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Stability Studies Plans (FY11)

E. Fredrickson,For the NCSX Team

NCSX Research Forum Dec. 7, 2006

NCSX

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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

• 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.

• Pellet-induced Te drop triggers island; then heals

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

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TFTR data suggests no stabilization

• 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.

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Island Width (cm) Measured (ECE) Resistive simulation Full neoclassical Outer neoclassical

B? ( )G

Island width evolution measured with ECE on TFTR

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 n=1 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?

PFC Boundary

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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 4–6 x 1013/cm3 for Alfvén mode studies:

– beam ions should be super-Alfvénic.

– fast/thermal > 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