Plasma Science & Fusion Center

Smaller & Sooner – Exploiting high magnetic
fields from new superconductors for a more
attractive fusion energy development path
D.G. Whyte, J. Minervini, B. LaBombard, E. Marmar, L. Bromberg,
M.Greenwald, B. Sorbom
1MIT Plasma Science and Fusion Center, Cambridge MA, USA

IAEA Fusion Energy Conference
Kyoto, Japan October 2016

Whyte, IAEA 20161

It is self-evident that smaller, modular fusion
devices will accelerate fusion’s development
Shippingport:+1954+“Pilot”+Fission+Plant++

ITER+Cost)/)volume)(M$/m3))

230+
60++
0.6+10+500++~1000+~+20+~+20+

Construction)time)(y))~+4+>+20+!

Pthermal)(MW))Core)volume)(m3))
Cost)(2012)US)B$))!

• Cost & time ∝ unit volume and mass
• ITER is an invaluable science experiment for burning plasmas but is
not an optimized size for modular fusion energy “pilots”
Ø ITER is a trial of just one fusion concept, fission pilot tried four different cores!

• Small size and modularity are self-reinforcing: pilots of complex
engineered systems as small as possible, yet sufficiently capable
Whyte, IAEA 20162

Confinement strongly physics favors high B toproduce fusion capable devices at smaller size
β N H 1.3 3R B
Gain nT τ E ∼2q*

Copper coilpulse ~ 10 sWhyte, IAEA 2016

V ∝ R3R (m)2.14V (m3)30Bo (T)10Qp>10Steadystate

NoTritium
breeding

No

Qelectric

0Pfusion β N 2 24 Power
 2 RB density
Swallq*Continuous /w
High-B
Superconductors?

3

Multiple, linked engineering design
challenges to smaller, modular path
Challenges

Opportunities /w new technology

Bcoil > 20 T

REBCO superconductors

SC Joints

REBCO: tape formREBCO: T~ 25 K

Demountable coils
Small radial build
neutronics

Immersion liquid blanket

Internal components withsmall space +high power density

Additive manufacturing ofsingle-unit VV/PFC withadvanced cooling capability

Whyte, IAEA 2016

4

The 20+ year-old magnet technology forces
ITER to be large

Whyte, IAEA 2016

5

REBCO opens up fusion energy gain atvery small scale for short time periods

ARCWhyte, IAEA 2016

6

ARC conceptual design example of “smaller, sooner”
fusion device using new superconductors
REBCO superconductor B = 9.2 T

Pfusion ~ 500 MW

ARC: R ~ 3.2 m
Whyte, IAEA 2016

x B4Copper, B = 3.5 T

Pfusion ~10 MW

JET: R ~ 3 m~4 years construction
7normalsuperconducting

103

jcrit

moderate field, large device
Nb-Sn operating space(e.g. ITER)(MA/m2)

A revolution insuperconductors
in last 5 years:
REBCO(Rare-Earth Barium
Cu Oxide) remainsuperconducting
at VERY
high B-field andabove liquid He
temperatureshigh-field target
for compact
fusion devices

102

20%
Superconductor 10%
fraction of coil

Superconductor
REBCO
Nb3-Snsupnormalerconduc tingpractical coil
design limit

4%
R=3 m, A=3, shield~0.8m
1.00.8structural stress limits
in stainless steel

Structurestress (GPa) 0.6
high-field coilprimarily limited by stress

0.40.2
12Whyte, IAEA 2016

14161820

22

Maximum B field on coil (Tesla)

248

REBCO: coated superconductors inrobust tape form, commercially available

REBCO tape composition(not to scale)•••

Strong in tension due to steel
Flexible
Outer Cu coating à simple
solder low-resistance joint• Stark contrast with NbSn
superconductor strand & CIC!
Whyte, IAEA 20169

Small REBCO coil for NMR science matches
most local requirements for ARC design

Bcoil(T)26.5

23Je (A/mm2)

400

400-500

T (K)

4.2

25

Materials

REBCO, SS316L

σmax (MPa)

593

660

Diameter (m)0.03~6Whyte, IAEA 201610

Large-bore challenge for high-B MFE magnet:
requires optimized geometry & superstructure
Peak stress ~ 0.67 Gpa~65% of limit for 316SS LN

Bcoil = 23 T

1. Support ring, 2. Top TF leg
4. Mechanical joint
6. Epoxy enforcement
Whyte, IAEA 2016

11Basic geometry favors demountable magnets toprovide modularity for internal components😀

😕
FNSF-AT V. Chan et al NF 2011
Whyte, IAEA 201612

Demountable TF coil: Evolving strategy à
Separation of mechanical and electrical joints
Initial electrical tests encouraging
REBCO“jumper”

REBCO
Conductor
terminations
Demountablecoil segment
Coilmechanicalattachments

F. Mangiorotti,
MIT Ph.D. thesis
Whyte, IAEA 201613

Demountable coils have a profound effect on
modularity and design of interior fusion “core”• Core is designed as a single
integrated unitØ Synergy with keeping
design of small total massand volume• Fabrication + qualification
done completely off-siteØ VacuumØ HeatingØ Cooling

Replaceable
solid “core”
module• No connections made inside
reactor volume!

Whyte, IAEA 201614

Demountable coils allow for much more robust andattractive nuclear technology for D-T neutrons:
The Immersion Blanket😀

Replaceable
solid “core”
moduleWhyte, IAEA 2016

😕

15

Demountable coils allow for much more robust andattractive nuclear technology for D-T neutrons:
The Immersion Blanket
RF, fuel

Replaceable Vacuum Vessel isimmersed in liquid blanket:

SupportAdvantages

•••
•
•

SimpleNeutronics/nuclear engineering atatmospheric pressure.
No gaps
Energy & tritium extraction withsingle-phase low-velocity flow
No damage limits in blanket
Minimized solid waste
Tub is robust safety boundary

Whyte, IAEA 2016

Plasma+ 14 MeV
neutrons

coil••

Liquid flow out

Liquid blanket•VV

Low-pressure tub
Liquid flow in16

Immersion blanket: high-T molten salt FLiBe
Single-phase, low-pressure flow withminimum MHD effects

Internal pumps

Externalmotor• TBR ~ 1.14
• High thermal efficiency ~ 0.4 - 0.5• Shielding: ~10 FPY coil lifetime

FLiBeWhyte, IAEA 201617New technologies provide access to synergistic
design advantages at high-B and small size:
Robust steady-state far from disruptive limits
DIII-DARIES-AT

ARC

q95

6.3

37 .2

H98

1.5

1.7

1.7

βN

3.7

5.4

2.6

G = βNH98/q2

0.14

0.90

0.09

fbootstrap

0.65

0.91

0.63

n / nGreenwald

0.5

0.9

0.65

Pfusion β N 2ε 2
∼ 2 RB 4Swall
q*nT τ E ∼β N H 1.3 3R B2q*

• Steady-state scenario using high safety-factor,moderate Beta approach• Scenario ACHIEVED in present moderate-B
devices (e.g. DIII-D)
Whyte, IAEA 201618

Due to lack of intrinsic size scaling in the heat width,
the high-field compact approach is attractive for
mitigating heat flux at high power density and gain
• Work by Eich, Goldston, et al showed that outer mipdlane
heat flux only scales like local poloidal magnetic field… notsize.
PSOL Bpol B
PSOL BPBq// ∼~~R λ BpolRBpol
R

But P/S~P/R2 is a requirement

But gain is also requirementpth τ E ∼ R1.3 B 3Whyte, IAEA 2016

⎛ P⎞q// ∼ ⎜ ⎟ B R ~ B R⎝ S⎠

111q// ~ B 3/1.3 ~ B 2.3 ~ 1.3
BBB
19

In addition the ability to operate at a higher absolutedensity + dissipative divertor regimes is a highly
attractive and/or necessary feature

1q// ~ 1.3
BGreenwalddensitylimit

Ipne ≤π a2

B
ne ~qR
Atomic physics + 2 point-model

Prad , divertor ∼ n ~ n2div4e445.3

PradB /R
B~ −1.3 −1 ~ 3q// / R B R
R
Divertor dissipation
Whyte, IAEA 201620

ARC conceptual design example of “smaller, sooner”
modular fusion electricity pilot plantusing newly available superconductors
Affordable Robust Compact• Demountable magnetic field coils• Single-unit vertical lift
• Net electricity ~ 200-250 MW

B. Sorbom et al
Fusion Eng
Design 2015
Whyte, IAEA 2016

Small, modular design
features generically attractive
to your favorite MFE choice:ST, stellarator, etc.
21Summary• Fusion is too important to rely on a single project or concept• The ability to make burning-plasma capable devices at smaller
size will accelerate fusion by allowing multiple pathways to beexplored• With a very modest amount of up front R&D, the industriallymature REBCO superconductors can make magnet coils thatallow us to pursue this path.
• A synergistic feature of REBCO is that small size + demountablecoils lead to device flexibility and new options for fusion nuclear
technology.

Whyte, IAEA 2016

22

YT
Plasma Science & Fusion Center

Thank you

Whyte, IAEA 201623