Compact Stellarator Research

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Compact Stellarator Research

• G. H. Neilson
• Princeton Plasma Physics Laboratory
• presented at
• Oak Ridge National Laboratory
• February 9, 2001

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The Compact Stellarator Team
Auburn U., Columbia U., New York Univ., LLNL,
ORNL, PPPL, U. Montana, UC San Diego,
U. Texas-Austin, U. Wisconsin Germany,
Switzerland, Russia, Japan, Australia, Spain
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Stellarators Offer Innovative Solutions to
Critical Problems of Magnetic Fusion

• Challenge Finding an attractive plasma
  configuration
• Steady-state without disrupting.
• Low aspect ratio, high beta ? high power density.
• Sustainable with a minimum of power ? high
  Pfusion/Precirculating.
• Advanced tokamaks (AT)
• Bootstrap current, current profile control, MHD
  mode control.
• High-aspect ratio stellarators
• Externally-generated helical B-field, 3D
  shaping, low power density.
• Compact stellarators, a hybrid of AT and
  stellarators
• Bootstrap current plus helical fields / 3D
  shaping.
• ?Low-aspect-ratio (4), high-? (5) toroidal
  configuration.
• ?Low recirculating power, high power density.

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Compact Stellarator Research AdvancesFusion
Science in Unique Ways

• Can limiting instabilities (e.g., external kinks,
  neoclassical tearing modes) be stabilized by
  external transform and 3D shaping? How are
  disruptions affected?
• Can the collisionless orbit losses traditionally
  associated with 3D fields be reduced by designing
  the magnetic field to be quasi-axisymmetric?
  (Nuehrenberg, Garabedian)
• Do anomalous transport reduction mechanisms that
  work in tokamaks transfer to quasi-axisymmetric
  stellarators? Do mechanisms that work in
  currentless stellarators transfer to hybrids?
• How do stellarator field characteristics such as
  islands and stochasticity affect the boundary
  plasma and plasma-material interactions?
• CS provides unique knobs to understand toroidal
  confinement
• fundamentals rotational transform, shaping,
  magnetic symmetry.

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Compact Stellarator VisionThe Best of
Stellarators and Tokamaks
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Outline

• Stellarators and Compact Stellarators
• Compact stellarator physics design NCSX.
• Some engineering.
• Our plan.
• A proposal for your consideration.

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The Worlds Stellarator Effort is Substantial
LHD shown under construction. Operating since
1998.

• Japans Large Helical Device (LHD) - a 1B-class
  facility
• R3.9 m, ?a?0.65 m, B3(4) T, P?40 MW
• All-superconducting coils for steady-state
  operation

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LHD Has Been Getting Good Results

• Confinement
• Enhanced confinement, 1.6?ISS95 (multi-device
  scaling like tokamak ITER-89P)
• High edge Te pedestal (Te0 /2)
• ?E up to 0.3 s.
• Beta
• ? up to 2.4, heating power-limited.
• Exceeds theoretical stability limit.
• Fluctuations are small (?B/B10-4), increase with
  ?, do not degrade confinement.
• Parameters
• Te4.4 keV, Ti2.7 keV, ne1020 m-3
• Pulse length over 1 minute.

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Stellarator Fields Can Suppress Disruptions

• External transform applied to current-
• carrying stellarator
• 3-fold increase in density limit.
• qlt2 with no disruptions.
• total ?(a) 0.35
• Ohmic current, low ?, high aspect ratio.
• WVII-A Team, Nucl. Fusion 20 (1980) 1093.

Stellarators typically do not disrupt if
conditions for global tearing stability are
satisfied. Experiments are needed to extend to
high ?, low aspect ratio.
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Numerically Optimized Stellarators
Germanys Wendelstein 7-X - a 1B-class facility
to open in 2006. R5.5 m, ?a?0.52 m, B3 T,
superconducting coils

• Computational advanced stellarator optimization
  at R/?a?11
• Transport reduction by drift-orbit omnigeneity
• No Pfirsch-Schlüter or bootstrap currents at
  finite beta (5)
• No shear
• Modular coils
• Principles studied in partially-optimized
  W7-AS experiment since 1988.

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Helically Symmetric Experiment (HSX) Exploring
Stellarator Transport Reduction via Magnetic
Symmetry at R/?a?8

• R1.2 m, B1 T
• Univ. of Wisconsin

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Status of Stellarator Research

• Broad world program university-scale to
  1B-scale experiments.
• Strong knowledge base exists.
• Experiments tokamak-like confinement times,
  enhanced confinement regimes, good parameters,
  well-heated and diagnosed.
• Theory physics-based numerical design
  capability.
• Engineering accurate 3-D coils and structures at
  a range of scales superconducting magnets.
• Current Research
• New large devices to study steady-state core and
  divertor physics.
• Plasma configurations optimized for high ?,
  well-confined orbits, no current.
• Large aspect ratios (R/?a? 5-12).
• Large reactors projected, e.g. R18-22 m advanced
  stellarator (Germany).

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Compact Stellarator Research Fills Important Gaps
In Stellarator Physics

• High beta (4-5 or more) combined with low
  aspect ratio (4 or less).
• Hybrid design, optimized with bootstrap current.
• Magnetic quasi-symmetry used to confine
  collisionless particle orbits.
• U.S. Stellarator Proof-of-Principle Program
• Medium-scale experiment, NCSX, quasi-axisymmetric
  capture tokamak physics benefits, too.
  (proposed)
• Smaller, complementary experiment QOS
  quasi-poloidal, lower aspect ratio. (proposed)
• Couple to small experiments at universities (HSX,
  CTH).
• Stellarator theory and design.
• Collaborate internationally on stellarator
  physics.

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National Compact Stellarator Experiment Mission

• Acquire the physics data needed to assess the
  attractiveness of
• compact stellarators. (a 10-year fusion program
  goal)
• Demonstrate
• Conditions for high-beta, disruption-free
  operation.
• Understand
• Beta limits and limiting mechanisms.
• Reduction of neoclassical transport by QA design.
• Confinement scaling reduction of anomalous
  transport by flow shear control.
• Equilibrium islands and neoclassical tearing-mode
  stabilization by choice of magnetic shear.
• Compatibility between power and particle exhaust
  methods and good core performance.

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Compact Stellarator Design Methodology

• Design a reference plasma, shaped to have
  desired physics properties
• at ?4, including the effects of bootstrap
  current.
• Design practical coils to preserve those
  properties.
• Contrasts with previous stellarators optimized
  for no net current and
• vacuum magnetics.
• Capable design tools were acquired or developed
• Improved 3D equilibrium codes- PIES and VMEC.
• Plasma currents incorporated into configuration
  optimizer.
• Stability, transport, bootstrap current, and coil
  engineering metrics incorporated to improve
  targeting of design objectives.
• Coil design innovations to reduce complexity and
  current density, heal islands, preserve good
  physics properties.

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NCSX Plasma Configuration Has Attractive Physics
Plasma Cross Sections

• 3 periods, R/?a?4.4, ???1.8
• Good magnetic surfaces.
• Quasi-axisymmetric low helical ripple transport.
• Stable at ?4.1 to ballooning, kink, vertical,
  Mercier modes.
• Limited by ballooning mode
• Rotational transform 0.4 ? 0.653/4 from
  external coilsneoclassical-tearing stable

LI383
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Hybrid Configuration Combines Externally-Generated
Fields with Bootstrap Current
Reference Current Profile
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Quasi-Axisymmetry Low Effective Ripple

• Effective ripple (?eff) for low collisionality
  neoclassical transport (?eff3/2) calculated with
  NEO code (Nemov-Kernbichler).
• ?eff 3.4 at edge.

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QAS Low Ripple ? Low Helical Transport
QAS at B1 T, Pheat5 MW, R1.75 m

• Helical transport (Shaing-Houlberg) sub-dominant
  with self-consistent Er.

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Neutral Beam Losses are AcceptableEven for
Counter-NBI
R1.7m
Counter-NBI

• Allows control of beam-driven current, including
  ability to avoid it.
• Assumed tangent to mag.-axis at oblate
  cross-section.

Co-NBI
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NCSX Modular Coils Provide Good Physics Capability

• Preserve physics properties of reference plasma
• stable to kink and ballooning modes at reference
  ? (4).
• modest increase in ripple.
• Good magnetic surfaces.
• Provide physics flexibility, in conjunction with
  auxiliary coils.

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Modular Coils Provide Good Magnetic Surfaces At
High Beta
Converged free-boundary PIES reconstruction of
reference (?4) state. Island removal method
employed in coil design process. Only small
islands remain.
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Modular Coils Provide Knobs to Vary Physics
Properties (I)

• External rotational transform controlled by
  re-shaping plasma.
• Can adjust to avoid iota0.5.
• Can also control magnetic shear at fixed ?(0).
• Trim coils are planned to maintain surface
  quality over flexibility range.

?0, full current
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Modular Coils Provide Knobs to Vary Physics
Properties (II)

• Can control magnetic shear to study, e.g.,
  kink-stabilization physics.

?0, full current
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Modular Coils Can Accommodate a Range of Profiles

• Quasi-axisymmetry maintained for current profiles
  from reference (?0) to peaked (?1)
• Kink and ballooning stable at ?3 for 0.0?0.5.
• Also robust to variations in pressure profile.
• Also robust to variations in ? and Ip with fixed
  profiles.

full current
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Layered Trim Coil Design Targets m5 and 6
Resonances
m5 ( outer )
m6 ( inner )
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NCSX Design Requirements (I)

• upgrade possibilities in ()
• Major radius 1.4 m., Magnetic field 1.2 ?(1.7) T,
  (gt2T at reduced ?external)
• Flexible coil set modular, poloidal, toroidal,
  trim.
• Plasma heating
• Neutral beam 3 (?6) MW w/ 2 (4) tangential
  beams, co- and ctr-
• (Ion cyclotron RF 6 MW mode conversion or
  high-harmonic).
• pulse length 0.2?(1) s.

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NCSX Design Requirements (II)

• Fueling gas injection, high-field-side pellet
  injection
• Power particle handling absorb heat loads,
  control neutral and impurity influx. Staged
  implementation.
• Wall conditioning bakeout carbon PFCs to 350C,
  glow discharge cleaning, boronization.
• Good diagnostic access.

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Limiter and Divertor Concepts

• Start with limiters.
• Add baffles and pumps as upgrades.

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NCSX Design Features

• Coil geometry numerically determined from physics
  requirements.
• Flexible copper conductor pre-cooled to liquid
  nitrogen temperature.
• Conformal structural shell for coil support.
• Conformal vacuum vessel with carbon first wall
  structures, bakeable to 350 C.
• Casting favored for major structural parts (odd
  shapes, accurate, modest forces, cost effective)

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Coil Winding Form and Structure
Coil 4
Coil 1
Coil 1

• Winding channel tied to shell segments.
• Shell segments are bolted to radial TF coil plates

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Vessel Configuration

• Shell material Inconel 625
• Thickness 3/8 inch
• All metal seals.

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Assembly of 3 Field Periods
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Machine Configuration
Cryostat
PF Coils
TF Coils
Vacuum Vessel
Structure
Modular coils structure
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Machine Configuration
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The Plan (I)

• PPPL and ORNL propose Construct NCSX, because
• Compact stellarators offer innovative solutions
  to make magnetic fusion more attractive
  steady-state without disrupting, compact,
  efficient.
• It advances fusion science in ways that are
  unique, interesting, and beneficial to
  understanding toroidal confinement the roles of
  3D shaping, rotational transform sources, and
  magnetic quasi-symmetry.
• Cost target 55M in FY-1999 dollars.
• The U.S. fusion community has supported compact
  stellarator
• physics research and concept development for 3
  years. Results
• A sound physics foundation has been established.
• Many plasma and coil configurations have been
  evaluated.
• Design choices are the best among many options
  considered.
• Engineering development is starting out on a
  sound basis.

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The Plan (II)

• A Physics Validation Review is planned for March
• Scientific merit.
• Programmatic benefit.
• Soundness of the physics basis, resolution of
  issues.
• Appropriateness of the physics requirements,
  plausible engineering.
• Project plans cost and schedule targets.
• Next Steps After a Successful Review
• Conceptual design (CDR in Spring, 2002)
• Start of Title I Design in FY-2003.

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Conclusion

• Compact Stellarators provide both interesting
  science and important
• solutions for fusion.
• A sound physics foundation has been established
• A strong team with good links to international
  stellarator research.
• Capable physics-based tools.
• Attractive configurations.
• Design requirements and concepts.
• Ready for the next phase, conceptual design.
• The NCSX would be a valuable asset for the fusion
  science program.
• Wanted Your interest, participation, and support.

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NCSX Modular Coils
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NCSX Modular Coils
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Modular Coils Provide GoodPhysics Performance

• Free-boundary equilibrium calculations (VMEC and
  PIES codes) validate
• physics properties of coils
• Can reproduce reference plasma shape.
• Stable to kink and ballooning modes at reference
  ? (4.1)
• Modest increase in ripple.
• Good magnetic surfaces non-stochastic at the
  edge, internal islands small.
• Flexible provide physics knobs, e.g. vary iota
  and shear.
• Trim coils being studied to maintain good
  equilibrium quality over the flexibility range.

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Modular Coil Free-Boundary Equilibria Reproduce
Reference Plasma Well

• Reconstructed physics properties validates coil
  design
• Shape deviation lt1 cm well within first-wall
  boundary.
• Stable to external kink and ballooning mode.
• Modest increase in ripple flow damping limits
  being evaluated.

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Diagnostic Access

• Location of flange interface on port extension
  depends on use

cryostat
Modular coil / shell
vessel
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Tangential NBI Access
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Visit NCSX on the Web!
www.pppl.gov/ncsx/