Results from Visible Light Imaging of Alfv

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Results from Visible Light Imaging of Alfvén
Fluctuations in the H-1NF Heliac

• J. Read, J. Howard, B. Blackwell, David Oliver,
  David Pretty

Acknowledgements Greg Potter, John Wach, Mark
Gwynneth, Horst Punzmann
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Outline

• What are Alfvén waves?
• The H-1NF Heliac
• Experimental setup and apparatus
• Results
• Conclusions and future research

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Toroidally confined waves

• Toroidal geometries enforce periodic boundary
  conditions on Alfvén waves
• ? (n/m-?m/R) vA
• ? ? 0 as ? ? n/m
• whale tail resonances.
• Generally global structures

TJ-II
W-7AS
D. A. Spong, Energetic particle physics for three
dimensional toroidal configurations
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The H-1 Heliac at The Australian National
University

• An experimental magnetic confinement device (of
  the stellarator class)
• Degree of twist can be finely controlled in H-1
  by setting ?h (Ihelical/Imain) the
  configuration parameter

Michael PhD thesis 2003.
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Waves and instabilities in H-1

• Instabilities exhibiting Alfvénic properties have
  been discovered in H-1
• Magnetic fluctuations correlate with electron
  density fluctuations typical of Alfvén
  instabilities
• Excitation mechanism unknown although theories
  exist

?h
?h
Taken from Observations of Alfvénic MHD Activity
in the H-1 Heliac, B. D. Blackwell
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Global Alfvén eigenmodes in H-1
Mode Structure (Blue-Gray)

• Mode structure and magnetic field line can be
  imagined as a double helix, each helix with
  different pitch angle
• Resonance occurs when the pitch angles are equal
  ? helices lie on top of each other
• ? (n/m-?m/R) vA
• Mismatch caused by differing pitch angles
  produces pressure gradient along line of force
• Propagation caused by interchange of magnetic
  (?B2/2µ0) and kinetic (?nkT) pressures

n/m-?
?
n/m
Field Line (Red)
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How do we sense the waves?
Toroidal Field Coils

• Mirnov (Magnetic) pick-up coils measure
  magnetic field fluctuations
• Broadband light emission indicator of density
  fluctuations (I ñe)
• 16 channel PMT detectors for profile measurements
• Multiple toroidal viewing locations

Poloidal Field Coil
Trace of Magnetic Field Line
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Dynamic sweeping of rotational transform

• Ability to obtain radial profiles in a single
  shot using the 16 channel PMT detectors.
• These studies performed using continuously
  scanned configurations over a single shot with
  the 16 channel PMT detectors this is the first
  time this has been done.
• We have 2 PMT arrays at different toroidal
  positions toroidal mode structure may be
  explored.

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New PMT detector gives rotation information

• Installed a 16 channel PMT for broadband light
  emission measurements.
• Views plasma at an angle which breaks the
  symmetry of previous light imaging multi-channel
  PMT detector.
• Forward modelling shows shear in the projections
  (which depends on poloidal rotation direction)
  which is not seen in the previous system.

Intensity (arbitrary units)
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Delayed field penetration

• The expected 5/4 (n/m) resonance is at ?h 0.4
• Observed ?h delayed or advanced depending on
  direction of Ihelical sweep ? inducing a current
  in the plasma delays field penetration Lenzs
  Law
• L/R 3.5ms ? R 2.8m? (L 10mH)

5/4
4/3
Time increasing
Log of Cross Power
Time increasing
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Phase flips about resonance

• Observed 180o phase shifts in ñe with respect to
  the magnetic fluctuations at the resonances
• What causes this?

Log of Cross Power
Time increasing
Phase Difference in Degrees
Time increasing
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Phase flips about resonance (cont...)

• The sense of the phase between the magnetic and
  light fluctuations changes about the resonance
• Possible cause is the change in sense in the
  mismatch between the helices (n/m - ?).

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Mode helicity

• Constant, steady phase difference between the two
  toroidally separated PMT arrays ? mode structures
  maintain their helicity (resonant structure) in
  varying configurations.

Toroidal angle 312.5o
Phase shear
Time increasing
Phase Difference in Degrees
Toroidal angle 240o
Time increasing
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Mode Rotation

• Light emission profiles were sheared, direction
  of shear dependent upon poloidal rotation
  direction of the wave
• Evidence of a counter propagating mode around
  resonance points
• Increased density fluctuations at these points
  indicate the presence of a sound wave which is
  not apparent away from resonances ? mode
  conversion at resonances

Intensity (arbitrary units)
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Conclusions and future research

• Obtained the first light emission profiles from
  a continuously varied magnetic field
  configuration
• Modes appear to convert from Alfvén to sound
  waves near resonances accompanied by phase
  reversals between magnetic and light fluctuations
• Intend to place more imaging systems at different
  poloidal and toroidal locations.
• Construct full models of the spatial mode
  structure using methods of tomography