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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. Overview.

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the role of the boundary plasma in defining the viability of magnetic fusion energy

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

overview
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
acknowledging the support of numerous colleagues in boundary pmi research
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
slide4
Fusion Boundary Challenge: Every Joule of energy is extracted through a distant surface with small area to volume ratio

Fission

Fusion

boundary challenge from physics viewpoint and physics does not stop at the material1
Boundary challenge from physics viewpoint:and physics does not stop at the material!

Progression of talk

boundary challenge from power plant viewpoint
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)

boundary challenge from power plant viewpoint1
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)

boundary challenge from power plant viewpoint2
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

boundary challenge from power plant viewpoint3
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

boundary challenge from power plant viewpoint4
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

boundary challenge from power plant viewpoint viability directly linked to boundary challenges
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

robust arguments hold regardless of configuration t 1000k p f s 3 4 mw m 2 30 000 000 s
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

slide17
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
slide18
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
slide19
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

slide20
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

slide21
Plasma power exhaust is a complex, dealing with many facets of plasma and confinement physicsOngoing efforts required for confidence in extrapolation
slide22
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

slide23
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

slide24
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

slide25
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

slide26
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)

slide27
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

plasma and pfc geometry critical in power exhaust tradeoffs between flux expansion tile alignment
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

slide29
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

slide30
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

slide31
A reactor requires high plasma pressure & energy density which will damage materials if transiently released
slide32
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

slide33
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

slide34
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

slide35
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

slide36
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

slide37
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

critical mfe reactor design goals for pfcs
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
power density duration
Power density + duration
  • Divertor PFC must be thin ~ 2-5 mm for heat conduction
    • q=10 MW/m2 ~ 100 K/mm for W
power density duration1
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
slide41
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.
slide42
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)

campaign integrated divertor tungsten erosion in present devices
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)

slide44
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

slide45
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

slide46
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

slide47
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)

slide48
The plasma-surface interface is perturbing & complex

e.g. the divertor surface is reconstituted ~100 times per second

Wirth, Whyte, et al MRS 2011

slide49
The plasma-material interface never comes to full equilibrium because the fusion environment perpetually evolves the surface and material properties
slide50
And even with a constant plasma, erosion/deposition changes material temperatures  Evolving, exponentially sensitive boundary response

Arrheniusrates

slide51
Tungsten nano-tendril “fuzz” clearly demonstrates how the reactor plasma + thermal environment “re-makes” materials

Reactor divertor: 5 mm W PFC with 10 MW/m2

Bubble Formation

W re-crystallization

Fuzz Formation

2,000 s

4,300 s

300 s

9,000 s

22,000 s

Ambient Twall for thermal efficiency

[1] S. Kajita et al. Nucl. Fusion 49 (2009) 095005

slide52
What is the impact of “fuzz” in a reactor? Integrated experiments in divertor environment such as C-Mod critical to extrapolation

After exposure

W Probe

Mo ramped tiles

G. Wright NF 2012

G. Wright GI-2 Tues AM

  • Micron layer formed in ~10 s
  • Rate of growth agrees with linear plasma devices
  • Consequences to erosion, dust, tritium retention, response to transient heating?
slide53
Quality models and measurements of material response in a fusion reactor environment are required to understand boundary evolution

Ion-beam analysis to measure He depth concentration in fuzz during growth

N. Juslin PSI 2012

K. WollerPSI 2012

Molecular Dynamics of He bubble &

Tungsten surface distortion

tritium control in reactor dual challenge of retention inventory
Tritium control in reactor: Dual challenge of retention & inventory

Retention in PFC

  • Yearly fluence of tritium in divertor
  • So total “recycling” of tritium in divertor
  • In-vessel safety limit: 1 kg
  • < 1 in 107 ions can be retained
tritium control in reactor dual challenge of retention inventory1
Tritium control in reactor: Dual challenge of retention & inventory

Tritium inventory

Retention in PFC

  • Yearly fluence of tritium in divertor
  • So total “recycling” of tritium in divertor
  • In-vessel safety limit: 1 kg
  • < 1 in 107 ions can be retained
  • Reactor consumes ~0.5 kg/day
  • ~1% burn fraction
  • 50 kg/day pumped & processed.
  • Recycling 100x pumping
  • 5000 kg/day recycled in PFC
  • Blanket TBR ~ 1.05  0.025 kg/day net tritium
  • < 1 in 5x106 can be lost to PFC to not affect supply.
tritium control in reactor dual challenge of retention fuelling
Tritium control in reactor: Dual challenge of retention & fuelling

Even in refractory metals, the viable option is high T

Retention in PFC

< 1 in 106

tritium control in reactor dual challenge of retention fuelling1
Tritium control in reactor: Dual challenge of retention & fuelling

And neutron damage  1% tritium storage capacity at defects

Y. Hatano PSI 2012

slide58
Tritium control, thermal efficiency and annealing of neutron effects are all linked/solved through high material T

Tritium storage capacity in

neutron-damaged W PFC

Whyte PSI 2008

ah the problem is you are using instead of which clearly solves your issues
Ah, the problem is you are using _____ instead of ______, which clearly solves your issues

Tokamak  Stellarator

  • Avoid disruptions but edge stability /w pedestal a concern?
  • Continuous core plasma easier BUTpower density, PFC lifetime, retention unchanged, the failures of which will also halt the reactor’s operation.
  • Non-axisymmetric engineering of divertor
  • 3-D knowledge of divertor/SOL required because limits are local, not global.
ah the problem is you are using instead of which clearly solves your issues1
Ah, the problem is you are using _____ instead of ______, which clearly solves your issues

Solid walls  Liquid lithium

  • Inherently solves erosion lifetime issue, quiescent and transient

BUT

  • Giving up solid phase  much more restrictive T range  thermal efficiency?

R. Doerner J. App. Phys. 92 (2004)

ah the problem is you are using instead of which clearly solves your issues2
Ah, the problem is you are using _____ instead of ______, which clearly solves your issues

Solid walls  Liquid lithium

  • Inherently solves erosion lifetime issue, quiescent and transient

BUT

  • Giving up solid phase  more restrictive T range  efficiency?
  • Almost zero recycling must be assured to avoid enormous tritium inventory & flowthrough
    • Re-imaging of entire core scenario [Zacharov]

R. Majeski GI-2 Tues AM

ah the problem is you are using instead of which clearly solves your issues3
Ah, the problem is you are using _____ instead of ______, which clearly solves your issues

Solid walls  Liquid lithium

  • Inherently solves erosion lifetime issue, quiescent and transient

BUT

  • Giving up solid phase  more restrictive T range  efficiency?
  • Almost zero recycling must be assured to avoid enormous tritium inventory & flowthrough
    • Re-imaging of entire core scenario [Zacharov]
  • This is not meant as a thorough critique of different
  • boundary design choices but key points are:
  • There are no magic bullets, there are tradeoffs to solving boundary challenges
  • The boundary continues to largely define the viability of the MFE reactor and may even completely reshape its design.
how do we address these gaps1
How do we address these gaps?

Exploit different PMI vs. neutron-only scale-lengths to attack the “Zero dpa” problem:

how do we address these gaps2
How do we address these gaps?

Exploit different PMI vs. neutron-only scale-lengths to attack the “Zero dpa” problem: Heat exhaust, film growth, retention, melting, fuzz, etc. will come into play well in advance of neutron induced effects which accumulate more slowly

+

HIGH TEMPERATURE

dimensionless scaling arguments could be used to address reactor boundary issues
Dimensionless scaling arguments could be used to address reactor boundary issues
  • Highly successful in core transport studies [Luce PPCF 2009]
  • Mid-90’s efforts to capture SOL plasma/Te response
    • Lackner: P/R to match edge atomic physics (not heat flux!)
    • Hutchinson/Vlases: q///B similarity for divertor plasma
  • If atomic physics is important to the boundary response, then PMI and material response are equally important Capture complex reactor boundary science in scaled-down devices.
developing dimensionless figures of merit gyro orbit redeposition
Developing dimensionless figures-of-merit:Gyro-orbit redeposition

P. Stangeby GI-2 Tues AM

slide70
A large subset of PMI dimensionless figures-of-merit can be matched with reactor-like divertor n, T, B, geometry and PMI material

Reactor

range

Description

Parameter

Dependencies

Vulcan Special Issue FED March 2012

slide72
Basic argument: If atomic physics is important in boundary plasma then surely PMI is too! Plasma & ambient T  material physics
slide73
Material science dimensionless figures of merit in reactor  Dependencies on local n, T, B, PFC species and ambient material temperature

Reactor

range

Description

Parameter

Dependencies

Vulcan Special Issue FED March 2012

slide74
The US and world will lose its first glimpse of a realistic reactor divertor environment due to the termination of C-Mod and its new high-T W divertor

S. Harrison JP8.00093 Tues AM

  • Comments

Innovative divertor design:

toroidally continuous aligned W surfaces  0.5 degree grazing incidence  can actually exploit high flux expansion vertical or snowflake topology

Bulk tungsten outer divertor from room temperature  600 C

/w reactor-like P/S, ne, Te, B

At the divertor

slide77
New “boundary-oriented” D-D confinement devices + dimensionless scaling  wind-tunnel experiments for complex & integrated issues

Vulcan (MIT)

800 C walls

Steady-state

NHTX (PPPL)

700 C walls

ST, non-inductive

J. Menard https://e-reports-ext.llnl.gov/pdf/349577.pdf

Vulcan Special Issue FED March 2012

very long pulse high t wall confinement devices could bridge key boundary chasms at small size
Very long pulse, high-T wall confinement devices could bridge key boundary “chasms” at small size
overview1
Overview
  • 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
  • We know the boundary chasms to bridge…. Let’s do it! with a balanced attack of PMI science, better boundary measurements and high-T material confinement devices.
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