A vision for an experimental stellarator program in the
U.S. that is a slingshot for a stellarator DT device
Oliver Schmitz
with input from and David T. Anderson, Aaron Bader,
Raymond J. Fonck, Chris C. Hegna, John S. Sarff, John C. Schmitt*
University of Wisconsin – Madison, Madison, WI, USA*Auburn University, AL, USA, on assignment at UW Madison

I acknowledge very helpful discussions with
P. Helander, T. Klinger and R. Wolf – IPP Greifswald
D. Maurer and S. Knowlton – Auburn University

An invited report to the National Academy of Sciences, Engineering and Medicine panel
on “A Strategic Plan for U.S. Burning Plasma”, March 23 2018, video conferencePreface
The vision offered is based on reports on a Stellarator Vision for the U.S.
Fusion Program which was generated in various community activities•

Report of the National Stellarator Coordinating Committee NSCC
STELLCON report: [“Stellarator Research Opportunities – A report of the National Stellarator
Coordinating Committee NSCC, various authors, 2017]
Published as [D. Gates et al., “Stellarator Research Opportunities – A report of the National
Stellarator Coordinating Committee, Journal of Fusion Energy 37 (2018) 1-44]•

Presentations in community meetings in Madison and Austin in 2017
[D.T. Anderson et al., “The Rationale for a Strong Stellarator Component in the US FESStrategic Plan”, U.S.MFRSD Madison Community Meeting, July, 2017][C.C. Hegna et al., “Stellarator research: Challenges and Opportunities”, U.S.MFRSD Madison
Community Meeting, July, 2017][D.A. Gates et al., “An invigorated US domestic stellarator program based on quasisymmetry”, U.S.MFRSD Madison Community Meeting, July, 2017]•

A dedicated white paper submitted as input to this NAS study[D.A. Gates, D.T. Anderson, C.C. Hegna, “Quasi-symmetric Stellarators as a Strategic
Element in the US Fusion Energy Research Plan”, 2018]

1

U.S. stellarator community consensus: a stepladder to a performanceextension facility through an intermediate scale device
Taken from STELLCON report: [“Stellarator
Research Opportunities – A report of the
National Stellarator Coordinating Committee
NSCC, various authors, 2017]

Mid-scale device
Large-scale device

What is the goal for these devices and which choices
should be made? This talk provides a vision!
2

The proposal in a nutshell
Exploit our leading experience in Quasi-Helically Symmetric (QHS)
stellarator research to engage into a rapid path to a DT stellarator concept.
Start design
immediately

QHS device
Combine neoclassical andturbulent transport
optimization with flexible
divertor test platform andcustom fit PMI

Steady state physics at W7-X
See talk by S. Lazerson

Sustained theory program to enable extrapolation

Enabling stellarator technology (A/M, HTSC)

3

Go to DT

Community
process
Accelerate
design effort
Commandthe risks

DT stellarator
A spectrum of DT &BPP science questions
can be addressed
Stellarator as stableand efficient system
will be made available
as a new option

Leverage ITER
knowledge

Such a bold approach based on the stellarator concept will
be a slingshot for U.S. leadership to accelerate fusion energy.

Outline• Stellarators: why?
Reasoning to consider stellarators as possible game changer• Stellarators: how?
Configuration aspects and prioritized research needs• Stellarators: what?
A concrete, one step initiative to a DT stellarator

4

Outline• Stellarators: why?
Reasoning to consider stellarators as possible game changer• Stellarators: how?
Configuration aspects and prioritized research needs• Stellarators: what?
A concrete, one step initiative to a DT stellarator

Energy efficiency: low recirculating power expected for stellarators
Pilot plant study comparing AT,
ST and CT (Q-A) stellarator

DEMO study in the E.U.

Similar size (Q-A stellarator)
Fixed B at coil (14T)

Lowest fusion power

Makes net electricity!
[H. Zohm, “ITER-DEMO FPP: a Stepladder to Fusion
Energy”, Plasma Seminar, UW Madison (2017)][J.E. Menard et al., Nuclear Fusion 51 (2011) 103014]

5[H. Zohm et al., Nuclear Fusion 57 (2017) 086002]

Efficient system with promise to be economically viable!

Shown to be free of current disruptions and MHD stable
High-b, quiescent operation
No current disruptions observed••

Discharge limited by power balance
Runaway electron issue not present

Stable system with promise to be a secure investment!
6

High density operation enables competitive performance

Stellarators are operated
at high density

Performance of stellarators isapproaching tokamak levels –W7-X is on a very promising route
ITER
W7-X OP1.2.aTokamaks
LHD
W7-AS[R. Wolf, private communication]

7

Reliable and high-gain system with promise forsufficient, steady state output.

[R. Wolf, Nuclear Fusion
57 (2017) 102020]

Recent success in stellarator research offers a new option
for the U.S. to accelerate fusion energy development

Stellarators offer unique features for an economic power plant
Low recirculating power

energy efficiency

Steady state operation

system reliability

Free of current disruptions

investment security

MHD stable plasma operation

supply security

High density operation

maximize system gain/output

The next frontier is to realize a stellarator which demonstrates an integrated
stellarator design: transport by design – divertor solution – custom fit PMI.
The proposal put forth a concrete idea that uses this challenge as research target.

8

Outline• Stellarators: why?
Reasoning to consider stellarators as credible alternative• Stellarators: how?
Configuration aspects and prioritized research needs• Stellarators: what?
A concrete, one step initiative to a DT stellarator

Quasi-Symmetry is an U.S. invention which offers
significant potential to further boost stellarator
performance
Quasi-isodynamic (QI)

Good confinement (NC & FP) at
nearly vanishing internal currents
and significant mean flow damping

Quasi-helical symmetric (QHS)

Good confinement (NC&FP) at moderate but
finite internal currents with facilitated mean flows[F.S. Anderson et al., Fusion
Technology 27 (1995) 273]

W7-X
See talk by S. Lazerson

HSX

Quasi-symmetric stellarator
optimization together with Quasi Axi-Symmetry (QAS)[J. Geiger et al., Wilhelm Hereaus
Seminar, 2017, Bad Honnef]

9Comparison between QI and QHS allows to assess advantages and roleof flows and internal currents in stellarators for device performanceFact chart optimization aspect – coarse grain (!!)
This is a coarse gain summary of the mean averaged cases made on these aspects in literature and discussions

10

AspectQAS

QHS

QI (W7-X, ongoing)

NC confinement

Expected to be good

Demonstrated good(e- thermal)

Expected to be good

FP confinement

Potential to be good

Potential to be good

Potential to be goodInternal currents

Expected highDemonstrated
reducedInitial evidence forlow currents

MHD stability

Open questions about
MHD stability

High expected b limits(high ieff)

b=6% target to getgood performance

Turbulence
optimization

Significant potential(e.g. Mynick PRL’10)

Significant potential(e.g. Hegna PoP’18)

Significant potential(e.g. Proll PPCF’16)

Facilitates flows

Expected

Low flow damping
demonstrated

Not expected

Divertor solution

Non-resonant possible

Two candidates
identified

Island divertor

Concept build andtestedNo

Yes

Yes (ongoing)Fact chart optimization aspect – coarse grain (!!)
This is a coarse gain summary of the mean averaged cases made on these aspects in literature and discussions

QHS

QI (W7-X, ongoing)

Expected to be good

Demonstrated good(e- thermal)

Expected to be good

FP confinement

Potential to be good

Potential to be good

Potential to be goodInternal currents

Expected highDemonstrated to be
reducedInitial evidence forlow currents

MHD stability

Open questions about
MHD stability

High expected b limits

b=6% target to getgood performance

Turbulence
optimization

Significant potential(e.g. Mynick PRL’10)

Significant potential(e.g. Hegna PoP’18)

Significant potential(e.g. Proll PPCF’16)

Facilitates flows

Expected

Low flow damping
demonstrated

Not expected

Divertor solution

Non-resonant possible

Two candidates
identified

Island divertor

Concept build andtestedNo

AspectQAS

NC confinement

Yes

Yes (ongoing)

Decision in QS branch
10• Three main physics advantages for QHS (on this level)• Only the QHS concept has been build andsuccessfully explored (20 years, $30M)

Fact chart optimization aspect – coarse grain (!!)

Next frontiersThis is a coarse gain summary of the mean averaged cases made on these aspects in literature and discussions

QHS

QI (W7-X, ongoing)

Expected to be good

Demonstrated good(e- thermal)

Expected to be good

FP confinement

Potential to be good

Potential to be good

Potential to be goodInternal currents

Expected highDemonstrated to be
reducedInitial evidence forlow currents

MHD stability

Open questions about
MHD stability

High expected b limits

b=6% target to getgood performance

Turbulence
optimization

Significant potential(e.g. Mynick PRL’10)

Significant potential(e.g. Hegna PoP’18)

Significant potential(e.g. Proll PPCF’16)

Facilitates flows

Expected

Low flow damping
demonstrated

Not expected

Divertor solution

Non-resonant possible

Two candidates
identified

Island divertor

Concept build andtestedNo

AspectQAS

NC confinement

Yes

Yes (ongoing)

Decision in QS branch
10• Three main physics advantages for QHS (on this level)• Only the QHS concept has been build andsuccessfully explored (20 years, $30M)

“Transport by design” is an exciting prospect for stellarators•

Neoclassical electron thermal transport was reduced in HSX by design[J.M. Canik et al., Phys. Rev. Letters 98 (2007) 085002]

High heat flux
towards plasma edge

QH reduces heat flux
at mid-radius•

Control the turbulence: next frontier to design for reduced turbulence levels

Reduced electron heat flux for HSX
by reducing TEM growth rate

Optimization
enabled by 3D
gyro-kinetic
codes
Non-linear ITG
turbulence seems
promising in QHS

11

Obviously: the underlying
anomalous transport has
to be the next target[J.Proll et al., PPCF 58 (2016) 014006]

Derivation of analytical metric for nonlinear ITG turbulence saturation[C.C. Hegna et al., Physics of Plasmas 25 (2018) 022511][P.W. Terry et al., Physics of Plasmas 25 (2018) 012308]

Such new metrics for turbulence
optimization are being developed and testedA flexible divertor test platform is needed to custom
fit the divertor to the optimized plasma core•Island divertor utilizes low order resonance in edge – equilibrium sensitivity
Provided access toHigh Density Hmode at W7-AS

Tunable volume divertor(heat dissipation, exhaust)•

Non-resonant divertor utilizes sharp magnetic edges for robust divertor concept

W7-X

12

[E. Strumberger et al., Nuclear Fusion 32 (1992) 737]

HSX

Robust against
equilibrium changes[A. Bader et al., Physics of Plasmas 24 (2016) 032506]

Outline• Stellarators: why?
Reasoning to consider stellarators as credible alternative• Stellarators: how?
Central research needs as background for a concrete initiative• Stellarators: what?
A concrete, one step initiative to a DT stellarator

A medium-size QHS device is proposed as the intermediate step

QHS device
Combine neoclassical and turbulent
transport optimization with flexible divertor
test platform and custom fit PMI

The QHS device proposed will start from a firmly explored
concept and is ready to take the next step.
Such a stepladder through a concept exploration (CE) phase
is a mandate for device development.
13

The unique features of QHS are compelling- and they have been demonstrated in HSX•

Constructed device matched the designed magnetic spectrum[J.N. Talmadge et al., “Experimental determination of the magnetic field spectrum in HSX using passing
particle orbits” Physics of Plasmas 12 (2001) 5165]•

Improved neoclassical electron confinement in QHS[J.M. Canik et al., “Experimental Demonstration of Improved Neoclassical Transport with Quasihelical
Symmetry”, Phys. Rev. Letters 98 (2007) 085002]•

Reduced flow damping with QHS[S.P. Gerhardt et al., Experimental Evidence of Reduced Plasma Flow Damping with Quasisymmetry, Phys.
Rev. Letters 94 (2005) 015002]•

Reduced Bootstrap and Pfirsch-Schlueter currents in QHS[J.C.Schmitt et al., “Modeling, measurement and 3-D equilibrium reconstruction of the bootstrap current in
HSX” Physics of Plasmas 21 (2014) 092518]•

Good trapped particle confinement of high-E electrons in QHS[D.T. Anderson et al, “Overview on recent results from HSX”, Fusion Science andtechnology 50 (2006) 171-176]

Clear goals were identified to complete the QHS
qualification as candidate for a DT stellarator device

14

Goal 1: qualify electron and ion neoclassical transport•

Complete validation of QHS neoclassical confinement performancerequires hot ions (~0.5-1keV) with Te ~ Ti at low collisionality•••

NBI route is versatile: heating, flow drive, core fueling, fast particle physics
Requires sufficient space for tangential beam injection
Requires target plasma of ~ 5 x 1019m-3 and low core neutral fraction

Larger corner coils
needed for NBI injection
New coil design tools
are being used!
I
NB

This is an initialexample and requires
re-optimization of HSX.

FOCUS: [C. Zhu et al., Nuclear
Fusion 58 (2017) 0168008]*collaboration PPPL

15

But: HSX in its present size is too small
for beam absorption & full e-i coupling & low neutral density in core

Goal 2: reduce turbulent transport through 3D shaping•

A new non-linear metric for ITG driven turbulence was developedand is being tested against GENE modeling[C.C. Hegna et al., “Theory of ITG turbulence saturation in stellarators: Identifying mechanisms
to reduce turbulent transport”, Physics of Plasmas 25 (2018) 022511]•

Reduced flow damping in QHS is promising to reduced turbulence•

Other metrics are emerging and being tested

Such metrics are being used in STELLOPT to optimize plasma
equilibrium for reduced turbulence – an active field of research

16•

Enables for the first time a perspective to design a magnetic confinement
device with deliberate optimization for reduced turbulent transport•

Coupling to flow physics in QHS device offers unique fundamental
science laboratory on link between turbulence and flows

Goal 3: divertor test platform to generate integrated system
No existing stellarator has a flexible divertor qualification capacity••

Island divertor: attractive volume divertor with sensitivity to equilibrium
Non-resonant divertor: focusing divertor but resilience against equilibrium effects
Non-resonant
divertor at HSX

In QHS, both types ofdivertors are accessible

Flexible island divertor configurations at HSX[A. Bader et al., Physics of Plasmas 24 (2017) 032506]

The divertor needs space,which makes it an integrated
optimization challenge[A. Bader et al., Nuclear Fusion 53 (2013) 113036]

17

Goal 4: custom fit a relevant plasma-wall interface
to the divertor of choice
Assumptions: hot ion (ion root) Ti>0.5 keV plasma, opaque to neutrals (ne> several 1019m-3) at Te > 1keV

It’s not a choice, but a mandate to have a suitable PMI concept

A stellarator on this level will be a top-notch PMI science facility
in stellarator geometry – focus on anything but graphite! G > 1020 m-2 s-1

qd > 3-5 MW m-2

Inward impurity pinch in ion root can beaddressed with realistic Er

Realistic magnetic pre-sheath
Hot ions give realistic sputtering conditions –not easily accessible in linear PMI facilities

Additive Manufacturing enables flexible first wall and divertor
interface to custom fit PMI interface to optimized plasma core18

A focused concept for a new, mid-scale QHS stellarator in the U.S.
•
•
•
•
•
•

19

Size: ~ HSX x 2
Neoclassical and turbulent transport optimization
NBI and ECH heating for high density (> 5 x 1019 m-3) plasma at Te~Ti > 0.5-1keV
Low core neutral content
Increased room for divertor test platform
Custom fit PMI and material test station for PMI studies

A focused concept for a new, mid-scale QHS stellarator in the U.S.
•
•
•
•
•
•

Size: ~ HSX x 2
Neoclassical and turbulent transport optimization
NBI and ECH heating for high density (> 5 x 1019 m-3) plasma at Te~Ti > 0.5-1keV
Low core neutral content
Increased room for divertor test platform
Custom fit PMI and material test station for PMI studies

Where are we in the design process? Do we have the tools? Results?
*collaboration PPPL

Hierarchy of design loop

19

Tools used in STELLOPT**collaboration ORNL & Auburn U

Equilibrium

VMEC

Transport

NC & Turbulence & (FP)*collaboration PPPL
& U Maryland

Coils

FOCUS/REGCOIL

Divertor/PMI

2-point model, analyticalerosion model

EMC3-EIRENE, GENE and other dedicated numerical tools
are available for specific verification*collaboration NIFS

*collaboration IPP
Greifswald & FZ
JuelichThe necessary tools are at hand and they are being exercised
addressing the central goals listed before
Coupling of new coil design tools andequilibrium optimization is working*Collaboration with U
Maryland & Auburn U

REGCOIL solutions pulls coils
away from HSX boundary[M. Landreman et al., Nuclear
Fusion 57 (2017) 046003]

20

The necessary tools are at hand and they are being exercised
addressing the central goals listed before
Coupling of new coil design tools andequilibrium optimization is working

REGCOIL solutions pulls coils
away from HSX boundary[M. Landreman et al., Nuclear
Fusion 57 (2017) 046003]

20

Three wave energy transfer*Collaboration with U
Maryland & Auburn U

A new, non-linear 3D turbulence metric isbeing developed – ongoing research[C.C. Hegna et al., Physics of Plasmas 25 (2018) 022511]

The necessary tools are at hand and they are being exercised
addressing the central goals listed before
Coupling of new coil design tools andequilibrium optimization is working

A new, non-linear 3D turbulence metric isbeing developed – ongoing research
Three wave energy transfer*Collaboration with U
Maryland & Auburn U

REGCOIL solutions pulls coils
away from HSX boundary[M. Landreman et al., Nuclear
Fusion 57 (2017) 046003]

EMC3-EIRENE predicts
robust divertor strikelines at HSX[A. Bader et al., Nuclear
Fusion 53 (2013) 113036]

20*Collaboration withIPP Greifswald[C.C. Hegna et al., Physics of Plasmas 25 (2018) 022511]

Medium Islands

Small Islands

Large Islands

Such a device can be constructed and operated as athrilling University based enterprise•Time line and cost estimate
Designtoday

Construct

2 years

Operate6 years

Design: $3M/a for 2 years
Construction: $10-25M/a for 4 years
Operation: ~$10M/a•

Strategy considerations:-

Highly collaborative effort for design and construction
Becomes USER FACILITY once completed (single owner approach before accelerates)-

Hosted by University makes it a thrilling enterprise for graduate student training-

Expertise across University(ies) is an asset for state of the art technology andengineering at high cost effectiveness
Such a facility would be scientifically as well as from theimplementation standpoint unique in the world!

21

The DT stellarator perspective addresses a spectrum of needs for fasterfusion energy development through design and operation security
Mission definition &risk assessment
in community process

A spectrum of DT missions to reduce risk
towards a demonstration power plant (DEMO)
science & technology
risk for new facility

W7-X & QHS device
with theory, continuous
design and technology
programFirst
ITE yearsoRDT p f thehase•a-particle
BPP

Qeng >1
Risk to go
to DEMO

Nuclear Science
Facility

Initial design efforts support scalability of QHS to reactor levels[SPP study led by UCSD, UCSD-ENG-004 (1997)]

The inherent features of the stellarator as reliable, steady state,high output system are transformational to provide a strong –maybe superior - candidate system for such facilities

22[J.E. Menard et al.,Nuclear Fusion 51(2011) 103014]

Summary of a concrete attempt to establish U.S. leadershipat the frontier of the stellarator concept
Exploit our leading experience in QHS stellarator research toengage into a rapid path to a DT stellarator concept.
Start design
immediately

Q-H device
Combine neoclassical andturbulent transport
optimization with flexible
divertor test platform andcustom fit PMI

Steady state physics at W7-X

Go to DTDT stellarator
Community
process
Accelerate
design effort

See talk by S. Lazerson

Sustained theory program to enable extrapolation

Enabling stellarator technology (A/M, HTSC)

Commandthe risksA spectrum of DT &BPP science questions
can be addressed
Stellarator as stableand efficient system
will be made available
as a new option

Leverage ITER
knowledge

Such a bold approach based on the stellarator concept will
be a slingshot for U.S. leadership to accelerate fusion energy.
23

APPENDIX

Summary: concrete mission elements of QHS facility
Approach: base facility on advantages of Q-H, whichwere demonstrated with the HSX device

QHS

Goal 1: qualify improved neoclassical transport with realistic ion temperature in Q-H
Goal 2: demonstrate for the first time “turbulence by design” approach in Q-H
Goal 3: Integrated divertor qualification for island and non-resonant divertor
Goal 4: custom fit PMI interface to divertor of choice using additive manufacturingLine of attack in a nutshell: size: ~ HSX x 2, Q-H, neoclassical and turbulent transport
optimization, room for divertor, NBI, low neutrals in core, flexible wall interface + A/M
Parameters: ~5 s discharges, nsep>5.0e19 m-3, Te,c~1-2keV, Ti,c~1keV
Not high priority guidance elements: MHD stability at high-b,
extensive energetic particle studies, steady state aspects
A-1

The long term goal must be a DT stellarator – a reasoning(1) With the QHS device and the strong program at W7-X as well as
experience from LHD, we must be able to make a significant stepThis has to be the goal of this strategic initiative in a fusion energy context!
(2) Argument is: reduced control needs + significantly lower recirculating power
make stellarator a more viable. This has to be shown by building a DT device faster!
(3) At the time, ITER will (hopefully) be operational, so additional BP physics
starts to become available -> risk mitigation through leverage(4) Tools used for QHS design and further development (sustained theory anddesign program!!) have to provide predictive capabilities(5) Advances in A/M and magnetic design (SPARC) expected which will possibly
aid stellarator design (high field magnets) and manufacture of 3D components-> consider dedicated stellarator HTSC magnet and A/M component R&D effort

A-2