Inductive Current Profile Control and Sustainment in the RFP

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Magnetic Fluctuation Induced Charge Transport
and Its role in Relaxation on MST
Weixing Ding, D.L. Brower, B.H.Deng and
T.Yates University of California, Los Angeles
A. Almagri, J. Anderson, A. Blair, M.
Cengher, B. Chapman, D. Craig, D. Demers , D.
Ennis, D. Den Hartog, F. Ebrahimi, D. Ennis,
G. Fiksel, C. Forest, S. Gangadhara, J. Goetz,
C. Hegna, D. Holly, B. Hudson , K. McCollam, V.
Mirnov, R. OConnell, S. Prager, J. Sarff, V.
Svidzinski, M. Wyman Center for Magnetic
Self-Organization in Astrophysical and Laboratory
Plasmas Department of Physics,University of
Wisconsin, Madison

RFP workshop, 2005 Madison This work is
supported by U.S.DOE
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Magnetic and Current Density fluctuations play
an important role in plasma relaxation in the
Reversed Field Pinch
Particle and Charge Transport Current Transport
(dynamo) Momentum Transport Energy Transport
?J// (r)
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Outline

• Hall Dynamo (review)
• Ion Momentum Balance
• Current Fluctuation Induced Charge Transport
• All three processes are coupled through current
  density fluctuations.
• Implications (1) radial electric field and flow
  generation,
• (2) momentum transport, viscous damping
• (3) ion heating

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Faraday rotation system measures mean and
fluctuating B J

• Faraday rotation angle

11-chord FIR laser

32 magnetic coils toroidal array
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Measured Magnetic Fluctuation Profile
Current fluctuation surges at sawtooth crash
Current density fluctuation peaks at resonant
surface with width 8 cm
Inversion see Ding, et al,
RSI, 75,3387(2004)
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(1) Measured Current Profile Relaxation
At crash, current profile flattens
200 ?s
A/cm2
At crash, electric field increases
ET V/m
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Hall Dynamo from Two-Fluid Theory
Parallel mean field components
MHD Dynamo
Hall Dynamo
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Hall Dynamo is balanced by Electric Field
Hall dynamo peaks at resonant surface but is
spatially extended (8 cm c/wpi)
Ding,et al PRL,93,045002(2004)
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(2) Ion Momentum Transport over Sawtooth Crash
At sawtooth crash ion momentum in the core drops
on timescale much faster than classical viscous
time.
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Ion Momentum and Torque over Sawtooth Crash
(?jx?b torque acts on ions)
Surprising Imbalance !!
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(3) Fluctuation-Induced Radial Charge Flux
On MST for a specified (m,n) mode
At mode resonant surface charge flux is zero,
but can be non-zero locally (near the resonant
surface) to form charge filamentation
R.E. Waltz, Phys.Fluid,25,1269(1982)
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Measured Charge Flux at sawtooth crash in the
MST
Charge flux is radially localized and changes
sign across resonant surface
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Charge Transport and Radial Electric
Field
See R. E.Waltz, Phys. Fluids,
25,1269(1982) K.Itoh and S. Itoh, Plasma Phys.
Control Fusion,38,1(1996).
Ion polarization current, (c/VA)2104
Charge flux
Ion polarization current acts to offset charge
flux
At sawtooth crash, radial electric field is
established due to non-ambipolar transport
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Localized Radial Electric Field and ExB Flow
Charge flux generates a local Er with spatial
scale 5 cm. (1) ExB generates flow and flow
shear which may strongly damp the mean flow.
(2) Electric field (or potential well) can lead
to ion heating.
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Effect of ExB flow shear on ion momentum
transport
Enhanced viscous damping by local flow shear
can reduce mean flow near resonant surface.
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Effect of local electric field on Ion
Heating
This potential well superposes on equilibrium
potential. Electric field enhances the
collision between ions
Majority ion will gain energy up to
e???200-300eV Impurity Ion will gain energy up to
Ze??1.2kV for C6 --------In qualitative
agreement with observation of ion heating
-------Impurity ions are heated stronger than
majority ions
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Summary
Measurements indicate the following coupled
processes
Current relaxation
Tearing mode
Momentum relaxation
Ion Heating
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Evidence of flow and potential structure ?
Potential measurement by Heavy Ion Beam Probe
(HIBP)
(Preliminary) Ion velocity measurement by Charge
Exchange Recombination Spectroscopy
Gangadhara,S, Craig,D et al, CMSO genreal
meeting, San Diego (2005)
Lei,J, et al, PRL, 89,275001(2002)