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Published byJake Bird Modified over 2 years ago

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MHD Dynamos in the Lab and Dynamos Beyond MHD

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The lab plasma dynamo does Generate current locally Increase toroidal magnetic flux Conserve magnetic helicity Act through alpha and other effects Arise from fluctuations superposed on the mean field Achieve a nonlinearly saturated steady state (with full backreaction) The lab plasma dynamo does NOT Generate magnetic field from a small seed field Increase magnetic energy (it redistributes magnetic field)

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in experiment E || j || radius additional current drive mechanism (dynamo)

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Measure quantities during discrete dynamo event Toroidal Magnetic Flux (Wb) MST time (ms)

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The MHD Dynamo

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The nonlinear dynamo energy source instability dynamo MHD dynamo analytical quasilinear theory, nonlinear MHD computation

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tasks Measurements of dynamo Active flow drive Connections to more general problem

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MHD Dynamo: Status of Past Measurements edge measurements with probes, core with passive Doppler spectroscopy Important in edge and core, but not full story

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time (ms) r/a = 0.9 MHD dynamo dominant at some radii, not everywhere r/a = 0.8 Measurement of MHD dynamo Volts m Volts m time (ms) r/a = 0.9 r/a = 0.8

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MHD Dynamo Measurements (~ 3 year project) Measure spatial profile of in MST good progress in CHERS for v - now debuting laser Faraday rotation improving (B) motional Stark effect in several years (B) probes for edge Measure dynamo and helicity changes in MRX probes

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SXR imaging of dynamo instabilities information on 3D structure of magnetic field, collaboration with RFX team (P. Franz, P. Martin, L. Mirelli) spatial resolution being improved

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Active Flow Drive Present experiments: Flow drive experiments Exploratory: flow drive via neutral beam injection, inject ~ 25 keV beam of neutral atoms, transfers momentum to plasma

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Test experimental feasibility in MST Beam installed; momentum transfer tested in few months If good absorption, then model with MHD computation, implement in MST and MRX from Novosibirsk

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Investigate why alpha effect ( ) large in lab, small in geometrically simpler computation Specifics of flow pattern?

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dynamo effects beyond MHD

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Effects from Two-Fluid Theory parallel mean-field component, Linear part, Using in top eqn, MHD Hall Pressureless MHD diamagnetic

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|| + dynamo = || Dynamotheory status MHDQL, NL comp HallQL Diamagneticvery little KineticRR, constraints from Landau resonances

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Status of non-MHD effects Diamagnetic dynamo observed in edge probe measurements under some conditions Hall dynamo observed in MST recently

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Hall dynamo measured by laser Faraday rotation (Ding, Brower, Deng) 11-chord FIR laser rotation angle

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Experimental Goal Determine locations and conditions at which different dynamo mechanisms dominate

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Experimental Plans Measure diamagnetic dynamo (probes in MST and MRX, explore Thomson scattering for p e ) Measure v e x B (= v x B + j x B/ne) (explore laser Fizeau effect for v e, ) Measure Hall dynamo with improved spatial resoluton (Faraday rotation, MSE, probes)

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Hall Dynamo Theory and Computation Expand quaslinear calculation to finite pressure Develop early nonlinear theory (Rutherford theory) Develop two-fluid computation for lab plasma (Nimrod)

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Related work: Two-fluid computation in a box shows Hall dynamo important under some conditions (Minnini, Gomez, Mahajan)

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Ongoing Assessment of astrophysical relevance of various dynamo effects

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