & total energy throughput: Extrapolation from present devices to FSNF/reactors at least x10,000 300 s 2,000 s 4,300 s 9,000 s Example of 10 micron W surface microstructure over ~1/4 day in PISCES lab plasma at 1100 K Baldwin et al PSI 2008 22,000 s Micron deep W fuzz grown in Alcator C-Mod divertor in ~10 seconds at 1500 K! Wright et al NF 2012 12

The PMI Science Challenge & Fusion Viability are inextricably linked Fusion Viability 1. Average neutron power loading ~ 4 MW/m 2 2. Continuous 24/7 power production. 3. Thermodynamics demand high ambient temperature. PSI Challenge 1. Global average exhaust power P/S ~ 1 MW/m 2 2. Global energy throughput > 30 TJ/m 2 delivered by plasma 3. Fundamental new regime of physical chemistry for plasmafacing materials. 13

Required High-T walls present a fundamentally new regime of physical chemistry for PMI science that has not even been approached in an integrated manner Arrenhius equation Rates exp exp E o T material 11,600K (500 1000)K 14

Example from PISCES test-stand: Nano- fuzz highly T dependent 900 K Tungsten surface after exposure to ~1 hour Helium plasma. 1120 K 1320 K Baldwin et al PSI 2008 15

The plasma-surface interface is perturbing & complex e.g. the divertor surface is reconstituted ~100 times per second Simplified Surface Picture + ordered crystal sputtered imurity atom mm surface nm Realistic Surface Picture + Long-range Ion impact Material Recycling Fuel Recycling material transport chemical removal secondary electron reflection emission implantation + + + sputteringte ionization γ excitation + redeposition chargeexchange + ionization + dissociation recombination + + + sheath potential - fuel codeposition surface fuel saturation fuel diffusion & permeation fuel trapping at defects vacancy/void defects from ion and neutron radiation bubbles & blisters amorphous film growth Electron + H/D/T fuel ion + PFC material ion H/D/T fuel neutral atom PFC material atom Redeposited PFC material atom Wirth, Whyte, et al MRS 2011 17

PMI/Boundary plasmas in a confinement device set by coupled, multi-scale processes 18

The Core of Multi-scale PMI Science is Hyper-Sensitive to Material Temperature Arrenhius rates Rates exp exp E o T material 11,600K (500 1000)K 19

The Plasma-Surface Interaction Science Center: addressing multiscale diagnosis http://psisc.org 20

The Plasma-Surface Interaction Science Center: addressing multiscale modeling & simulation http://psisc.org 21

We seriously think we can figure out this mess by measuring surfaces every year or so in tokamaks?* * What we do now http://psisc.org 22

AGNOSTIC: Proof-of-principle diagnostic development on Alcator C-Mod to provide first shot-to-shot diagnosis of plasma-facing surfaces (4) Advanced in-vessel neutron and gamma spectroscopy, unoflred with GEANT4, maps all surface properties (depth resolved!)" Hartwig, et al, Rev. Sci. Instrum 2010 23

AGNOSTIC* requires leading edge nuclear transport modeling and simulations Full 3-D model of tokamak GEANT4 simulation of Scintillation detection *Accelerator-based Gamma and Neutron Observing Surface-diagnosing Tool for In-situ Components 24

Example: Complete synthetic diagnostic of Boron film thickness in Alcator C-Mod 25

Proposal: Use dimensionless similarity to study coupled issues of edge plasma, PMI and materials in a scaled-down device* Dimensionless parameter scaling techniques are a powerful tool to study complex physical systems (e.g. wind-tunnel for aeronautics) Ø Especially in tokamak fusion experiments where full-size cost is prohibitive. Objective: provide similarity for critical parameters in reactor while avoiding technology limits in scaled-down device Ø Full similarity is not possible Ø The well-known P/R divertor scaling does not meet these objectives A new P/S scaling (actually a set of requirements) provides fidelity to reactor divertor conditions in a small device which is used as the physics basis for Vulcan. * Special Issue on the Vulcan Conceptual Design, Fusion Engineering Design March 2012 27

Lessons about using dimensionless similarity in core Critical dimensionless parameters are posited based on physical reasoning (without proof), for example Kadomtsev constants M i M p ~ a R q ~ B T B P β ~ nt B 2 ν * ~ nr ρ * ~ T 1/2 T 2 BR Leads to size scaling of plasma parameters n ~ R 2 T ~ R 1/2 B ~ R 5/4 But now the reality of the scaling effort must be accounted 1) Magnetic field B has a hard technology limit at fixed aspect ratio 2) Reactor must max. B since power density ~ B 4 Therefore full matching is not practically useful: what to relax? One chooses rho* based on physical reasoning Far below unity and therefore avoids any threshold effect. Is practically difficult to vary in one device. N.B.: this practical strategy leads to experimental validation* * Luce et al PPCF 50 (2008) 29

The challenge is that many more parameters become important in boundary / PMI. Cleverness in similarity is mandatory Lackner and others (90 s) made reasonable argument that atomic physics important in SOL: posited T/E atomic =cst. à T = cst. Global power balance // Spitzer conduction // Pressure balance P/R scaling Much is implicit in P/R scaling! Radial power width λ r ~ R, which requires q // ~ 1/R!! This guarantees cannot implement P/R scaling in a scaled-down device since power density must be near technology limit ~10 MW/m 2 Aspect ratio must be matched (ST does not simulate AT reactor) Density must be much larger in smaller device (current drive)? Lackner, Cont. Plasma Physics 15 (1994), Whyte, et al Fus. Eng. Des. (2012) 30

The challenge is that many more parameters become important in boundary / PMI. Cleverness in similarity is mandatory Lackner and others (90 s) made reasonable argument that atomic physics important in SOL: posited T/E atomic =cst. à T = cst. Global power balance // Spitzer conduction // Pressure balance P/R scaling N.B. much implicit in P/R scaling! Radial power width λ r ~ R, which requires q // ~ R -1!! This guarantees cannot implement P/R scaling in a scaled-down device since power density must be near technology max. ~ GW/m 2 in reactor Not practical Aspect ratio must be matched (ST does not simulate AT reactor) Density must be much larger in smaller device (current drive)? 31

Basic argument: If atomic physics is important in boundary plasma then surely PMI is too! Which dimensionless parameters? 32

Basic argument: If atomic physics is important in boundary plasma then surely PMI is too! Material removal through sputtering E D+ E B ~ T e E B Y phys ~ f ( T e E B, M D M W ) Y chem ~ f ( T e E B, M D M W, T W E B ) 33

Basic argument: If atomic physics is important in boundary plasma then surely PMI is too! Electrostatic redeposition ( λ MFP ~ M E W W ) 1/2 n e S W λ MFP ~ E 1/2 W M 1/2 W n 1/2 e T 1/2 1 e S W L Debye λ MFP L Presheath ~ λ MFP ρ H + ~ B E 1/2 W M 1/2 W n 1 e T 1/2 1 e S W 34

Basic argument: If atomic physics is important in boundary plasma then surely PMI is too! Gyro-orbit redeposition λ MFP ρ W + ~ B M 1 W n 1 1 e S W Reactor divertor n ~ 10 21 m -3 T ~ 10 ev B ~ 6 T One surface atom can theoretically undergo ~billion of these cycles in one year. 35

PMI figures of merit in reactor à Must match divertor n, T, B in scaled down device to avoid thresholds in figures of merit à But relaxed divertor collisionality OK Vulcan Special Issue FED 2012 36

Basic argument: If atomic physics is important in boundary plasma then surely PMI is too! Plasma & ambient T à material physics T W E W ~ n T 3/2 B B δ W κ W + T ambient σ Thermal σ Yield ~ n T 3/2 B B δ W R W D H in W exp E W, H T W 37

Basic argument: If atomic physics is important in boundary plasma then surely PMI is too! Plasma & ambient T à material physics TW 3/2 B δ W ~nt + Tambient EW B κw σ Thermal 3/2 B δ W ~nt σ Yield B RW DH in W EW, H exp TW 38

Material physics figures of merit in reactor à Must match divertor n, T, B AND ambient temperature in scaled down device Vulcan Special Issue FED 2012 39

Proposed P/S scaling rules provide matched divertor/ SOL parameters in scaled-down device à reactor PMI wind-tunnel VULCAN 1. Non-inductive steady-state operation (arbitrary long pulses) 2. Areal heating power density P/S (~1 MW/m 2 ) 3. Magnetic field B (~6-7 Tesla) of reactor { λ p ~ R through ballooning limit} 4. Geometry matched: R/a, q, L // /R, etc. 5. Core density: n ~ R -2/7 6. Ambient wall temperature matched (> 500 C) With an implicit 7 th requirement that embodies the philosophy of the scaling law: 7. The scaling laws must actually allow for the construction and operation of the scaled down device (duh!) 40

P/S scaling : A practical approach to providing a high fidelity reactor PMI wind-tunnel P/R inherently fails to match atomic physics (n~r -1 ) & cannot be operated due to violation of heat flux limits 41

Vulcan design scope: R=1.2 m, P LHCD ~20 MW Vulcan Special Issue FED 2012 42

Double-can vacuum vessel: High Temperature wall Points here 43

And the MOST important material in magnetic fusion? 44

And the MOST important material in magnetic fusion? The MAGNET! Points here YBCO high-t superconductors coils could revolutionize magnetic fusion by up to x2 increase in B 45

YBCO Superconductor tapes à Demountable SC coils à Vertical lift-off maintenance Points here 46

YBCO Superconductor tapes à Demountable SC coils à Vertical lift-off maintenance Points here 47

Double-can vacuum vessel: High T-wall + eliminate sector maintenance Points here 48

Double-can vacuum vessel: High T-wall + eliminate sector maintenance Points here 49

VULCAN The 24/7 PMI wind-tunnel p.s. we should design ST and stellarator versions too! Points here Vulcan Special Issue FED 2012 50

Vulcan addresses the PMI chasms to FNSF/reactors 51

The US and world will lose its first glimpse of a reactor divertor environment with the C-Mod termination on the eve of the hot W divertor Comments Bulk tungsten outer divertor from room temperature à 600 C /w reactor-like P/S, n e, T e, B Innovative divertor design: toroidally continuous aligned W surfaces à 0.5 degree grazing incidence à can actually exploit high flux expansion vertical or snowflake topology 52

We desperately need coherent data and a validated model for the SOL width Recent experiments across US devices indicates width only depends on poloidal field à 1 mm widths in ITER (like C- Mod) λ SOL ~ a I p ~ 1 B p Makowski, et al APS 2011 54

We desperately need cohesive data and a validated model for the SOL width Recent experiments across US devices indicates width only depends on poloidal field à 1 mm widths in ITER (like C- Mod) Yet the separatrix pressure is well constrained Ø Pedestal stability: p sep ~ 5% p ped Ø Power exhaust: P ~ λ SOL p sep T 1/2 A 1 mm SOL width in ITER would require a separatrix pressure equal to that at the top of the pedestal?? 56

The inevitable x2-3 increase in areal energy density from ITER à reactor will disallow any significant instability W th A wall τ 1/ 2 ~ pv A(R /c s ) ~ P 1/ 2 1/ 2 fusion ε R 1/ 2 ARIES-AT ARIES-ST Limit ARIES-RS Material thermal limits Material T max (K) Limit MJ m -2 s -1/2 Be 1550 8 C 4000 42 W 3680 45 ITER 30 mm 57

ELMs will not be allowed à ELMy H-mode is not a reactor relevant confinement regime à extremely high priority to develop intrinsically ELM-free pedestals (QH, I-mode) Tungsten Before exposure W th pv 1/ 2 1/ 2 ~ ~ P ε R fusion Awall τ 1/ 2 A(R /c s )1/ 2 ARIES-AT Limit After 5 large ELMs ARIES-ST ARIES-RS ITER 30 mm ~30 MJ/m2/s1/2 58

ELMs will not be allowed à ELMy H-mode is not a reactor relevant confinement regime à Extremely high priority to develop intrinsically ELM-free pedestals (QH, I-mode) Tungsten Before exposure After 5 large ELMs ~30 MJ/m2/s1/2 59

Magnetic Fusion Plasma Design Design issues from core à edge Report Card Experimental Demonstration Validated Predictive Theory/Simulation Core pressure/kink limits Current drive and bootstrap Pedestal stability boundary Self-regulated pedestal w/o ELMs X SOL heat width? X Divertor T and heat flux below limits X PMI & PFC response @ T > 500 C X X Erosion / redeposition control for 30,000,000 seconds + 20 dpa X X 60

Take away messages The boundary plasma and its material interface will continue to grow in importance and challenges for integrated fusion devices à reactor This is not simply a technology issue, there is no unobtainium, rather we must push ourselves to the knowledge frontiers of boundary plasma and material science. Fusion theory and computation must become more than plasma theory and will be critical in achieving success. 61

Extra Comments 62