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MHD Dynamos in the Lab and Dynamos Beyond MHD

MHD Dynamos in the Lab and Dynamos Beyond MHD. 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

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MHD Dynamos in the Lab and Dynamos Beyond MHD

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  1. MHD Dynamos in the LabandDynamos Beyond MHD

  2. 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)

  3. in experiment E|| j|| radius additional current drive mechanism (dynamo)

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

  5. The MHD Dynamo

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

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

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

  9. Measurement of MHD dynamo 0 Volts m -10 r/a = 0.9 r/a = 0.9 -20 Volts m 0 -10 r/a = 0.8 r/a = 0.8 -20 time (ms) -0.5 0 0.5 time (ms) MHD dynamo dominant at some radii, not everywhere

  10. MHD Dynamo Measurements(~ 3 year project) • Measure spatial profile of <v x B> 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

  11. 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

  12. 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

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

  14. Investigate why alpha effect (<v x B>) large in lab, small in geometrically simpler computation Specifics of flow pattern?

  15. dynamo effects beyond MHD

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

  17. <E>|| + dynamo = <j>|| Dynamo theory status MHD QL, NL comp Hall QL Diamagnetic very little Kinetic RR, constraints from Landau resonances

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

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

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

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

  22. 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)

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

  24. Ongoing Assessment of astrophysical relevance of various dynamo effects

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