Stability Studies Plans (FY11)

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Stability Studies Plans (FY11)
NCSX

• E. Fredrickson,
• For the NCSX Team

NCSX Research Forum Dec. 7, 2006
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Outline

• Equilibrium Reconstruction (FY11)
• Stability Physics (FY11)
• Magnetic Islands
• Neoclassical affect on intrinsic current driven
  islands
• External kink modes
• Initial survey Alfvén Eigenmodes (AE).
• Disruptivity (does MHD limit ??)

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Equilibrium Studies in FY11

• Goal is to apply 3D equilibrium reconstruction
  code to NCSX
• Benchmark V3Fit (being developed under separate
  grants)
• Validate modeling of PIES and other codes
• Document configuration variation with plasma
  pressure and current
• Experiments to address goals in FY11
• Test consistency of V3Fit with full diagnostic
  set
• Magnetics (extensive saddle coil array and B
  probes)
• Mapping of profile diagnostics (Thomson
  scattering, Chers, SXI)
• Contact with PFCs (thermal imaging)
• Look for effects correlated with low-order
  resonant surfaces
• Compare with modeling by PIES and other codes

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Bootstrap current in reversed shear should
stabilize islands

• Pellet-induced Te drop triggers island then heals

• Evidence for bootstrap current suppression of
  islands on LHD.
• Saddle coils might detect change in JBS
• Soft x-ray camera and Mirnov coil diagnostics,
  together with Thomson, Chers, etc. should be
  adequate diagnostic set.

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TFTR data suggests no stabilization
Island width evolution measured with ECE on TFTR

• Simulations find best agreement with no
  neoclassical contribution in negative shear
  region.
• Double tearing mode non-linear evolution has
  similarities to error-field induced islands on
  Stellarators.
• Outer island acts as "error field" in inner
  island growth, and vice versa.
• "error field" plays important role in island
  growth
• Rotation/sheared rotation should decouple and
  stabilize islands.

Fredrickson, et al., Phys. Plasmas 7 (2000) 4112.
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Goals for neoclassical island studies

• Effect of bootstrap current on intrinsic islands
• Make vacuum islands or excite n1 islands with
  trim coils
• Will the bootstrap current heal intrinsic vacuum
  islands?
• Islands are stationary and stellarator symmetric
  growth detectable in saddle coil data?
• With small islands, plasma rotation may
  stabilize.
• Effect of bootstrap current on ?' islands
• Use OH current drive to make ?' unstable plasmas
• Use ? scan to vary neoclassical stabilization
  term
• Islands might rotate (sxi Mirnov) or be
  stationary (saddle coils)

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Coil Flexibility Gives Control of Kink ?-limit

• Simulations predict external-kink critical-?
  changed from 3 to 1 by modifying plasma shape
• either at fixed shear or fixed edge-iota !
• Assume free-boundary equilibria, fixed pressure
  and current profiles
• In unstable configuration, is beta limited by
  kink onset?

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Goals/Requirements for external kink stability
studies

• Test understanding of 3D effects in theory
  determine role of iota-profile
• Determine impact of unstable external kink on
  plasma performance.
• Vary external transform, look for affect on
  disruptivity
• Can we do similar study for ballooning modes?
• Soft x-ray camera, Mirnov coils and/or saddle
  coils to detect external kink.
• Some capability for equilibrium reconstruction
  and mapping of equilibrium data profiles.

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Density window for TAE studies

• Density scan controls ?fast and VAlfvén.
• Window around 46 x 1013/cm3 for Alfvén mode
  studies
• beam ions should be super-Alfvénic.
• ?fast/?thermal gt 30 .
• Early TFTR beam driven TAE studies were at
   3 x 1013/cm3, 110 keV deuterium beams,
  Btor  1T Vfast/VA  ?fast/?thermal  1

Assume ISS04v3 confinement, 1.2 T and 3 MW of 45
kV hydrogen beams
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Goals for TAE studies

• Configuration scans (shear) explore continuum
  damping.
• Density and beam power scans to identify
  threshold for excitation.
• Required diagnostics would be equilibrium profile
  data and Mirnov coils with 200 kHz bandwidth.

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We will have the tools to perform exciting
research

• Program to develop stellarator equilibrium code
  is moving forward
• Benchmarking the code will be done in FY11
• The diagnostic set planned for FY11 operation is
  sufficient for initial stability studies of
• Neoclassical tearing modes in negative shear
• Configuration-dependence of stability limits
• Initial investigations of fast ion driven
  instabilities
• Acquire preliminary information on disruptivity