the future of boundary plasma and material science · the future of boundary plasma and material...
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1 Sherwood Whyte April 2012
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
2 Sherwood Whyte April 2012
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.
3 Sherwood Whyte April 2012
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.
COMMENTS ���
- Boundary/PMI science is too broad to be inclusive of every topic of interest���
- The following comments reflect my personal views on critical paths forward in both experiment, theory and computation
4 Sherwood Whyte April 2012
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.
5 Sherwood Whyte April 2012
Demo constants: T > 1000K, Pheat/S ~ 1 MW/m2 for 30,000,000 seconds.���
ITER falls far short ITER ARIES-AT ARIES-CS ARIES-ST
Duration (s) 400 3x107 3x107 3x107
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
Pfusion / S (MW/m2) ~1 4.3 2.6-5.4 4.9
P/S (MW/m2) 0.21 0.85 0.7-1.1 0.99 P/Adiv (MW/m2) 2.4 10 >20 20
Adivertor / S ~ 5-10%
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Boundary/PMI Science “Gap” to FNSF/Reactors is More like a 3-D Chasm
7 Sherwood Whyte April 2012
Boundary/PMI Science “Gap” to FNSF/Reactors is More like a 3-D Chasm
Why these axes?
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The PMI Science Challenge & Fusion Viability are inextricably linked
Fusion Viability���
1. Average neutron���power loading ~ 4 MW/m2
PSI Challenge���
1. Global average exhaust power��� P/S ~ 1 MW/m2
9 Sherwood Whyte April 2012
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/m2 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.
10 Sherwood Whyte April 2012
The PMI Science Challenge & Fusion Viability are inextricably linked
Fusion Viability���
1. Average neutron���power loading ~ 4 MW/m2
2. Continuous 24/7 power production.
PSI Challenge���
1. Global average exhaust power��� P/S ~ 1 MW/m2
2. Global energy
throughput ���> 30 TJ/m2 delivered by plasma
11 Sherwood Whyte April 2012
Erosion limits are set by complex PMI interplay ���& total energy throughput:
Extrapolation from present devices to FNSF/reactors at least x10,000
300 s 4,300 s 9,000 s 2,000 s 22,000 s
The wall surface never truly equilibrates because erosion cannot be turned off at all surfaces.
Tungsten main-wall: ~1-10 tons of erosion from charge-exchange neutrals
12 Sherwood Whyte April 2012
Material limits set by complex PMI & total energy throughput: Extrapolation from present devices to FSNF/reactors at least x10,000
300 s 4,300 s 9,000 s 2,000 s 22,000 s
Example of 10 micron W surface microstructure over ~1/4 day in PISCES lab plasma at 1100 K
Micron deep W fuzz grown in Alcator C-Mod divertor in ~10 seconds at 1500 K!
Baldwin et al PSI 2008 Wright et al NF 2012
13 Sherwood Whyte April 2012
The PMI Science Challenge & Fusion Viability are inextricably linked
Fusion Viability���
1. Average neutron���power loading ~ 4 MW/m2
2. Continuous 24/7 power production.���
3. Thermo-dynamics demand high ambient temperature .
PSI Challenge���
1. Global average exhaust power��� P/S ~ 1 MW/m2
2. Global energy
throughput ���> 30 TJ/m2 delivered by plasma���
3. Fundamental new regime of physical chemistry for plasma-facing materials.
14 Sherwood Whyte April 2012
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
Rates ∝ exp − Eo
Tmaterial
⎛⎝⎜
⎞⎠⎟
≈ exp − 11,600K(500 −1000)K
⎛⎝⎜
⎞⎠⎟
Arrenhius equation
15 Sherwood Whyte April 2012
Example from PISCES test-stand: Nano-”fuzz” highly T dependent
Tungsten surface after exposure to ~1 hour Helium plasma.
900 K
1120 K
1320 K Baldwin et al PSI 2008
16 Sherwood Whyte April 2012
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.
17 Sherwood Whyte April 2012
The plasma-surface interface is perturbing & complex e.g. the divertor surface is reconstituted ~100 times per second
ordered crystal
Simplified Surface Picture
sputteredimurity atom
Realistic Surface Picture +
-
sheath potential
chemicalremoval
implantationreflection
secondaryelectronemission
sputteringte gnggngg
ionization
redeposition
charge-exchange
Fuel RecyclingMaterial RecyclingIon impactLong-rangematerial transport
ionization
dissociation
recombination
++
+ +
++
++
+ +
vacancy/void defectsfrom ion andneutron radiation
fuel diffusion &permeation
surface fuel saturationbubbles &blisters
amorphousfilm growth
H/D/T fuel ion PFC material atomH/D/T fuel neutral atomPFC material ionElectron Redeposited PFC material atom+ +
fuel trappingat defects
fuelcodeposition
nm
mm
surface
excitatio
n!
Wirth, Whyte, et al MRS 2011
18 Sherwood Whyte April 2012
PMI/Boundary plasmas in a confinement device set by coupled, multi-scale processes
19 Sherwood Whyte April 2012
The “Core” of Multi-scale PMI Science is ���Hyper-Sensitive to Material Temperature
Rates ∝ exp − Eo
Tmaterial
⎛⎝⎜
⎞⎠⎟
≈ exp − 11,600K(500 −1000)K
⎛⎝⎜
⎞⎠⎟
Arrenhius rates
20 Sherwood Whyte April 2012
The Plasma-Surface Interaction Science Center:���addressing multiscale diagnosis
http://psisc.org
21 Sherwood Whyte April 2012
The Plasma-Surface Interaction Science Center: addressing multiscale modeling & simulation
http://psisc.org
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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
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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
24 Sherwood Whyte April 2012
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
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Example: Complete synthetic diagnostic of Boron film thickness in Alcator C-Mod
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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.
27 Sherwood Whyte April 2012
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
28 Sherwood Whyte April 2012
Lessons about using dimensionless similarity in core
• Critical dimensionless parameters are posited based on physical reasoning (without proof), for example Kadomtsev constants
Mi
M p
~ aR
q ~ BTBP β ~ nT
B2ν* ~ nR
T 2 ρ* ~ T1/2
BR
n ~ R−2 T ~ R−1/2 B ~ R−5 /4
Leads to size scaling of plasma parameters
29 Sherwood Whyte April 2012
Lessons about using dimensionless similarity in core
• Critical dimensionless parameters are posited based on physical reasoning (without proof), for example Kadomtsev constants
Mi
M p
~ aR
q ~ BTBP β ~ nT
B2ν* ~ nR
T 2 ρ* ~ T1/2
BR
n ~ R−2 T ~ R−1/2 B ~ R−5 /4
Leads to size scaling of plasma parameters
• 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 ~ B4
• 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)
30 Sherwood Whyte April 2012
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/Eatomic=cst. à T = cst.
Global power balance // Spitzer conduction // Pressure balance
“P/R” scaling
• Much is implicit in P/R scaling! • Radial power width λr ~ 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/m2
• 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)
31 Sherwood Whyte April 2012
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/Eatomic=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 λr ~ 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/m2 in reactor
• Aspect ratio must be matched (ST does not simulate AT reactor) • Density must be much larger in smaller device (current drive)?
Not practical
32 Sherwood Whyte April 2012
Basic argument: If atomic physics is important in boundary plasma then surely PMI is too!
Which dimensionless parameters?
33 Sherwood Whyte April 2012
Basic argument: If atomic physics is important in boundary plasma then surely PMI is too!
Material removal through sputtering
ED+
EB
~ TeEB
Yphys ~ f (T e
EB
, MD
MW
)
Ychem ~ f (T e
EB
, MD
MW
,TWEB
)
34 Sherwood Whyte April 2012
Basic argument: If atomic physics is important in boundary plasma then surely PMI is too! ���
Electrostatic redeposition
λMFP ~MWEW( )1/2ne SW
λMFP
LDebye~ EW
1/2 MW−1/2 ne
−1/2 Te−1/2SW
−1
λMFP
LPresheath~ λMFP
ρH+
~ B EW1/2 MW
−1/2 ne−1 Te
−1/2 SW−1
35 Sherwood Whyte April 2012
Basic argument: If atomic physics is important in boundary plasma then surely PMI is too! ���
Gyro-orbit redeposition
λMFP
ρW +
~ B MW−1 ne
−1 SW−1
Reactor divertor n ~ 1021 m-3 T ~ 10 eV B ~ 6 T
One surface atom can theoretically undergo ~billion of these cycles in one year.
36 Sherwood Whyte April 2012
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
37 Sherwood Whyte April 2012
Basic argument: If atomic physics is important in boundary plasma then surely PMI is too! ���
Plasma & ambient T à material physics TWEW
~ n T 3/2 B⊥
BδWκW
+Tambient
σ Thermal
σ Yield
~ n T 3/2 B⊥
BδWRW
DH inW ∝ exp −EW , HTW
⎛⎝⎜
⎞⎠⎟
38 Sherwood Whyte April 2012
Basic argument: If atomic physics is important in boundary plasma then surely PMI is too! ���
Plasma & ambient T à material physics TWEW
~ n T 3/2 B⊥
BδWκW
+Tambient
σ Thermal
σ Yield
~ n T 3/2 B⊥
BδWRW
DH inW ∝ exp −EW , HTW
⎛⎝⎜
⎞⎠⎟
39 Sherwood Whyte April 2012
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
40 Sherwood Whyte April 2012
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/m2) 3. Magnetic field B (~6-7 Tesla) of reactor { λp ~ 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 7th 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!)
41 Sherwood Whyte April 2012
“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
42 Sherwood Whyte April 2012
Vulcan design scope:���R=1.2 m, PLHCD~20 MW
Vulcan Special Issue FED 2012
43 Sherwood Whyte April 2012
Double-can vacuum vessel:���High Temperature wall
• Points here
44 Sherwood Whyte April 2012
And the MOST important material��� in magnetic fusion?
45 Sherwood Whyte April 2012
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
46 Sherwood Whyte April 2012
YBCO Superconductor tapes à Demountable SC coils à Vertical lift-off maintenance
• Points here
47 Sherwood Whyte April 2012
YBCO Superconductor tapes à Demountable SC coils à Vertical lift-off maintenance
• Points here
48 Sherwood Whyte April 2012
Double-can vacuum vessel:���High T-wall + eliminate sector maintenance
• Points here
49 Sherwood Whyte April 2012
Double-can vacuum vessel:���High T-wall + eliminate sector maintenance
• Points here
50 Sherwood Whyte April 2012
VULCAN ���The 24/7 PMI “wind-tunnel”���
p.s. we should design ST and stellarator versions too!
• Points here
Vulcan Special Issue FED 2012
51 Sherwood Whyte April 2012
Vulcan addresses the PMI chasms ���to FNSF/reactors
52 Sherwood Whyte April 2012
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, ne, Te, 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
53 Sherwood Whyte April 2012
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.
54 Sherwood Whyte April 2012
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)���
λSOL ~aI p~ 1Bp
Makowski, et al APS 2011
55 Sherwood Whyte April 2012
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)���
• Yet the separatrix pressure is well constrained Ø Pedestal stability: psep ~ 5% pped Ø Power exhaust: P ~ λSOL psep T1/2
56 Sherwood Whyte April 2012
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: psep ~ 5% pped Ø Power exhaust: P ~ λSOL psep T1/2���
• A 1 mm SOL width in ITER
would require a separatrix pressure equal to that at the top of the pedestal??
57 Sherwood Whyte April 2012
The inevitable x2-3 increase in areal energy density from ITER à reactor will disallow ���
any significant instability
30 mm
�
Wth
Awall τ1/ 2 ~
pVA(R /cs)
1/ 2 ~ Pfusion1/ 2 εR1/ 2
Material Tmax (K)
Limit MJ m-2 s-1/2
Be 1550 8 C 4000 42 W 3680 45
ITER
ARIES-RS
ARIES-AT ARIES-ST
Limit
Material thermal limits
58 Sherwood Whyte April 2012
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)
30 mm
�
Wth
Awall τ1/ 2 ~
pVA(R /cs)
1/ 2 ~ Pfusion1/ 2 εR1/ 2
ITER
ARIES-RS
ARIES-AT ARIES-ST
Limit
Tungsten���Before���
exposure
After���5 “large”
ELMs ~30 ���
MJ/m2/s1/2
59 Sherwood Whyte April 2012
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
60 Sherwood Whyte April 2012
Magnetic Fusion Plasma Design ���Report Card
Design issues from ���core à edge
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
61 Sherwood Whyte April 2012
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.
62 Sherwood Whyte April 2012
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