The Future of Boundary Plasma and Material Science · The Future of Boundary Plasma and Material...

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


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.


Outline



•  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


Outline



•  Developing a dimensionless parameter “wind-tunnel” for fusion PMI •  Critical needs for boundary plasma understanding & prediction.


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%


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


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

Why these axes?


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


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.





2.  Continuous 24/7 power production.

PSI Challenge��


2.  Global energy

throughput ��> 30 TJ/m2 delivered by plasma


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


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





2.  Continuous 24/7 power production.��

3.  Thermo-dynamics demand high ambient temperature .

PSI Challenge��


2.  Global energy

throughput ��> 30 TJ/m2 delivered by plasma��

3.  Fundamental new regime of physical chemistry for plasma-facing materials.


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


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


Outline





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


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


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


The Plasma-Surface Interaction Science Center:��addressing multiscale diagnosis

http://psisc.org


The Plasma-Surface Interaction Science Center: addressing multiscale modeling & simulation

http://psisc.org


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


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


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


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


Outline





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


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


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)


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)


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


Basic argument: If atomic physics is important in boundary plasma then surely PMI is too!

Which dimensionless parameters?


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

)


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



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.


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



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

⎛⎝⎜

⎞⎠⎟


Material physics figures of merit in reactor à Must match divertor n, T, B AND ��

ambient temperature in scaled down device



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!)


“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


Vulcan design scope:��R=1.2 m, PLHCD~20 MW



Double-can vacuum vessel:��High Temperature wall

•  Points here


And the MOST important material�� in magnetic fusion?


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


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

•  Points here


Double-can vacuum vessel:��High T-wall + eliminate sector maintenance

•  Points here


VULCAN ��The 24/7 PMI “wind-tunnel”��

p.s. we should design ST and stellarator versions too!

•  Points here



Vulcan addresses the PMI chasms ��to FNSF/reactors


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


Outline





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


We desperately need coherent data and a validated model for the SOL width


•  Yet the separatrix pressure is well constrained Ø  Pedestal stability: psep ~ 5% pped Ø  Power exhaust: P ~ λSOL psep T1/2


We desperately need cohesive data and a validated model for the SOL width


•  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??


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


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


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


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


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.


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The Future of Boundary Plasma and Material Science · The Future of Boundary Plasma and Material...

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