1. Magnetic Fluctuations In High Density Pulsed Plasma
Sarvenaz Sarabipour
Applied and Plasma Physics, The School of Physics, The University of Sydney, NSW, Australia, 2006

2. Outline What is a plasma; Pulsed arc plasma source setup;
Mirnov coils;
Results;
Conclusions and future work;

3. So What is a Plasma?! The fourth state of matter.
99% of visible matter in the Universe.
Examples are: Stellar interiors and atmospheres, nebulae, lightning, fluorescent tubes, plasma displays and fusion plasmas.
Definition: A form of quasi-neutral electrified gas with the atoms dissociated into positive ions and electrons.
Figure 1: The interior and atmosphere of the Sun is in the plasma state (x-ray view).

4. Magnetically Confined Plasmas
Historically, plasma physics driven by fusion research, e.g. Tokamaks.
More recently, plasma processing uses plasmas for material applications.
Important plasma parameters: Toroidal B-field (BΦ), poloidal B-field (Bq ) and plasma current.
Figure 2: Example of a magnetically confined plasma in a Tokamak.

5. Pulsed Arc Vacuum Vessel
Consists of a half torus.
A magnetic plasma duct inside a quarter of the torus.
Major radius of the vessel is 0.44 m.
Duct radius is m
Plasma source
Plasma Duct
Substrate
R.Davies 2003
Figure 3: The Experimental Setup
for the vessel.

6. Pulsed Cathodic Arc Very high density, up to 10²³ m-³.
50 mm
Very high density, up to 10²³ m-³.
Almost 100% ionised
Positively biasing the magnetic duct assists plasma transport
Plasma duct
windings x y z
Anode mouth
Figure 4: 8 μs exposure image of plasma taken 300 μs after arc initiation (False colour image).

7. Average diameter of ~ 8.37 mm
Mirnov Coils.
Vertically Mounted
Mirnov Probe
Directly measure dBθ/dt & dBΦ/dt to detect magnetic fluctuations
An active integrator integrates the signal initiating form the probe (integrates over dB/dt to gives us BΦ & Bθ)
11 Turns,
Average diameter of ~ 8.37 mm
Figure 5: schematic of a Mirnov coil
Horizontally Mounted
Mirnov Probe
Figure 6: Mirnov Probes Mounted
on the vessel.

8. Location of Mirnov Probes in the vessel
duct
By locating Mirnov probes in this position (1cm inside the duct) the phase shift, rotation and rotation angle between BΦ & Bθ has been measured.
Mirnov 1
Plasma
Mirnov 3
Figure 7: Schematic of the experimental setup for Mirnov probes.

9. Typical Mirnov Signal

10. Scanning across The Plasma
centre
Plasma
edge
No Plasma

11. Observation of oscillations.
KHz

12. Mirnov Probes Detecting Rotation
Three Mirnov probes used to detect rotational behaviour of plasma.
Analysis in μs time interval.
The top Mirnov coil consistently led the horizontal and bottom vertical Mirnovs → clockwise rotation.
Rotational velocities have been measured for various duct biases and duct (toroidal) magnetic field.
Mirnov 2
Mirnov 1
Plasma
Mirnov 3
Figure 8: Direction of Rotation of Bθ

13. Plot of Plasma Rotation

14. Fluctuation frequency, [kHz]
Discussion.
Region of low rotational velocities in the V range and 15 mT.
Else where velocities up to 1000 krads-1! Consistent with literature.
Other measurements show:
Low oscillations at high magnetic field and high duct bias
Strong oscillations 30-50kHz at low magnetic field and high duct bias
Fluctuation frequency, [kHz]

15. Project Summary We measured: 1)The physical dimensions of the plasma
2)The strength and fluctuations of the BΦ & Bθ fields.
● We found:
1) Plasma diameter ~ 4 cm.
2) The magnetic field oscillations rotate clockwise with velocities of krad.s-1
3) there are strong magnetic oscillations in the
plasma with an inverse relationship
between angular velocities and the
amplitude of the oscillations.

16. Improvements & Further Directions
Using a series of Mirnov coils (e.g. 16) in a circle or semi-circle or placing the coils along the torus, to elucidate further the structure of magnetic fluctuations in the plasma.
Further experiments to determine whether the plasma is diamagnetic or paramagnetic.

17. Acknowledgments The University of Sydney:
This project has been greatly assisted by:
The University of Sydney:
A/Prof. Rodney Cross,
Prof.Marcela Bilek,
Prof.David Mckenzie,
Dr.Richard Tarrant (supervisor),
Daniel Andruczyk (supervisor),
Luke Ryves,
John Pigott,
Phil Denniss,
The Australian National University:
Dr.Mattew Hole
Prof. Boyd Blackwell.

18. Any Questions?