Ion Transport and Fluctuation Suppression in Sheared Zonal Flow

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Ion Transport and Fluctuation Suppression in
Sheared Zonal Flow

• R. McWilliams, D. Edrich, and L. Chen
• Dept. of Physics and Astronomy
• University of California, Irvine

Supported by DoE Grant DE-FG03-99ER54542
Slides at http//HAL9000.ps.uci.edu
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Goals of Research Program

• Study ion transport, particularly cross-field
  diffusion and convection, in a region of sheared
  flow.
• Study fluctuation suppression and generation in
  same region of sheared flow.
• Analytic and simulation theory effort in
  conjunction with experiments.

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Laser-Induced Fluorescence (LIF)
fion(x,v,t)

• Time- space-resolved ion distribution functions
• All moments of fion are calculable. Flow fields,
  heating, phase-space diagrams,
• Of most interest for current experiment,
  transport D, D? , Dll, u (spatial diffusion,
  convection) Dv v , Cv (Fokker-Planck
  coefficients for velocity-space diffusion
  convection)

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A double LIF system Two pump ion lasers, in
background, drive two tunable dye lasers (one
with cover removed).
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How does LIF work? Fluoresce ion. G. Severn,
D.Edrich, R.McWilliams, Rev.Sci.Inst.69,10(1998)
Then scan laser frequency, or create test ion
1.5 GHz 1 km/s for argon
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Classical cross-field ion diffusion(Coulomb,
?n/n0lt0.002) with small turbulent
contribution. R.McWilliams, M.Okubo, Phys. Fluids
30, 2849 (1987)
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Turbulent (electrostatic) spatial ion diffusion
R.McWilliams, M.Okubo, N.Wolf, Phys. Fluids B2,
523 (1990)
Note linear relation. But, what about small
fluctuation levels?
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Turbulent diffusion does not depend quadratically
on fluctuation amplitude for small fluctuations.
Blue line is a quadratic fit to data.
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Optical tomography in velocity space shows
non-Maxwellian convecting at about 4 km/sec
below. M.Zintl, R.McWilliams, Rev. Sci. Instrum.
65, 2574 (1994)
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Velocity-space ion diffusion in quiet and
turbulent plasmas. J.Bowles, R.McWilliams,
N.Rynn, Phys. Plasmas 1, 3814(1994)
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Experiment Design

• Create simple cartesian-coordinate, steady-state
  sheared flow geometry
• Spatially localized electric field created by
  injection of e-beam sheet along magnetic field
• Measure electric field and fluctuation structures
  with probes (emissive, rf, Langmuir)
• Measure ion sheared flow (convection) and
  diffusion with laser-induced fluorescence (LIF)

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Irvine Torus

• R0.56m, a0.035m
• 0 lt Bo lt 0.25 T
• 1015 lt nionlt 1018 m-3
• 5 lt Te lt 15 eV
• 0.1 lt Tion lt 2 eV
• Argon

• wpe (1017 m-3) 2 x 1010s-1
• wpi 7 x 107 s-1
• wce(1kG) 2 x 1010 s-1
• wci 2 x 105 s-1
• lD (Te 10 eV) 7x10-5 m
• Vth e 2 x 106 m/s
• Vth i (0.1 eV) 1 x 103 m/s

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Experiment Geometry

• Horizontal electron current sheet creates local
  vertical E

• Vertical shear in horizontal plasma velocity, v,
  due to E x Bo

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E x B Induced Convection Shear

• Electron beam produces vertical potential profile
• Vertical E field combined with background
  horizontal Bo produces horizontal plasma drift
• Vertical Shear in horizontal plasma velocity due
  to varying E x Bo

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Electron BeamThe beam path is visible in photo
through a vacuum port window due to argon ion and
neutral excitation by the beam electrons
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Localized Electric Field Structure has been
created

• Red curve shows vertical (y-direction) electric
  field created by sheet e-beam at y0.

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Sheared Ion Flow Field has been measured

• Convective flow, vx, due to local E shows zones
  of flow and shear in y direction
• Vi,x Vi,x(y) and

vx
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Turbulence and Its Suppression

• Fluctuation spectrum as a function of vertical
  position moving through shear layer.
• Notice suppression of turbulence in shear layer

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• Low frequency (10 16 kHz) turbulence is
  suppressed in the shear region
• Turbulence suppression of 90 easily produced in
  shear layer

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Accomplishments and Future Work

• Sheared Flow has been established
• DC electric field structure and fluctuation
  spectra can be measured
• Fluctuations are suppressed in sheared region
• First ion diffusion measurements show D 0.58
  m2/s

• Need to measure ion diffusion with spatial
  resolution and spatial dependence through shear
  layer
• Need to measure fluctuation wavenumbers
• Need theory linking shear, fluctuations,
  transport (Liu Chen)
• Study over varied shear parameters