progress, prospects, and opportunities in stellarator v1€¦ · in stellarator physics and 3d...
TRANSCRIPT
Progress, Prospects, and Opportunities in Stellarator Physics and 3D Plasma
Confinement Research
Presented by D.T. Anderson on behalf of the US Stellarator Community2013 Experimental Plasma Research Workshop, Fort Worth TX, February 14, 2013
or
To steal a phrase from Krstic:
“If 3‐D effects are so important, why did it take decades to recognize?”
Topics
• Why Stellarators?• Achievements and Status of Major Experiments• Collaboration Activities and Physics Opportunities– Fully 3‐D systems– 3‐D physics in “2‐D” systems– Code development/application and education
• Needs and Opportunities for Improvement• Concluding Remarks and Discussion
Why Stellarators?• Stellarators have confinement similar to
tokamaks• Inherently steady‐state• Startup on well‐formed magnetic surfaces• Capable of nearly currentless operation• No virulent current‐driven or pressure‐
driven instabilities that abruptly terminate the plasma
• Good normalized pressure ()• Minimal profile control or current drive
requirements• Big advantage – capable of high density
operation– Desirable for divertor operation– Faster damping of energetic particles– No current => no conflict with current drive– Density limit set by simple power balance
N /(Iequiv/aB)
LHD [next three slides from H. Yamada 2012 report to IEA]
W7X [next five slides from Bosch Oct.2012 presentation to DOE]
Will come back to these later
Oak Ridge and Princeton major players in design and assembly of the superconducting coil current lead components
Collaboration Activities and Physics Opportunities
– Fully 3‐D systems• W7X (Trim coils, divertor, thermal imaging)• LHD (Spectroscopy, flow effects on islands)• Disruption suppression with 3‐D fields (CTH)
– 3‐D physics in “2‐D” systems• ELM suppression in tokamaks• Equilibrium reconstruction across all toroidal systems
– Selective comments on code developments/applications– University programs are making world‐class fundamental contributions
• Trim coil system constructed by PPPL consists of 5 water‐cooled copper coils with independent power supplies.
• Coils produces mainly an n =1 radial field with variable spatial phase• Use to regulate(time dependently) non‐uniform hot spots due to error fields, alignment
offsets.• Important “knob” to use in verification of heat load modeling and control.
US on track to deliver all Trim Coil equipment by end of FY‐13.
W7X Trim Coil Project
No talk/poster on this: Talk to Jeff Harris for more details
New ‘Scraper Element’ being designed
14
MW/m2
• Issue: W7‐X magnetic configuration evolves as plasma bootstrap current increases– Configuration evolution can result in overload of main target edges, >10MW/m2 on sections rated for 5MW/m2
– Occurs on ~40s timescale, effectively steady‐state thermal loads
• Solution: New ‘scraper element’ intercepts heat and particle fluxes to protect main targets– Element will be built using actively cooled carbon fiber composite (CFC) monoblocks (design qualified for ITER)– Steady‐state heat fluxes ~10MW/m2 (rated for 20MW/m2)
• Successful design PDR held in July
W7‐X Divertor Module
Heat loads without scraper
Heat loads with scraper
Overloaded elements (>10MW/m2, rated for 5)
Field lines carrying heat to overloaded target sections are intercepted by scraper element Scraper
element
Main targets
Scraper element
Poster J. Lore: This afternoon
Views of the C‐Mod divertor tiles
IR camera system tested on C-Mod for W7X• A new high speed infrared camera was procured and tested
this summer on the Alcator C‐Mod tokamak. Export control documents are in the approval process with DOD and the State Department. The camera will first be used at IPP to confirm resolution tests of the prototype endoscope system, and for real‐time imaging software development [LANL/ Talk to Glen Wurden for more information]
X‐RAY IMAGING CRYSTAL SPECTROMETER (XICS) on LHD:PROVIDES FULL PROFILES OF Ti, Te & Vθ
• Provides profile measurements of:– Ion Temperature– Electron Temperature– Poloidal Flow Velocity
– Time resolution: 20ms, Spatial resolution: 2cm.– Non‐perturbative to the plasma (no NBI).
Can provide profiles in ECH and IRCF plasma.
• Tomographic inversion allows local values to be inferred from line integrated measurements.– Requires 3D equilibrium reconstructions
(STELLOPT).
• Two complementary systems are installed which use emission from Ar17+ and Ar16+– Coverage in nearly all LHD plasma conditions.
• Development led by Novimir Pablant at PPPL.
ION
TEMPERATU
REELECTRO
NTEM
PERATURE
ARGON 16+
BRIGHTN
ESS
See poster by N. Pablant (P2.029, Thursday Afternoon)
Plasma flows heal islands in stellarators
3 40
1
2
0 0.5 1.0 1.5 2.0
-505
R[m]
Te[keV] #43619 t=2.13[s]Growth
[rad]
b1n=1/Bt[10-5]3 4
0
1
2
0 0.5 1.0 1.5 2.0
-505
Te[keV]Healing
R[m]
#67923 t=2.4[s]
b1n=1/Bt[10-5]
[rad]
• LHD performed experiments with large vacuum islandsUnder some operating conditions, the islands spontaneously healed Not predicted by 3‐D MHD equilibrium codes
• Theory suggests flows physics is crucial (Hegna, NF ‘11) Plasma flows induce island healing currents Theory predicts critical conditions for healing
• 3‐D equilibrium codes undulypessimistic in predicting surface fragility
• Extended MHD modelingrequired
170 0.2 0.4 0.6 0.80
1
wvac=116mm C1=50(ne/1019)0.5(h*@/2=1)1/4(1+C1h*@/2=1)-1/4
[%]@/2=1
crit prediction consistent with LHD data
Poster this afternoon by Hegna
2‐D/3‐D Connections
• 3‐D codes and analysis are being applied to understand plasma behavior in nominally 2‐D systems and examine benefits of 3‐D shaping
• Subject of next three talks– Maurer on disruption suppression in CTH– Schlutt on application of NIMROD to profile evolution in CTH[for more on CTH see poster by Knowlton this afternoon]
– Chapman on 3‐D equilibria in the MST RFP
Mechanism for RMP suppression of ELMs in tokamaks is not understood
• RMP produces 3‐D MHD equilibrium alters stability and transport properties – Model developed to explain transport in H‐mode with RMP
– Edge plasma flows of sufficient magnitude to suppress resonant field penetration –> stochastic transport not viable
– Small 3‐D distortions can lead to substantial modificationsto the local shear KBM Stability boundaries– Distortion sensitive toproximity to rational q values– Dramatic changes tolocal micro‐instability properties(Bird and Hegna, NF, ‘13)
19
No presentation this workshop; see Hegna for more details
20
We can now reconstruct 3D equilibria from experimental data for any device with flux
surfaces
IAEA 2012 paper: J. D. Hanson and 19 co‐authors=> V3FIT University led effort! <=
US: Auburn, GA, ORNL, PPPL, WisconsinInternational: NIFS (Japan), RFX (Italy)
Multiple US‐origin reconstruction codesV3FIT, STELLOPT both use VMEC as equilibrium engine
Use in ongoing physics studies & diagnostic interpretationStability: ELMs & disruptions, hi‐res profiles, helical states
Future developmentsAutomation, acceleration in algorithmsIslands (PIES, SIESTA, SPEC, HINT2, etc.)W7X steady state: real‐time for control?Unique contributions to ITER from lab + university team
US team leads world in 3D equilibrium reconstruction
21
STELLOPT has been developed for stellarator equilibrium reconstruction
• Development led by Sam Lazerson at PPPL.
• STELLOPT optimizes the input parameters for an VMEC equilibrium solution.– Searches for a best match to input diagnostic data.– Assumes good flux surfaces (no stochastic field lines).
• Accurate reconstructions are important for:– Diagnostic interpretation.– Beam deposition calculation.– Transport analysis.– Study of performance limits.
• Used routinely for XICS analysis on LHD.
• Integration into TASK‐3D underway.
• Uses VMEC equilibria (with nested surfaces) as initial starting point to calculate equilibria with islands
• SIESTA could eventually be used to replace VMEC as the equilibrium engine for:– Initializing extended MHD simulations– Stellarator optimization– Equilibrium reconstruction
• Scalability: Allows for massively parallel simulations
SIESTA: a nonlinear MHD equilibrium solver
M=6 island chain
PENTA Restores Momentum Conservation
• Originally developed by Don Spong (ORNL) – Uses DKES coefficient calculations– Corrects for momentum conservation with method of Sugama and Nishimura
• Has been expanded at HSX (J Lore now at ORNL) for– Multiple ion species of arbitrary mass, charge, temperature (HSX impurity transport)– Arbitrary expansion order (improves accuracy, allows for convergence checks)– Particularly important experimental comparisons of flow (ChERS) and bootstrap current
• Effects of parallel flow, interspecies collisions included• Expressions used analytically reproduce intrinsic ambipolarity in
symmetric limit• In principle, this method can be applied to the full range of
configurations:tokamaks rippled tokamaks quasi-symmetric conventional stellarators
Increasing effective rippleH. Sugama and S. Nishimura, Phys. Plasmas 9, 4637 (2002).
University programs are making world‐class fundamental contributions
• Quasisymmetry• Extended MHD • 3D reconstruction• Disruption suppression with 3‐D fields• 3‐D modeling and optimization• Overseas collaborations/exchanges help bothprograms, e.g.– HSX student to LHD for impurity studies– W7X student to HSX for magnetic diagnostics and equilibrium reconstruction
Opportunities for Configuration Improvement
• Previous optimization targets and demonstration of success
• Identified areas where improvement is needed (and what can we do)– Divertors– Energetic ion confinement– Turbulent transport– Impurity transport and accumulation– Coil simplification and maintainability
Optimizations for W7X, HSX, and NCSX
• W7X was optimized for good magnetic surfaces, 5% , low parallel current and reduced collisionless transport (operational 2014)
• HSX was optimized for very low 1/ transport, marginal well, and avoidance of resonances; high effective transform; quasihelical symmetry (operational 2001)
– eff = N‐m = 3 in HSX (q=1/3)• NCSX optimized for compactness, MHD stability and neoclassical transport; large bootstrap current; quasiaxisymmetry
Successes in HSX with Quasisymmetry
Collector Disk
• Collector plate in direction of electron B drift shows large negative potential when quasisymmetry broken.
• Larger HXR flux in QHS configuration also observed.e
-
ECRH
Floating Potential vs Density
QHS
Mirror
0 1 2 3 4 5 60.7
0.8
0.9
1
1.1
1.2
1.3
(Radians)
|B| (
T)
0 1 2 3 4 5 60.7
0.8
0.9
1
1.1
1.2
1.3
(Radians)
|B| (
T)
MirrorQHS|B| along field line |B| along field line
Good single particle confinement of trapped particles with QHS
Quasisymmetry in Thermal Plasmas
0 0.5 10
0.5
1
1.5
2
2.5
3
r/a
T e (keV
)
QHS
Mirror
100 kW ECRH input
φ
θ
QHS
Large flows measured by CXRS in the direction of
symmetry
Increase in central electron temperature with symmetry (from Thomson scattering)
Divertors in Stellarator Geometries• An effective divertor is critical for core performance and control of the plasma edge
• Stellarators can operate at high density, but have severe geometrical constraints– Helical divertors as in LHD– Island divertors as in W7X– Both have drawbacks
• Reliable divertor modeling is essential for design of new systems and divertor concepts and optimization toward a stellarator DEMO
Critical area! Universities can fit in through detailed edge
measurements and modeling
EMC3 used to model HSX‐like plasmas
EMC3 is a Monte-Carlo edge code that solves the fluid equations on open and closed field lines in fully 3D geometries.
Adjusting the edge transform in an HSX-like QHS plasma, can alter the presence and size of magnetic islands in the edge plasma.
The results can be used to help design future experiments and to compare with edge measurements on HSX.
Aaron Bader’s poster yesterday!
Measurements of flows on HSX can inform edge transport models
Edge measurements with a 6-pin Gundestrup probe are already underway on HSX.
Additional experiments will look at other plasma parameters in the edge and heat/particle fluxes to the targets.
Energetic ion confinement
• ALL of Wendelstein 7X, HSX and NCSX were designed to have good confinement of energetic ions in their ideal magnetic configurations
‐in their realizations they all failed!
• Small ripples introduced by the finite coils to produce the configuration resulted in significant predicted losses‐In the ARIES‐CS compact stellarator reactor study ‐particle losses were approximately 10%‐Perturbation of the quasiaxysymmetry by addition of a toroidal mirror term cut these losses in half.
• The effects of small ripples depend on where they are located:‐Particles trapped in a ripple where the drift direction is within a magnetic surface do not contribute to the radial transport and loss.
Configuration and coil design needs to more properly account for energetic particle losses than simply ensuring good
quasisymmetry in the ideal design
Optimization for Turbulent TransportSee Poster by Harry Mynick this afternoon
• HSX results show that when neoclassical losses are sufficiently reduced, the electron thermal transport is anomalous
• There is a need to incorporate reduction of turbulent transport in the optimization process.
• Groundbreaking work by Mynick has developed a method for including optimization targets for the reduction of turbulent transport into the STELOPT code
• STELOPT has generated configurations that have heat fluxes calculated by GENE reduced by factors of 3‐4
Reduction of turbulent transport without relying on rotation has far ranging implications for device design.
Impurity Transport and Accumulation
• Stellarators in ion root discharges (inward directed radial electric field) are predicted to have impurity accumulation
• This can lead to radiative collapse of the plasma• Divertors and edge control will help, but cannot alleviate this
occurrence• Conditions have been identified, but not understood, where
impurities are eliminated from the core plasma without the presence of ELM’s• HDH mode in W7‐AS at high density (> 1014)• “Impurity hole” on LHD (also at high density)• New occurrence at high Ti at 1013 (Slide4)
There is a need to understand the conditions and mechanisms for impurity exclusion from
stellarator core plasmas
“Simpler” Coils
• Stellarator coils have been manufactured and used successfully
• Very low aspect ratio drives complexity• Coils which have reduced curvature and more plasma/coil spacing are desirable for ease of manufacturing and space for an effective divertor
• Flexibility in the coil structure to accommodate optimization needs such as energetic particle confinement, anomalous transport reduction and maintainability could result in more compelling designs.
• Interesting avenue for investigation
[Poster this afternoon by Josh Breslau on new method for coil modeling]
Concluding Remarks• The stellarator is a player in the quest for fusion energy • There exists an excellent fundamental research program in 3‐D
physicsEquilibrium reconstruction, extended MHD, edge divertor physics, configuration optimization
• University programs can and are contributing significantly to large programs
Big opportunities at many levels to get involved
• Collaborations work best from bottom up• Large space for improvements in stellarator design
Large increase in capabilities over last 25 years!New targets: divertors, anomalous transport, energetic particlesWhat are tradeoffs in optimization?How good can we make it?
Discussion!