First-wall, plasma-materialinteraction, liquid metals, andstrategic elements for advancing
liquid metal science and technology
M.A. Jaworski
Princeton Plasma Physics Laboratory
Presentation to the NAS Committee for a Strategic Plan for
U.S. Burning Plasma Research
April 12th, 2018

Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

Outline of material• Overview of charge letter• Recent community assessments of the state offirst-wall and PMI challenges• Potential for liquid metal concepts to solve
fusion challenges• Some current US research highlights on firstwall and liquid metal concepts• Strategic elements for advancing liquid metal
science and technology
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

2Charge letter requested information inseveral categories• Guidance from Co-chairs on content for talk received
3/26/18– Directed to give a talk “as a member of the community”
who happens to be a PPPL employee (apologies in advanceif highlights are, at times, parochial)– Views expressed are my own• Three topical areas of the charge letter:– First-wall– Plasma-material interaction (PMI) challenges, and– Liquid metals (LMs)• Strategic elements for advancing liquid metal
technologies
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Does the community think it has sufficientknowledge in first-wall and PMI challenges?
• PMI community workshop
report issued 2015
• Essential picture:understanding the impact ofboundary conditions on thesystem– Material interactions
mediated by edge plasma– Core-edge coupling• Community identified 5priority research directions(PRDs) and cross-cutting
opportunities

PMI report. Fig. I-1, p. 2.

Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Community workshop highlights need todevelop tools that can extrapolate to a reactor• PRD-A: Identify the present limits on power and particle handling… forsolid and liquid plasma-facing components (PFCs)… (including liquids)• PRD-B: Understand, develop and demonstrate innovative
dissipative/detached divertor solutions… (including liquids)• PRD-C: Understand, develop and demonstrate innovative boundary
plasma solutions for main chamber wall components, including tools forcontrollable sustained operation…
• PRD-D: Understand the science of evolving materials at reactor-relevant
plasma conditions and… enable improved plasma performance (including
liquids)• PRD-E: Understand the mechanisms by which boundary solutions andplasma facing materials influence pedestal and core performance…
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

5Community workshop highlights need todevelop tools that can extrapolate to a reactor• PRD-A: Identify the present limits on power and particle handling… forHandling(PFC Material+ Design)
solid and liquidPowerplasma-facing
components(PFCs)…
(including liquids)• PRD-B: Understand,developand(Plasma
demonstrate
innovativePower
Handling
physics
Design)dissipative/detached divertor solutions… (including+liquids)• PRD-C: Understand, develop and demonstrate innovative boundary
Main Chamberand(Plasmaphysics)
plasma solutions
for mainErosion
chamberwallTransportcomponents,
includingtools forcontrollable sustained operation…
• PRD-D: Understand the science of evolving materials at reactor-relevant
Material(Materials
science)
plasma conditions
and… Evolution
enable improved
plasmaperformance (including
liquids)• PRD-E: Understandthe mechanisms
by which boundary solutions andCore-Edge
Coupling
plasma facing materials influence pedestal and core performance…
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Power exhaust challenge previously covered inpresentations to this committee• Two presentations by EU authors highlighted thefusion power-exhaust challenge– T. Luce, “The Science of ITER”, Feb. 2018
– G. Federici, “EU DEMO Design and R&D Activities”, Feb. 2018

• Highlighted points from Luce:– Empirical scaling of exhaust width indicates heat flux challenge
is 3-4x as difficult as initial design (p. 24)– Full-scale ELM transients exceed surface melt limit of ITER
tungsten divertor targets (p. 28)• Highlighted points from Federici:– “ITER solution may be marginal for DEMO” – dedicated, €500M
machine Divertor Test Tokamak seeks advanced solutions (p. 16)– Conventional divertor technology is one driver to R>7m (p. 19)
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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PRD-C, -D, and –E deal with main-chamber materials,
their evolution, and core-edge coupling• PRD-C: first-wall erosion andtransport

Reflected H0,
Eroded First-Wall– Neutrals at edge undergo “chargeexchange” (C-X) and can erode
first-wall– Plasma transports erodedmaterial• PRD-D: fusion reactor modifies allmaterials over time

CORE
PLASMAC-X neutrals,“Blobby” plasma– Plasma continuously destroys andcreates material surfaces– Neutrons continuously transmute
materials in a reactor• PRD-E: the boundary conditions
affect core performance

Neutrons
J. Nichols, PhD Thesis,
Princeton U. 2018

Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Material erosion and evolution issignificant and inconvenient• Neutron transmutation converts W toalmost 10% Re in the first year

Tungsten Transmutation
100%• Steady erosion of the first wall estimatedto move ~8000 kg of W per full-power
year!

10%
1%

• Timescale of modification is inconvenient!
– Too slow to allow system to equilibrate– Too fast to ignore; may force maintenance

Forrest, et al.,“Handbook of Activation
Data…” UKAEA
Report FUS 552,
March 2009.
Stangeby,
J. Nucl. Mater.
415 (2011)

Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

9Outline of material• Overview of charge letter• Recent community assessments of the state offirst-wall and PMI challenges• Potential for liquid metal concepts to solve
fusion challenges• Some current US research highlights on firstwall and liquid metal concepts• Strategic elements for advancing liquid metal
science and technology
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Potential for liquid metal PFCs
1. What advantages are common to all liquid metals?
2. Which liquid metals are commonly considered?
3. How could liquid metal PFCs enable greater power
exhaust?
4. What are concepts for integrating such PFCs in areactor?
5. What confinement impacts have been observed withlithium-conditioned PFCs?
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Liquid metals present intrinsic advantages
over solid PFCs in several areas• Liquid metals provide a selfhealing/renewable plasma-facing material

Cracking after thermal
shock loading– Immune to thermo-mechanical stresses– Returns to equilibrium after perturbations– Replenishment eliminates net-reshaping byplasma bombardment• Separates neutron damage effects fromplasma-material interactions• Fundamentally alters material time-scales
and enables continuous control

Wirtz, et al., JNM 2013

CMOD W-lamellae• Potential for increased power handlingcapability
Coenen, et al., JNM 2013
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Liquid metal options cover wide rangeof atomic number• Three metals most often
discussed– Li (3), Ga (31), Sn (50)
– Sn-Li alloy also considered• Lithium most studied –lowest Z, relatively benign
in coreMajeski, PPPL-4480, Jan. 2010

• Tin features largest
temperature range forliquid concepts

LiquidAblating

Lithium
LiquidTin

Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Liquid metal PFCs provide additional
pathways for energy transport
Energy Transport Mode

Solid PFCs

Heat Conduction
to Substrate• Conventional, solid PFCs
utilize extrinsic impurities
to enhance radiation• Demonstration of surface
stability is key for allconcepts• Significant differences intechnologicalrequirements; expected
operating temperatures
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Liquid metal PFCs provide additional
pathways for energy transport
Energy Transport Mode

Evaporating
Slow-flowSnnon-evaporatingslow-flow

Fast-flow

Evaporative andRadiative Cooling

Heat Conduction
to Substrate

Heat Convection
by Liquid Metal• Conventional, solid PFCs
utilize extrinsic impurities
to enhance radiation• Demonstration of surface
stability is key for allconcepts• Significant differences intechnologicalrequirements; expected
operating temperatures

Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Liquid metal PFCs can absorb more heat
flux than do leading tungsten technologies•

Fast-flow systems advect power away fromheating zones

Perpendicular Heat Flux Limit– Limiting temperature, heating size, andvelocity determine limiting heat flux
– Li, Ga, Sn are all possible metals for use•

Slowly flowing liquid targets recently
demonstrated in multiple configurations?
Ablative Li

Actively cooled tungsten expected tosurvive 5-15 MW m-2 steady-state

Solid W
Sn CPS•

Fast Flow
Steady vapor-shielding Sn– Non-vaporizing, water-cooled tin
demonstrated at
20 MW m-2 in Magnum-PSIa– Vapor-shielded tin achieved self-regulated
temperature up to 22 MW m-2 in MagnumPSIb
aMorgan, 2017 Nucl. Mater. Energy; bS. Van Eden, et al., Nature Comm. 8(2017) 192.
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Liquid metal concepts range from
~10 m/s to ~few mm/s velocities•

LM concepts fall into two broad categories: fast andslow flow (“hybrid” in FESAC TEC report) concepts– Fast-flow typically >1cm thick– Slow-flow typically capillary-restrained, <1mm thick•Fast-flow, first-wall anddivertor concept

Fast vs. slow approaches differ in maturity of physics andtechnology– Fast flow: less mature technology, less physics maturity
for surface stability– Slow flow: more mature technology,
less physics maturity
for ablating targets•

Reactors expected to featurelarge areal coverage andcontinuous flow
Evaporation
Condensation

Abdou, et al., FED 2001; Golubchikov,
et al., JNM 1996; Goldston, et al., Phys Scr. 2016

Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

High-temperature,lithium divertor
conceptc.f. Mirnov 2009 JNM“emitter-collector”
17Lithium conditioning observed to increaseenergy confinement time in multiple machines• Confinement improvement attributed
to reduced recycling• LTX coated metallic PFC + Li pool islatest demonstration• Follows historical trend in wallconditioning dating to TFTR
– See also FTU, T11-M, HT-7, EAST, DIIID, TJ-II, RFX
TFTRStrachan, et al., J. Nucl. Mater. 217 (1994)

LTX

D. Boyle, et al., Phys. Rev. Lett. 119 (2017) 015001.

NSTX

R. Maingi, et al., Phys. Rev. Lett. 107 (2011) 145004.

Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Outline of material• Overview of charge letter• Recent community assessments of the state offirst-wall and PMI challenges• Potential for liquid metal concepts to solve
fusion challenges• Some current US research highlights on firstwall and liquid metal concepts• Strategic elements for advancing liquid metal
science and technology
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Some highlights from current US research
1. Liquid Metal Fusion Energy System Study activity
2. Ongoing confinement-device studies in
LTX-β (PPPL), EAST (CN), and
HIDRA (U-Illinois)
3. First-wall evolution code validation and materialtransport studies underway at PPPL and GA(Solid PFC research areas covered by J. Rapp)
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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LM-Fusion Energy System Study (FESS) broadly
examining issues with LM deployment•

System study group considering Fusion Nuclear Science Facility (FNSF) with liquid
metal PFCs
– Liquid metal choice (Sn, Li, Sn-Li, Pb-Li, etc.)– Range of PFC concepts (fast, slow/hybrid, jets)– Plasma response and power loads (e.g. Li vapor divertors)– Interactions with substrates and sub-systems (e.g. corrosion, embrittlement)– Integration issues (e.g. tritium recovery, LM filtration, maintenance impacts)•

Example: can the power extracted by fast-flow liquid metal be converted toelectricity?
– Cycle efficiency favors large temperature rise in LM– Initial analysis indicates fast-flow tin better integrates than fast-flow Li

Kessel, 2017, ICFRM.
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Confinement-device impact examined in
LTX(-β) and EAST-PMI projects• LTX-β will explore lithium on
metallic substrate impact inauxiliary-heated ST• EAST-PMI project deploying
mid-plane limiters in
addition to Li evaporations

M. Lucia, PhD Thesis, Princeton University. 2015.

EAST– Studying impact on machine
performance and operations– Technology prototyping andtesting in U-Illinois on HIDRA
alongside EAST tests

Hu, et al., Nucl. Fusion 56 (2016) 046011.
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Atomistic- thru machine-scale theory and modelingvalidation studies of coatings and solid materials• Laboratory studies of mixed-materials directlycompared with ab-initio models(theory + laboratory studies)• MAPP diagnostic successfully deployed in NSTX-U
2016 run campaign – part of validation data set forwhole-device transport modelling• Whole-device material transport studies also
conducted on DIII-D (e.g. metal-ring campaign)

NSTX-U

MAPP
Surface Science Laboratory

L. Buzi, et al., J. Nucl. Mater. 502 (2018).

In-vacuo surface
science (U-Illinois)

J. Nichols, Princeton Univ. PhD. 2018

Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

23

Outline of material• Overview of charge letter• Recent community assessments of the state offirst-wall and PMI challenges• Potential for liquid metal concepts to solve
fusion challenges• Some current US research highlights on firstwall and liquid metal concepts• Strategic elements for advancing liquid metal
science and technology
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Decision for a liquid metal reactor needs
answers to these questions
1. What does the integrated scenario look like for agiven liquid metal/temperature combination?
2. Can the tritium breeding ratio be maintained,
including any LM fuel-retention effects?
3. What is the life-time limit of the LM components?
4. Can these systems be made safe, reliable, andmaintainable?
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Integrated scenario provides assessmentof liquid metal choice and technology• Fusion core performance to determining overall plant sizeand capacity (e.g. compact vs. large scale)• Core-edge integration is linked to the operating
temperature (e.g. absorbing vs. ablating Li, other metal)• Control of material temperature limited by technicalcapabilities (e.g. fast/slow flow and plasma configuration)• Power extraction potential impacted by ability to utilize
liquid metal heat (cycle efficiency)
Core-edge integration can only be experimentallydemonstrated with a confinement device
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Tritium breeding essential tosuccessful D-T reactors• New mass-flows in and out of vacuum vessel
create potential losses for fusion fuel(common to all LM concepts)• Lithium absorbs hydrogen so requires a robust
strategy for closing the fuel cycle– New technologies in development at SRNL
– Slow-flow concepts target higher concentrations
to ease extraction
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Plant component lifetime and maintenanceschedule impacts cost of operating• Important value proposition for liquid research:– If solid component lifetime justifies liquid usage, thenliquid component must exceed solid lifetime– Technical solution must truly eliminate plasma-based
erosion• Liquid metal interactions with substrate material(including transmuted materials) could limit
component life
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Operations and licensing of a fusion plant depends
on safety, reliability, and maintainability• No one will agree to an unsafe nuclear reactor• No operator will keep a reactor that isn’t
profitable• Sodium fast-reactors provide a dataset of thechallenge of large-scale, alkali metal, nuclear
systems
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Range of facilities is needed to develop
critical data set• Definitive contributions• Supporting contributions• Hashed: no single, definitive contribution
Test-Stands, Experimental
Laboratory Linear Plasma Confinement
Scale
Devices
Devices

Theory,Design, andModeling

Integrated Scenario Development
First Wall and Divertor PFC
Design & Operation
Power and Particle Handling
Plasma Scenario Assessment
Tritium Breeding Ratio and Fuel
RecoveryLifetime of Components
Safety, Reliability, Maintainability
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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DOE views universities, national labs, and industry as
essential partners in energy innovation• Laboratory-scale experiments
and studies can be conducted
at universities• High-power confinement
devices operated by NationalLabs and industrial partners• Sub-components could be basisof spin-off technologies forindustrial partners– e.g. spray coating development
for slow-flow PFCs

Fig. 2-2, “Annual Report on the State of the DOE National
Laboratories”, DOE Report, Jan. 2017

Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Potential “show-stoppers” in all criticalareas suggests parallel effort• Integrated assessments could eliminate liquid
metal and technology choices• TBR process challenges could eliminate metals• Corrosion/life issues could make liquids metals
unattractive• Safety and reliability could make liquid metals
unattractive
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Sustained R&D progress could be enabledwith Shared R&D or User Facility model• Following DOE model: National labs could provide LMcapable devices for tests of new concepts andtechnologies– Core-edge integration assessment implies large areal
coverage, high-power (>10MW/m2), multiple confinement
times (> 1s)– High-heat flux, pre-filled lithium targets could be
implemented into a confinement device in ~3-5yr timeframe– Fast-flow concepts require additional development time, butfacility would be prepared for LM tests– Implies acceptance of machine redirection• Similar R&D facility model could be implemented for TBR,
and corrosion/lifetime, and safety assessments
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Less tangible aspects of aggressive R&D…
• CRENEL report: DOE sets forth the mission;labs and others execute the mission• Experimental results can eliminate ordemonstrate concepts –have to be willing to move on• Need to accept that there is RISK in doing newthingsJaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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There is a Way; is there a Will?
• Community assessment indicates there is development needed forreactor extrapolations– EU colleagues emphasized power exhaust challenge– Plasma- and neutron-induced material evolution is significant• Liquid metal plasma-facing components provide many intrinsic
advantages– Provide means of control for long-time scale evolution of material– Do require a lot of technical development to give clear comparison withsolid PFCs

• Critical questions facing LM reactor proposed with facility mix– University, National Lab, and Industrial partners are all needed– Liquid metals are not a “less-trodden” path,they are a never-trodden path
Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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Thank you for this opportunity

Jaworski – First-wall, PMI, LM, and Strategic elements for Fusion
April, 12th, 2018 – Princeton, NJ

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