Plasma-Materials
and Divertor
Options for Fusion

Presented to:
National Academy of Sciences Panel
A Strategic Plan for U.S. Burning Plasma Research

J. Rapp
ORNL is managed by UT-Battelle
for the US Department of Energy

Lifetime of divertor will deterimine
fusion reactor availability
TF coils

Coolant manifold(permanent)

Upper ports(modules and coolant)

Blanketmodules
5-6 yrs lifetime

Cost ofelectricity is
proportional
to (1/A)0.6

Cool shield
30cm(permanent)

Central ports(modules)Vacuum vessel
70cm(permanent)

Divertor plates
2 yrs lifetime goal

Lower ports(divertor)

Main driver of scheduled maintenance: divertor (and blanket)
2 Juergen Rapp

Outline• Plasma-Material Interaction (PMI) challenges• Potential Plasma-Facing Materials (PFMs) and Components (PFCs)• Current status of U.S. PMI research• Facilities needed for the development of PFCs

• Strategic elements to accelerate U.S. burning plasma research• A proposed high-level R&D program and roadmap for PMI
3 Juergen Rapp

Outline• Plasma-Material Interaction (PMI) challenges• Potential Plasma-Facing Materials (PFMs) and Components (PFCs)• Current status of U.S. PMI research• Facilities needed for the development of PFCs

• Strategic elements to accelerate U.S. burning plasma research• A proposed high-level R&D program and roadmap for PMI
4 Juergen Rapp

Challenges for materials: fluxes andfluence, temperatures
JET

ITER

FNSF

Fusion Reactor

50 x divertor ion fluxes
5000 x divertor ion fluence

up to 5 x ion fluence

106 x neutron fluence (1dpa)

up to 100 x neutron fluence (150dpa)

P/R about the same

5-10 x P/R

5 Juergen Rapp

Plasma Material Interactions (PMI) in
fusion reactor
Re-deposition
Co-deposition ofhydrogen

Worst case erosion rate
~ m/yr !

t
rge
a
t
bon
car

Erosion(chemical and physical)
Implantation Ablation
Melting (metals)

Strongly Coupled regime:

1) Eroded material is trapped in plasma (highly collisional) near target, and re-deposited on surface
due to incoming flows, electro-static acceleration and motion in magnetic field
2) Long exposure to damaging plasma flux Þ thick layers of re-deposited material
Every surface atom is displaced ~ 107 times in a divertor lifetime
Ø Material in a reactor divertor is NOT what was installed, we need a way to create and test
plasma-reformed surfaces
6 Juergen Rapp

Challenge:
material
choice for PFCs
Plasma Surface
Interactions
v Quite some materials have been tested as PFM over the years:
StSt (TEXT, PLT), Mo (Alcator-A, TFR), W(DOUBLET-II, ORMAK), Al (ST), Al2O3 (PETULA),
B4C (TFR), Be (ISX-B, JET), Au (DIVA), Ti (PDX,
DITE), Li (CDX-U), TiC (W-AS), TiB2 (ISX-B), Cu(ASDEX), C (CFC, graphite)…
v PLT used a carbon limiter

50% increase in Te

v Following those results, C (graphite, CFC) became the material choice for most devices

v Only recently the interest in high-Z PFCs is growing again, mainly because of theobserved Tritium retention during TFTR and JET DT experiments.

ITER, material choice

Beryllium

Tungsten

Carbon, now Tungsten
8 Juergen RappBe

low radiationC

non-melting CFC

Whigh melting point, low erosion by D, T

Power exhaust challenge
PPCS AARIES-ACT1

ITER

JET

AUG

Pheat/R [MW/m]

130

65

19.8

11.4

14

f*rad wo br, syn rad

0.64

0.67

0.54

0.76

0.87

P*heat B/R [MW T/m]

651

306

80

39

35

PLH B/R [MW T/m]

202

105

bN

3.5

4.75

1.77

1.6

3

D Maisonnier et al., FED 2006 C Kessel et al., FST 2015

Kallenbach, NF 2009

Issues• A significant part of the radiation is not in the SOL, PSOL/R ~ 7
NF 52 (2012)
has been achieved on AUG so far (ITER: PSOL/R ~ 12) Kallenbach,
122003• High P/R for DEMO is challenge
Ø High Pheat/PLH does allow for significant core radiation in
DEMO, ARIES-ACT1 (frad, core~ 70 - 80%); AUG has demonstrated
70% core radiation without loss of confinement
Juergen
9Rapp,
DEMORappworkshop 2011

Ø PSOL B / R could be reduced to 100-200Power exhaust with impurity seeding:what about confinement?
• H98(y,2) has been found to be bN
dependent• Impurities can improve confinement

H98(y,2)

JET

M. Wischmeier, IAEA 2014
A. Huber, EPS 2014

bN
1.2

AUG

H98(y,2)H98(y,2)

1

0.8

0.6

0.4

0.2

J. Rapp, Nucl. Fusion 52, 2012, 122002
0
0

0.5

1

1.5

bN
N

10 Juergen Rapp2

2.5

Ø Despite bN scaling and impurity effecton core confinement, it is uncertain if
high H98(y,2) of 1.2 or 1.6 can bereached with strongly radiating
mantle and plasma core

Power exhaust: advanced divertors• If radiative dissipation of power is not sufficient, advanced
divertors might help.

Courtesy, B. LaBombard

11 Juergen Rapp

Challenges for materials: fluxes andfluence, temperatures
JET

ITER

Fusion DEMO

50 x divertor ion fluxes
5000 x divertor ion fluence

up to 5 x ion fluence

106 x neutron fluence (1dpa)

up to 100 x neutron fluence (150dpa)

PSOL B/R about the same

3 x PSOL B/R

Materials need to be developed and tested under fusion prototypic conditions:High fluxes, high ion fluence, high neutron fluence
12 Juergen Rapp

Reactor: high plasma performance andhigh PFC lifetime requires strong redeposition to ensure low net erosion
Main chamber erosion due to ionsand high energy CX neutrals(ITER: ECX ~ 500 eV; DEMO = ??)Divertor plasma temperature inthe ~ 10 eV range where
GROSS sputtering yield oftungsten drops to ~ 10 X greaterthan the required NET sputtering
yield.

D ®X

Reactor divertor lifetime ~108 s
requires net erosion rate of 10-6

net
erosion~ 100 X
required
net yield

If re-deposition of W atmain chamber is notincreased, massivenet
amounts of W migrate todivertor (t/yr) Behrisch J. Nucl. Mater deposition~ 10 X

erosion~1X

313 (2003) 388

How does W surface evolve with strong deposition of W?
Grainsize, crystal structure, dust?
13 Juergen Rapp

Krieger J. Nucl. Mater 266 (1999) 207

High fluence and frequent ELMs mightchange W erosion processes
TungstenTungstenTungstenTungsten

100000 pulses@ 0.3 MJ/m2

Tungsten
T Loewenhoff et al., Nucl. Fusion
55 (2015) 123004

S Lindig et al., Phys. Scr. T145
(2011) 014039

MJ Baldwin et al., Nucl.
Fusion 48 (2008) 035001

M Wirtz et al., J. Nucl. Mater. 420(2012) 218

Consequences:High energy density
plasma changes:

Chemical and physicalerosion yieldSurface area; Surface
roughness

Relation between grosserosion and net erosion

Surface potential(unipolar arcing may
occur)
Surface temperature(loosely bound layers,
He bubbles)
Surface chemical
activity
14 Juergen Rapp

Y Ueda et al., Fus. Sci.
Technol. 52 (2007) 513

M Tokitani et al., Nucl. Fusion 51(2011) 102001

J Coenen et al., Nucl. Fusion 51(2011) 083008

Whole grain ejection
can causemacroscopic erosion

Unipolar arcing, canpossibly create W
dust of nm size

Meltlayer splashingcreates W dust ofµm size

Neutron irradiation will likely enhance macroscopic erosion

Dust production mightoccur due tomacroscopic erosion of
surface structure andmeltlayer splashingTritium retention
Projected T-retention in ITER

Fluence dependence D retentionin W

J Roth et al., J. Nucl. Mater. 390 (2009) 1

Issues•••••Fluence dependence
Flux dependence
Effect of surface temperature
Effect of impurities (He, N, Ne, Ar) on T-transport in W
Neutron irradiation effects

15 Juergen Rapp

R Doerner et al., Nucl. Mater. Energy (2016)

Neutron irradiation will influence PMI
Neutron irradiation
damageConsequences on PMI

Thermal conductivity

Temperature operation
window, less tolerance totransient heat loads, erosion
yield

Chemical composition(transmutation)

Hydrogen retention, thermal
conductivity indirectlyInterstitials, vacancies,
dislocations, voids

Hydrogen retention

Swelling and irradiation
creep at intermediate
temperatures

Tolerance in PFC alignment will
become larger, hence power
handling capability lower

Loss of high-temperature
creep strength

Reduced temperature
operation window

Ductile to Brittle
Transition Temperature

Reduced temperature
operation window

He, H embrittlement

Erosion and dust production
will be enhanced

Synergies of microstructural changes
between neutron andplasma irradiation

Increased erosion due toincreased surface roughness

16 Juergen Rapp

14 MeV, high He/dpa
up to 150 dpa for blankets
up to 50 dpa for divertor
Accumulation of He can have major implications forthe integrity of plasma-facing- and structuralcomponents
Voids in F82H
9dpa, 380 appm He

Grain boundaryØ Neutron irradiation will weaken grain boundaries
and possibly leading to increased macroscopic
erosionT-retention in refractory metals andimpact of irradiation• Most studies today rely on high energy ion irradiation (selfimplantation)–

Time scales of dpa creation are vastly different in those experiments–

Self implantation leads to shallow damaging zones

Lipschultz, ITPADivSOL 2010; HFIR: M.
Shimada, et al., Nucl.
Fusion 2015

HFIR irradiations• Some studies with HFIR irradiations started (up to 0.3 dpa)–

Plasma exposure is limited to low fluxes and low fluence• Deuterium retention is higher for irradiated tungsten• Deuterium retention is lower in mixed D-He plasmas
Ø Suggests changed transport of D in the presence of HedpaØ Suggest the need to test neutron irradiated samples athigh dpa (>> 0.3 dpa)

He

Ø Investigation of neutron irradiated materials with relevant
He/dpa ratio is required
17 Juergen Rapp

Alimov, J. Nucl.Mater. 420 (2012) 370 and other similar:
M. Baldwin et al, Nucl. Fusion (2011)
W. R. Wampler et al, Nucl. Fusion (2009)

Ion irradiations important (but do not
simulate neutrons)
Xu et al.,Acta Mater., 2015

W-2%Re33 dpa ions 500°C(Atom probe atom map)
P. Edmondson, ORNL

Pure W (99.9+%)
2.2 dpa HFIR 750°C
Now 5%Re-7%Os bulks-phase interconnected ribbons(Atom probe isodensity surface)
18 Juergen Rapp(Same scale)

How will tritium, helium, heat,etc., permeate this structure?

Could neutron irradiation lead to higher
physical sputtering?
• He ion irradiation has shown to change micro-structure in tungsten significantly(very high He/dpa, factor 100 too high).
• Grazing incidence of plasma could lead to enhanced physical sputtering ofroughened surface.
• What is expected with neutron irradiation at relevant He/dpa ratio?

V.S. Koidan et al., IAEA 2010

Irradiation borderDh [µm]4
20irradiated a + plasma
27 dpa by 4 MeV He

plasma-2-4
0
19 Juergen Rapp

250

500

L [µm]

750

Challenges of free-flowing liquid metal
PFCs• Fast flowing system: Kelvin-Helmholtz andRayleigh-Taylor instabilities• Slow flowing system: high temperature ->evaporation• Surface composition change due to impurities• Tritium retention due to gettering by oxygen (Li)• Helium pumping• Vapor shielding• Thinning of liquid metal layer• Irradiation damage of substrate material• Corrosion
M Jaworski et al., PSI 2016, Nucl. Mater. Energy (2016)20 Juergen Rapp

Outline• Plasma-Material Interaction (PMI) challenges• Potential Plasma-Facing Materials (PFMs) and Components (PFCs)• Current status of U.S. PMI research• Facilities needed for the development of PFCs

• Strategic elements to accelerate U.S. burning plasma research• A proposed high-level R&D program and roadmap for PMI
21 Juergen Rapp

Development of materials for PFCs
• Mechanically alloyed tungsten
Ch. Linsmeier et al., Nucl. Fusion. 57 (2017) 092007• Laminates
Ch. Linsmeier et al., Nucl. Fusion. 57 (2017) 092007• Fiber-reinforced composite materials
J.W. Coenen et al., Fus. Eng. Des. 1244 (2017) 964• Self-passivating tungsten, “smart”, alloys• Functionally graded materials for fusion• Alternatives: ceramics

22 Juergen Rapp

A. Litnovsky et al., Phys. Scripta T170 (2017)

Novel composite materials: Wf/W
J. Riesch et al., Report Max-Planck-IPP (2013)

Pseudo-ductile behavior of tungsten fiber reinforced tungsten
Properties rely on energy dissipation mechanisms• Fiber pullout• Crack bridging• Crack deflection
Pure W fiber might not retain strength under irradiation
Can we modify fibers accordingly?
23 Juergen Rapp

W-matrix
Tungsten fibre

J.W. Coenen et al., Fus. Eng. Des. 1244 (2017) 964

Exploring ceramics as PFM option
Material Properties

Pure
Tungsten

CarbideCVD SiC

Isotope-separated
Diborides
Zr11B2

Ti11B2MAX
Ti3SiC2

Atomic Number

High

Low

MediumMediumMediumMelting Point (°C)

3,422

2,730

3245

3225

~3,000Max. Operating Temperature (°C)

~ 1,100~1,400 (?)

Unknown

Unknown~ 1,000 (?)

Thermal Cond. (W/m-K) Unirr - RT/1000°C
Irradiated Degradation

180/110Moderate

120/100
Unknown

96/78
Unknown

40/50
Small

Radiation Tolerance

Poor(?)

Good

Unknown

Unknown

Fair(?)

Tritium Permeability

MediumLow

Unknown

Unknown

Unknown

Tritium Retention

Low

High?

Unknown

Unknown

Unknown

Neutron Absorption

High

Low

Low

Low

MediumShort-term Activation

High

Low

MediumMediumMediumLong-term Activation

Low

Low

MediumMediumMediumLOCA Safety

Poor

Good

Fair(?)

Fair(?)

Good

24 Juergen Rapp

400/80Harsh
but with NFB*

Advanced Materials enabled by newtransformative technologies
Diffusion barriers,
Permeation barriers
Functionally gradedmaterials

Self-healing materials

Additive
Manufacturing
Materials-by-Design
driven by
Artificial Intelligence
Self-passivatingHigh heat transfer
technologies

In-situ repair of PFCs
materials

25 Juergen Rapp

Transformative enabling technology
What is the effect of innovation?
• Higher heat fluxes• Larger temperature operation window• Larger stress resilience• Better compatibility with plasma• Better accident tolerance

What to expect in the future frominnovation?
• Smaller• Faster• More complex• More precise• Diffusion barriers / permeation barriers tolower tritium retention

Example for current limitation:• Defect barriers to improve irradiation
resistance• Atom Layer Deposition (2D-layer) possible
at low speed• 3D-structures possible with Additive
Manufacturing as small as 50µm

Materials-by-design on a micro- or nano-structure level to enablebulk/surface PFC properties in complex geometries in a single graded system
26 Juergen Rapp

Complex heat transfer systems will bebenefit from additive manufacturing
Microjets might open opportunity for power exhaust of up to 30 MW/m2

Temperatureq//
W faceplate
Outletplenum
W jetbody

Inlet plenum

27 Juergen Rapp

200 µmjets

Advancedmanufacturing
Required!

Micro
116 jets

HEMJ

D Youchison, FST (2014)

HTC

Opportunities for emerging materials
Design of radiation-resistant andradiation tolerant materials enabledby additive manufacturing• Adaptive self-healing materials• Complex hierarchical composites• Complex alloys• Hybrid liquid/solid systems

Wire EDM texture

P. Rindt, PFMC 201728 Juergen Rapp

Ghoniem and Williams, 2017

Flame sprayOutline• Plasma-Material Interaction (PMI) challenges• Potential Plasma-Facing Materials (PFMs) and Components (PFCs)• Current status of U.S. PMI research• Facilities needed for the development of PFCs

• Strategic elements to accelerate U.S. burning plasma research• A proposed high-level R&D program and roadmap for PMI
29 Juergen Rapp

StatusPISCES
TPE

LTX

W7-X

EAST

U.S. PMI R&DInternational PMI R&DWorld-wide unique capabilities to study Be, T
effects in linear devices. They are ideal for single tofew effects studies and benchmarking of PMI
computational models. Leadership in PMI science.

Recently, high flux, high fluence linear devices
became operational (Magnum-PSI)

Various small scale devices (LTX, HIDRA) etc. offer
unique capabilities to test liquid metal PFCs inparticular liquid Lithium.

Liquid metals are also studied on Magnum-PSI,
FTU, TJ-II and in the future on COMPASS
Upgrade.

Well diagnosed divertor plasmas. Leadership in
Div/SOL science. Design of devices allows in
principle for testing divertor concepts to various
degrees of closure. Load-lock systems (DIMES,
MAPP) allow exposure of material samples.

International devices have more relevant wall anddivertor materials installed: W and Be (JET, AUG,
EAST, WEST). Some devices also offer uniquecapabilities to test new divertors (MAST, TCV), andin future COMPASS Upgrade. Furthermore DTT in
Frascati on the horizon.

In collaboration with international long pulse
devices, U.S. develops actively-cooled PFCs andsteady-state scenarios compatible with PFMs withrespect to confinement, erosion/re-deposition (dust
production), T-retention.

Obviously home institutions of steady-state toroidal
devices have advantage.

Excellent tools to develop nuclear materials.
Leadership in material science and neutronscience. World leading neutron sources.

Capabilities spread around the world. If IFMIF or
DONES will be built, leadership could move tointernational facilities.

WEST

30 Juergen Rapp

Outline• Plasma-Material Interaction (PMI) challenges• Potential Plasma-Facing Materials (PFMs) and Components (PFCs)• Current status of U.S. PMI research• Facilities needed for the development of PFCs

• Strategic elements to accelerate U.S. burning plasma research• A proposed high-level R&D program and roadmap for PMI
31 Juergen Rapp

Development of PFCs requires devices
with increased capabilities to test PMI
at reactor relevant level
Transport in plasma

Transport in material

Classical Debye vs. Chodura sheath

Non-linear evolution of surface
as well as bulk effects
Ion implantation,
fuzz
10-100 nm
Blisters
10-50 µm

Parallel impurity transport (entrainment)

32 Juergen Rapp

Cracksmm

Increased capabilities with MPEXMPEX planned
capabilities

New plasma source concept (Helicon, EBW, ICRH) for independent
control of Te and Ti for entire divertor plasma parameter range.

Steady-state magnetic field [T]1-2

Steady-state high power flux to target[MW/m2]> 10

Steady-state high power plasma flux
on tilted (5 degree) target [MW/m2]

3

Target Te, Ti [eV]1 - 15

Target ne [m-3]60"

W erosion at 10 eV

DEMO

50"

40"

30"

MPEX

0"
1.E+06"33 Juergen Rapp

1.E+05"JE
T

10"

1.E+04"

JT
60
EA -SA
ST
IT
ER

FNSF

1.E+03"

1.E+02"

Annual"gross"erosion"of"W"[m]"1.E+01"1021 1019

Source Te, Ti [eV]20-30

Reactor relevant ion flux [m-2s-1]

1024

Annual fluence1031

Transients (laser, ET source, e-beam)Underassessment

Neutron irradiated samples

Y

Test of divertor component mock-ups

Y

20"

Cm
DI o dIID

Maximum"W"damage"by"neutrons"[dpa]"

Tungsten"material"damage,"lifeFme"invesFgaFon"

1.E+00"Outline• Plasma-Material Interaction (PMI) challenges• Potential Plasma-Facing Materials (PFMs) and Components (PFCs)• Current status of U.S. PMI research• Facilities needed for the development of PFCs

• Strategic elements to accelerate U.S. burning plasma research• A proposed high-level R&D program and roadmap for PMI
34 Juergen Rapp

Strategic elements for U.S. PMI program• An Advanced Linear Plasma Device• Fusion Prototypic Neutron Source• Whole device modeling capability to beable to make reliable predictions on
power exhaust• A DTT ??
35 Juergen Rapp

Outline• Plasma-Material Interaction (PMI) challenges• Potential Plasma-Facing Materials (PFMs) and Components (PFCs)• Current status of U.S. PMI research• Facilities needed for the development of PFCs

• Strategic elements to accelerate U.S. burning plasma research• A proposed high-level R&D program and roadmap for PMI
36 Juergen Rapp

Some milestones for PMI and PFC R&DLong term milestones within 15 years• Contribute to second generation divertor of ITER (higher heat flux > 10 MW/m2, higher
fluence, few dpa)
• First divertor and first wall components for US next step device (high heat flux >10 MW/m2,~10 dpa)• Second generation divertor for US next step device (high heat flux > 20 MW/m2, highfluence, high dpa > 50dpa)

Short term milestones within 5 years• Build advanced linear plasma device MPEX
• Down-selection between solid PFCs and liquid metal PFCs
• Decision on need for a DTT (on basis of knowledge derived from experiments, modelingand theory).

37 Juergen RappNear term R&D priority for PMI (5 years)• Develop solid material PFC technology• Scope liquid metal PFC technology• Develop advanced manufacturing methods and tools for fusion applications as PFCs (e.g.
additive manufacturing, materials by design utilizing AI)• Assess free-flowing liquid metal PFCs in LTX and NSTX-U• Assess the science of evolving surfaces with high flux, high fluence linear devices• Assess hydrogen retention in candidate materials with high flux, high fluence linear devices• Assess material migration of candidate novel PFMs in existing toroidal devices• Assess power exhaust scenarios with highly radiative plasmas and novel divertors on
existing toroidal devices (DIII-D and international e.g. MAST, TCV, COMPASS Upgrade,
AUG, JET, EAST, WEST)• Develop integrated (whole device) modeling tools (AToM) to interpret power exhaustexperiments to enable extrapolations to high magnetic field (low lq and high PSOL),
essentially to required PSOL B / R

38 Juergen Rapp

Long term R&D priority for PMI (15 years)• Develop advanced materials for fusion (self healing, irradiation and erosion resistant)• Build low-cost fusion prototypic neutron source (e.g. accelerator driven neutron sources) for
material development• Assess advanced materials under fusion prototypic conditions (high fluence, high flux, highirradiation damage)• Deploy next generation advanced solid or liquid PFCs to long pulse devices• Contribute to the design of the 2nd generation divertor for ITER
• Deploy new PFC systems to US Next Step Device

39 Juergen Rapp

Roadmap: PMI and divertor development
today 2020

2030

2040

Solid Materials Technol.
Liquid Metal Technol.
Advanced Materials Technol.
Existing linear devices

PISCES, TPE, P-MPEX, Magnum
National toroidal devices

LTX
NSTX-U
DIII-D

Liquid Li
Recovery

Liquid metal

Power exhaust, material migration

International short pulse toroidal devices

JET, AUG, MAST, TCV

Power exhaust

Fusion prototypic neutron source

Design Constr.

MPEX

Construction

Operations

Operations

International long pulse toroidal devices

EAST, KSTAR, WEST, W7-X

Material migration, dust

ITER

Construction

Operations

DTT
Fusion Demonstration Device/FNSF
40 Juergen Rapp

Stellarator Option

DesignConstruction

Operations