Transcript Document 7491840

Compact Stellarator Research
G. H. Neilson
Princeton Plasma Physics Laboratory
presented at
Oak Ridge National Laboratory
February 9, 2001
The Compact Stellarator Team
D. Anderson, U. Wisc.
R. D. Benson, ORNL
L. A. Berry, ORNL
B. Blackwell, Australia
A. H. Boozer, Columbia U.
A. Brooks, PPPL
T. G. Brown, PPPL
J. Chrzanowski, PPPL
M. Cole, ORNL
W. Cooper, CRPP
M. Drevlak, IPP-Greifswald
L. Dudek, PPPL
M. Fenstermacher, LLNL
G. Y. Fu, PPPL
P. Garabedian, NYU
A. Georgievskiy , consultant
R. J. Goldston, PPPL
P. Goranson, ORNL
A. Grossman, UCSD
R. Hatcher, PPPL
P. Heitzenroeder, PPPL
D. Hill, LLNL
S. P. Hirshman, O RNL
S. Hudson, PPPL
M. Isaev, Kurchatov
D. Johnson, PPPL
C. E. Kessel, PPPL
S. Knowlton, Auburn U.
L.-P. Ku, PPPL
H. Kugel, PPPL
E. Lazarus, ORNL
J. Lewandowski, PPPL
J. F. Lyon, ORNL
R. Majeski, PPPL
P. Merkel, IPP-Greifswald
D. Mikkelsen, PPPL
W. Miner, U. Texas
P. Mioduszewski, ORNL
D. A. Monticello, PPPL
H. Mynick , PPPL
C. Neumeyer, PPPL
N. Nakajima, NIF S
G. H. Neilson, PPPL
B. E. Nelson, O RNL
C. Nührenberg, IPP-G.
M. Okamoto, NIF S
A. Pletzer, PPPL
N. Pomphrey, PPPL
M. H. Redi, PPPL
W. T. Reiersen, PPPL
A. H. Reiman, PPPL
R. Sanchez, Spain
J. A. Schmidt, PPPL
J. Schultz, MIT
R. T. Simmons, PPPL
D. A. Spong, ORNL
D. Strickler, ORNL
P. Valanju, U. Texas
A. Ware, U. Montana
K. Y. Watanabe, NIF S
R. B. White, PPPL
D. A. Williamson, ORNL
R. Woolley, PPPL
M. C. Zarnstorff, PPPL
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 Advances
Fusion 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 Vision:
The 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 World’s Stellarator Effort is Substantial
LHD shown under
construction.
Operating since
1998.
Japan’s Large Helical Device (LHD) - a $1B-class facility
R=3.9 m, a=0.65 m, B=3(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/B~10-4),
increase with , do not degrade
confinement.
Parameters
• Te≤4.4 keV, Ti≤2.7 keV, ne≤1020 m-3
• Pulse length over 1 minute.
 0.079 a
 ISS95
E
2.21 0.65 0.59 0.51 0.83 2 0.4
R
P
n
B
( 3)
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Stellarator Fields Can Suppress Disruptions
External transform applied to currentcarrying stellarator:
• 3-fold increase in density limit.
• q<2 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
Germany’s Wendelstein 7-X - a $1B-class facility to open in 2006.
R=5.5 m, a=0.52 m, B=3 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
R=1.2 m, B=1 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. R=18-22 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  0.65
~3/4 from external coils
neoclassical-tearing stable
LI383
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Hybrid Configuration Combines ExternallyGenerated 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:
B=1 T,
Pheat=5 MW,
R=1.75 m
Helical transport (Shaing-Houlberg) sub-dominant with self-consistent Er.
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Neutral Beam Losses are Acceptable
Even for Counter-NBI
R=1.7m
• Allows control of beam-driven
current, including ability to avoid it.
• Assumed tangent to mag.-axis at
oblate cross-section.
Counter-NBI
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
Island removal method
employed in coil design
process. Only small islands
remain.
normalized radius
Converged free-boundary
PIES reconstruction of
reference (=4%) state.
poloidal angle
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1.0
–0.5
0.0
Elevation
1.5
2.0
2.5
0.5
1.0
2.0
2.5
0.6
0.8
Rotational Transform Profiles
1.0
1.5
2.0
2.5
Major radius
=0, full current
0.0
0.2
0.4
Rotational Transform (iota)
1.0
0.5
Elevation
0.0
–0.5
–1.0
0.5
1.5
Major radius
1.0
Major radius
• Can also control magnetic
• Trim coils are planned to
maintain surface quality
over flexibility range.
1.0
–1.0
0.5
• Can adjust to avoid
iota=0.5.
shear at fixed (0).
0.5
1.0
0.5
–1.0
–0.5
• External rotational
transform controlled by reshaping plasma.
0.0
Elevation
Modular Coils Provide
Knobs to Vary
Physics Properties (I)
0.0
0.2
0.4
0.6
Rel. Toroidal Flux
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0.8
1.0
1.0
1.0
–0.5
1.0
1.5
2.0
–1.0
0.5
0.0
Elevation
0.5
0.5
0.0
Elevation
–1.0
–0.5
Modular Coils
Provide Knobs to
Vary Physics
Properties (II)
2.5
0.5
1.0
2.5
0.6
0.8
Rotational Transform Profiles
0.2
1.0
1.5
2.0
2.5
Major radius
0.0
0.5
2.0
0.4
Rotational Transform (iota)
1.0
0.5
Elevation
0.0
–0.5
–1.0
• Can control magnetic shear
to study, e.g., kinkstabilization physics.
1.5
Major radius
1.0
Major radius
0.0
0.2
0.4
0.6
0.8
Rel. Toroidal Flux
=0, full current
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1.0
Modular Coils Can
Accommodate a
Range of Profiles
• Kink and ballooning stable at
=3% for 0.0≤≤0.5.
0.4
Rel. Parallel Current Density
• Quasi-axisymmetry
maintained for current
profiles from reference (=0)
to peaked (=1)
0.6
0.8
0.2
 = 0.0
 = 1.0
• Also robust to variations in
pressure profile.
• Also robust to variations in 
and Ip with fixed profiles.
full current
Rel. Toroidal Flux
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Layered Trim Coil Design Targets m=5 and 6 Resonances
m=5
( outer )
m=6
( inner )
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NCSX Design Requirements (I)
upgrade possibilities in ()
• Major radius 1.4 m., Magnetic field 1.2 (1.7) T, (>2T 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.
Operating Points at B=1.2T
Day-1
(Upgrade)
=2.6%, P=2.5 MW
(=4%, P=5 MW)
*=0.25 ne-limited *=0.25 ne-limited
5
12
7
17
n (1019 m-3)
T (keV)
1.1
0.45
1.2
0.50
HISS95
2.7
1.7
2.7
2.2
HITER-89P
1.1
1.0
1.2
1.1
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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 Good
Physics 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 <1 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/
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