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MHD Dynamos in the Lab and Dynamos Beyond MHD PowerPoint PPT Presentation


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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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MHD Dynamos in the LabandDynamos 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

  • 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)


in experiment

E||

j||

radius

additional current drive mechanism (dynamo)


Measure quantities during discrete dynamo event

Toroidal

Magnetic

Flux

(Wb)

MST

time (ms)


The MHD Dynamo


The nonlinear dynamo

instability

energy

source

dynamo

MHD dynamo

analytical quasilinear theory, nonlinear MHD computation


tasks

  • Measurements of dynamo

  • Active flow drive

  • Connections to more general problem


MHD Dynamo: Status of Past Measurements

  • edge measurements with probes,

    core with passive Doppler spectroscopy

  • Important in edge and core, but not full story


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


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


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


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


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


Investigate why alpha effect (<v x B>) large in lab, small in geometrically simpler computation

Specifics of flow pattern?


dynamo effects beyond MHD


Effects from Two-Fluid Theory

parallel mean-field component,

Linear part,

Using in top eqn,

MHD

Hall

Pressureless MHD

diamagnetic


<E>|| + dynamo = <j>||

Dynamotheory status

MHDQL, NL comp

HallQL

Diamagneticvery little

KineticRR,

constraints from

Landau resonances


Status of non-MHD effects

  • Diamagnetic dynamo observed in edge probe measurements under some conditions

  • Hall dynamo observed in MST recently


Hall dynamo measured by laser Faraday rotation

rotation angle

11-chord

FIR laser

(Ding, Brower, Deng)


Experimental Goal

Determine locations and conditions at which different dynamo mechanisms dominate


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)


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)


Related work:

Two-fluid computation in a box shows Hall dynamo important under some conditions

(Minnini, Gomez, Mahajan)


Ongoing

Assessment of astrophysical relevance of various dynamo effects


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