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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)

E||

j||

radius

additional current drive mechanism (dynamo)

Toroidal

Magnetic

Flux

(Wb)

MST

time (ms)

The MHD Dynamo

instability

energy

source

dynamo

MHD dynamo

analytical quasilinear theory, nonlinear MHD computation

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

- edge measurements with probes,
core with passive Doppler spectroscopy

- Important in edge and core, but not full story

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

- 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

information on 3D structure of magnetic field,

collaboration with RFX team (P. Franz, P. Martin, L. Mirelli)

spatial resolution being improved

- Present experiments:
- Flow drive experiments

Exploratory: flow drive via neutral beam injection,

inject ~ 25 keV beam of neutral atoms,

transfers momentum to plasma

Beam installed;

momentum transfer tested in few months

from Novosibirsk

If good absorption, then model with MHD computation, implement in MST and MRX

Specifics of flow pattern?

dynamo effects beyond MHD

parallel mean-field component,

Linear part,

Using in top eqn,

MHD

Hall

Pressureless MHD

diamagnetic

Dynamotheory status

MHDQL, NL comp

HallQL

Diamagneticvery little

KineticRR,

constraints from

Landau resonances

- Diamagnetic dynamo observed in edge probe measurements under some conditions
- Hall dynamo observed in MST recently

rotation angle

11-chord

FIR laser

(Ding, Brower, Deng)

Determine locations and conditions at which different dynamo mechanisms dominate

- 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)

- Expand quaslinear calculation to finite pressure
- Develop early nonlinear theory
(Rutherford theory)

- Develop two-fluid computation for lab plasma (Nimrod)

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

(Minnini, Gomez, Mahajan)

Assessment of astrophysical relevance of various dynamo effects