mhd dynamos in the lab and dynamos beyond mhd. the lab plasma dynamo does generate current locally...
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![Page 1: MHD Dynamos in the Lab and Dynamos Beyond MHD. The lab plasma dynamo does Generate current locally Increase toroidal magnetic flux Conserve magnetic helicity](https://reader035.vdocuments.us/reader035/viewer/2022062318/55151874550346a80c8b5ed8/html5/thumbnails/1.jpg)
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
-0.5
0.5
1.0
1.5
2.0
V/m
0.0
0.0 0.2 0.4 0.6 0.8 1.0ρ/a
E||
ηneo J||(Zeff = 2)
€
E ≠ η j
E||
ηj||
radius
additional current drive mechanism (dynamo)
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Measure quantities during discrete dynamo event
ToroidalMagneticFlux(Wb)
MST
time (ms)
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The MHD Dynamo
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The nonlinear dynamo
€
⟨E⟩
€
⟨ j⟩
⟨B⟩
€
⟨ ˜ v ⟩,⟨ ˜ B ⟩energysource
instability
dynamo
€
E + ˜ v × ˜ B = η jMHD
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
€
⟨ ˜ v × ˜ B ⟩
€
η⟨ j⟩− ⟨E⟩
MHD dynamo dominant at some radii, not everywhere
r/a = 0.8
Measurement of MHD dynamo
0
-10
-20
0
-20
-10
Volts m
Volts m
-0.5 0 0.5time (ms)
r/a = 0.9
r/a = 0.8
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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
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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)
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
spatial resolution being improved
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Active Flow Drive
• Present experiments:
• Flow drive experiments
€
j ⇒ ˜ v ⇒ B
€
v ⇒ B
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 (<v x B>) 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,
€
E + v × B −j × B
ne−∇pe
ne= ηj
€
E||+ ˜ v × ˜ B
||+
˜ j × ˜ B ||
ne= η j
||
€
˜ v −˜ j
ne≈
˜ E × B
B2 −
∇˜ p e × B
B2
€
E||+
˜ E ⊥˜ B ⊥
B2 +
(∇⊥˜ p e ) ˜ B ⊥
B2 = η j
||
MHD Hall
Pressureless MHD
diamagnetic
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<E>|| + dynamo = η<j>||
Dynamo theory statusMHD QL, NL comp
€
˜ v × ˜ B
€
˜ j × ˜ B
€
˜ B ⊥∇˜ p ⊥
€
˜ B r ˜ p ||
Hall QL
Diamagnetic very little
Kinetic RR, constraints fromLandau 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
€
~ nB ⋅dl∫
(Ding, Brower, Deng)
11-chordFIR 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 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)
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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