Experimental Study of Radial Wire Arrays with Added Axial Magnetic Fields

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Experimental Study of Radial Wire Arrays with
Added Axial Magnetic Fields
Jet Experiments, Simulations and Theory

• F. Suzuki Vidal, S.V. Lebedev, S.N. Bland, S.C.
  Bott,
• J.P. Chittenden, G.N. Hall, A. Marocchino, J.B.A.
  Palmer
• Imperial College London

A. Ciardi LObservatoire de Paris

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Outline

• Laboratory astrophysics
• Radial wire arrays
• Experimental setup
• Results
• Radial wire arrays with added BZ
• Experimental setup
• Results
• Comparison with no-BZ case
• Conclusions

• Future aims

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Laboratory astrophysics

• Plasma jets are observed in large scale phenomena
• Supernovae, Active Galactic Nuclei,Young Stellar
  Objects

• Laboratory astrophysics with z-pinches (small
  scales!)
• Conical wire array
• Radial wire array
• Dimensionless parametersare similar in both
  cases
• Mach number, density contrast, cooling
  parameter, b parameter, etc.

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Experimental setup

• MAGPIE generator
• Current of 1 MA in 240 ns

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Experimental setup

• MAGPIE generator
• Current of 1 MA in 240 ns

• Radial wire array
• Z-pinch configuration
• 16 tungsten wires (13mm diameter) held
  horizontally
• 6.3 mm diameter cylindrical cathode

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Experimental setup

• MAGPIE generator
• Current of 1 MA in 240 ns

• Diagnostics
• XUV imaging
• X-ray detectors
• Laser shadowgraphy and schlieren
• Optical streak camera
• Magnetic probes

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Radial wire array Dynamics
(Lebedev et al, 2005)
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Radial wire array Results

• Formation of bubbleand axial jet
• Typical z-pinch MHDinstabilities observed on
  the central plasma jet

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Radial wire array Results

• Measured expansion velocities of emitting
  boundary
• VR 36 km/s
• VZ 144 km/s

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Radial wire array with added BZ

• Why BZ?
• An axial magnetic field can stabilise the z-pinch
  from instabilities (Felber, 1988)
• Jet rotation? (Lebedev et al, 2002)

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Radial wire array with added BZ

• BZ introduced with solenoids, using the same
  current passing through the wires
• 2 configurations

1. Solenoid replacing the standard returns posts
2. Solenoid replacing the central cathode
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Added axial BZ Setup

• Calculated magnetic field on axis for maximum
  current

• Return solenoid has a more constant magnetic
  field along the axis
• Cathode solenoid has a stronger BZ, decaying
  faster with distance
• In both cases, BZ field is small compared with
  Btoroidal
• BZ lt 10 Btoroidal

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Added BZ Results

• XUV emission

• Bubble behaviour ismaintained

• Expansion velocities increased with BZ

• Features from the jet are observed with soft
  x-ray filters

Soft x-ray emission
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Added BZ Results (comparison)

• XUV emission

• Central jet diameterincreased with appliedBZ
• MHD instabilities are still observed

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Added BZ Results (comparison)

• XUV emission

• Evolution of features formed in the jet can be
  measured
• Features moving fasterwith axial distance?

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Radial wire array Results

• Dynamics of the bubbleare observed from
  streakimages
• X-ray emission nearcompression of the
  centralplasma column

Optical streak camera
R
t
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Added BZ Results

• Optical streak camera

• With added BZ
• Axial jet is stable for longer times
• Larger diameter of central plasma jet
• Decrease of x-ray emission

• Bounce of compressing plasma
• Compression of magneticfield?
• Jet rotation?

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Conclusions

• Experiments with radial wire arrays have been
  performed on MAGPIE, producing plasma jets
  magnetically confined by a predominant toroidal
  field.
• Additional axial magnetic fields BZ were applied
  to a wire radial array with and compared with a
  no applied BZ case.
• The dynamics of the magnetic bubble were
  maintained, observing an increase in the diameter
  of the central plasma jet. This suggests a
  compression of the added axial field BZ by the
  toroidal magnetic field or possible jet rotation.

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Conclusions

• Further analysis from present and future results
  will provide measurements of parameters for
  comparison with simulations (A. Ciardi and A.
  Marocchino) and for obtaining dimensionless
  parameters.

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Future aims

• New experiments are being conducted with
  different cathode diameters and wire materials
• Jet dynamics and expansion velocities of bubble
• X-ray emission study
• Improvement of magnetic diagnostics to measure BZ
• Measurement of mass density profiles in the
  ambient plasma.
• Have the dimensionless parameters changed
  significantly from previous experiments?
• New geometries for adding BZ!

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Future aims

• Twisting of anode structure to produce BZ

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Thank you!