Magnetic Fluctuations In High Density Pulsed Plasma

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 atmosphereof 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 0.072 m Plasma source Plasma Duct Substrate R.Davies 2003 Figure 3: The Experimental Setup for the vessel.

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

7. Vertically Mounted Mirnov Probe Horizontally Mounted Mirnov Probe MirnovCoils. • 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 Figure 6: Mirnov Probes Mounted on the vessel.

8. Location of Mirnov Probes in the vessel Mirnov 2 • By locating Mirnov probes in this position (1cm inside the duct) the phase shift, rotation and rotation angle between BΦ & Bθhas been measured. duct Mirnov 1 Plasma Mirnov 3 Figure 7: Schematic of the experimental setup for Mirnov probes.

9. Typical Mirnov Signal

10. Scanning across The Plasma Plasma centre Plasma edge No Plasma

11. Observation of oscillations. 0 V 30 - 50 KHz

12. Mirnov Probes Detecting Rotation Mirnov 2 • Three Mirnov probes used to detect rotational behaviour of plasma. • Analysis in 200 - 300 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 1 Plasma Mirnov 3 Figure 8: Direction of Rotation of Bθ

13. Plot of Plasma Rotation

14. Discussion. Fluctuation frequency, [kHz] • Region of low rotational velocities in the 40 -60 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

15. 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 200-1000 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. Project Summary

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 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?