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

HeliacJ. Read, J. Howard, B. Blackwell, David Oliver, & David

Pretty

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

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

3D. A. Spong, Energetic particle physics for three dimensional toroidal configurations

W-7AS

TJ-II

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

Taken from Observations of Alfvénic MHD Activity in the H-1 Heliac, B. D. Blackwell

Degree of “Twist”

Magnetic Fluctuations

Degree of “Twist”

Electron Density Fluctuations

• 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

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h

h

Global Alfvén eigenmodes in H-1

• 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

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Field Line (Red)

Mode Structure

(Blue-Gray)

n/m-

n/m

How do we sense the waves?• Mirnov (Magnetic) pick-up coils

– measure magnetic field fluctuations

• Broadband light emission – indicator of density fluctuations (Ĩ ~ ñe)

• 16 channel PMT detectors for profile measurements

• Multiple toroidal viewing locations

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Toroidal Field Coils

Trace of Magnetic Field Line

Poloidal Field Coil

• 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.

Dynamic sweeping of rotational transform

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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.

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Intensity (arbitrary units)

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 – Lenz’s Law

• L/R = 3.5ms R = 2.8mΩ (L = 10mH)

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5/4 4/3

Time increasing

Time increasing Log of Cross Power

Phase flips about resonance• Observed 180o phase shifts in ñe with respect to the

magnetic fluctuations at the resonances

• What causes this?

11Time increasing

Time increasing

Log of Cross Power

Phase Difference in Degrees

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

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Mode helicity

Toroidal angle 312.5o

Toroidal angle 240o

Phase shear

Time increasing

Time increasingPhase Difference

in Degrees

Mode Rotation

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• 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)

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

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