U.S. Research on
International Stellarators
Samuel A. Lazerson
Princeton Plasma Physics Laboratory
Presented to:
NAS Committee for a Strategic Plan for U.S. Burning
Plasma ResearchOverview

Overview
1. The motivation and options forinternational stellarator researchOverview
1. The motivation and options forinternational stellarator research
2. My experience as a collaborator on theWendelstein 7-X stellarator

Overview
1. The motivation and options forinternational stellarator research
2. My experience as a collaborator on theWendelstein 7-X stellarator
3. How to enhance international stellarator
research within the United StatesOverview
1. The motivation and options forinternational stellarator research
2. My experience as a collaborator on theWendelstein 7-X stellarator
3. How to enhance international stellarator
research within the United States
4. Strategic elements that might broadly
advance fusion research in the U.S. andpromote leadership in the field.

Stellarators provide a clear path toward fusion energy

The stellarator concept poses the challengeof achieving a burning plasma state, as apurely design problem.
•Steady-state is intrinsic•

Fully non-inductive operation•

Stability limits are soft•

Transients are non-disruptive•

The challenge is to build in the confinement

Stellarator performance hinges on new devices withincreased capability

Experimental Data

Theory100Device Construction

101Simulation

Stellarators allow us to address the needs of burning
plasmas nowProblems they solve

Topics to be addressed- Steady-state- High-performance- Transient-free- Impurity confinement- Non-inductive- Plasma-wall interactions- Tritium Breeding- Magnet DesignWhy international stellarator research?
Of the operating stellarators in the world only two are capable of long pulse, high betaoperation.
W7-X
HSX

CTH

SCR-1

CNT
HIDRA

LHD
TJ-II
Heliotron J

The U.S. has a growing international collaborative
effortIn the past decade the U.S. has had active (funded)
collaborations on both the LHD and W7-X devices.
LHD

W7-X

-Xray Imaging Crystal Spectrometer- Xray Imaging Crystal Spectrometer-Equilibrium Reconstruction- U.S. Built Trim Coil System- Equilibrium reconstruction- Divertor Scraper Element Program- Phase Contrast Imaging System- Island divertor physics programWendelstein 7-X
Wendelstein 7-X is the worlds first high-performanceoptimized stellarator• Quasi-isodynamic optimization• 1800 second plasmas planned• 10 MW steady-state heating• 3D divertor geometry• Stored energies in excess of 1 MJ
Operation started in 2015

• 27 m3 plasma volume

W7-X has already achieved 1% beta
• 5 MW X-mode discharge• Low density heating mode
• Electron gradients steep
core-edge• Possible MHD limit observed

Slide courtesy Sergey Bozhenkov (IPP-HGW)

W7-X has already achieved 1% beta
World record stellarator triple product• 5 MW X-mode discharge• Low density heating mode
• Electron gradients steep
core-edge• Possible MHD limit observed

Slide courtesy Sergey Bozhenkov (IPP-HGW)

The U.S. fills key roles in the W7-X experimental
program•Assessment of error fields in stellarators•Role of 3D fields in power exhaust•Confirmation of optimization and confinement•Development of divertor concepts for steady-state power exhaust•Direct input to the planning of the experimental campaign

U.S. Trim Coils play major role in W7-X scientificexploitation• First confirmation of coil assembly accuracy• First actuation of limiter heat loads• First assessment of coil deformation due to EM loads• Essential to long-pulse operation through divertor
load symmetrization

IPP engineers and scientists standing
inside a trim coil after delivery.

The trim coils are 5 water cooled
copper steady-state coils on W7-X.

Lazerson et al. 2016 Nucl. Fusion 56 106005

Superposition of 5 flux surface images,
showing actuation of configuration by
the trim coils

X-Ray Imaging Crystal Spectrometer (XICS) isconfirming optimization of W7-X
• First measurement of ion temperature in W7-X
• First measurements of poloidal flows on W7-X
• Actively being used to assess impurity
transport• Provides data essential to confirming
optimization

Geometry of the XICS viewing chords in W7-X.

Pablant et al. 2018 Phys. Of Plas. 25 022508

Ion temperatures routinely measured in W7-X byXICS.

High resolution infrared camera program played keyrole in the limiter campaign• Provided first measurement of heat flux to limiters in first operational
campaign• Provided data key to scrap-off-layer modeling• Provided first confirmation of plasma-wall interactions

Typical image from LANL FLIR camera oflimiter during first experimental campaign.

FLIR images (b) confirmed ability to use both field line connection length(a) and EMC3_EIRENE simulation (c) for predictive estimates of heatloads

Wurden et al. 2017 Nucl. Fusion 57 056036

The U.S. is poised to tackle serious reactor-relevant
issues using W7-X in the next 5 years• Steady-state fueling (Pellet Injector Program)• Steady-state wall conditioning (Boron Powder System)• Transport control (XICS Program, Heavy Ion Beam Program)• Steady-state heat exhaust (Divertor Program)

Fast camera image of high fieldside pellet injection

IR camera view of scraper
element installed in W7-X

Depiction of boron powder
head for manipulator

U.S. divertor program poised to make a large impact
in the upcoming campaign (OP1.2b 2018)• First test of divertor protection element(scraper)• IR cameras to assess heat loads• Langmuir probe array to assess plasma edge
• Filter-scope system to assess impurities

One of the 10 W7-X divertor modules showing thescraper element installed.

Lore et al. 2014 IEEE Trans. On Plas. Sci. 42 3, p.539

Current status: Ready!

Simulations showing protection of divertor edges
by scraper element.

The U.S. program on W7-X is a partnership• The ‘one-team’ approach• On-site, on-time, on-needs• Bilateral success metrics

Error field correction = Trim Coils
Ion Temperature = XICSDivertor protection = Scraper Element
Wall conditioning = Boron Powder Dropper
Fueling = Steady-State Pellet Injector• Committed team of scientists and engineers

We are not alone!

Stellarator research greatly benefits from collaboration
on international devices in the near futureW7-X and LHD provide capabilities to address criticalissues in burning plasma science before ITER
operation begins- Steady-state operations- A facility to assess plasma-wall interactions- Assess the role of 3D fields on confinement- Operations in a regulated, reactor-like, environmentFuture U.S. collaboration opportunities exist at W7-X
• Steady-state pellet injector• Heavy-ion beam probe program• Energetic Particle Physics• 8 MW of NBI coming online in 2018
• 1-2 MW of ICRH in 2020
• Divertor Physics• 10 MW/m2 for 30 minutes 2020
• Steady-state wall conditioningAn international view on a 20 year strategy
Goal: U.S. delivery of a world-class burning plasma science facility by 2040.
1. Aggressively pursue collaboration on the international superconductingdevices•

W7-X: Long pulse, high ß, island divertor, fueling, and PMI•

LHD: Deuterium campaign, high ß, helical divertor

2. Initiate national design and optimization activity for an improved quasisymmetric stellarator
3. Develop a conceptual design for a next-step mid-sized US facility to extend
quasi-symmetry into the hot ion regime•

Focus on benefits of flow and symmetries which cannot be investigatedon the large international facilities•

Define the minimum scope, needs and capabilities of such a system