1. Flinders University of South Australia Leon Mitchell Nathan Prior University of Sydney Brian James Alex Samarian Felix Cheung

2. X_ Program Introduction Experimental setup Results and discussion on the rotation of various dust plasma crystal configurations: Large Small Annular Future research Summary of experimental results Conclusion

3. X_ What is a Dust Plasma Crystal? A well ordered and stable array of highly negatively charged dust particles suspended in a plasma Relatively new scientific field with many interesting properties discovered Theoretical prediction Ikezi (1986) Experimental confirmation Lin I et el (1994), Thomas H et el (1994) Recent interests include: Dynamical behavior (oscillations, waves, convective motion) Solid State (melting, crystal growth, structural packing)

4. X_ The Basic Ingredients Melamine formaldehyde polymer Spherical, micron-sized and mono-dispersed Electrons: Small, light and fast moving Frequent collisions with particles Ions: Big, heavy and slow moving Minimal interaction with particles Neutrals: Damping in system Argon plasma Dust particles

5. X_ Gravitational Force Electrostatic Force Dust Plasma Crystal Production Produce an argon plasma Disperse particles with shaker Particles will then: Charge negatively Fall under gravity Balance at sheath-plasma interface Mg = eZE s Arrange to form crystal structure

6. X_ Experimental Setup Observation window Teflon rod Shaker RF coil Viewing area Quartz window Turbo-molecular vacuum pump Electrode and coil on mount He-Ne laser Rail Adjustable mount Signal generator Matching network Baratron Penning gauge ENI amplifier Magnetically coupled manipulator Turbo-molecular vacuum pump controller RF coil set in Araldite Vacuum Chamber Argon gas inlet Connection pin Mesh wire Grounding pin Backing pump

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8. X_ Observation window Teflon rod Shaker RF coil Viewing area Quartz window Turbo-molecular vacuum pump Electrode and coil on mount He-Ne laser Rail Adjustable mount Signal generator Matching network Baratron Penning gauge ENI amplifier Magnetically coupled manipulator Turbo-molecular vacuum pump controller RF coil set in Araldite Vacuum Chamber Argon gas inlet Connection pin Mesh wire Grounding pin Backing pump Half cylindrical lens

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10. X_ Experimental Setup

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12. X_ Side View A Closer Look… Aluminum electrode mount Nickel-plated steel magnetic coil Copper electrode Top View

13. X_ Crystal Structure Hexagonal closed packed expected Simple hexagonal structure observed Hexagonal patterned (top) Vertical alignment (side) Simple HexagonalHexagonal Close Packed

14. X_ Crystal Structure Structure not solely Coulomb force dependent Other factors: Ion drag (ion focusing) Neutral damping Gravity Neighbor interaction Confining potential Thermophoretic force Source: Ishihara O. and Vladimirov S.V. (1997) Wake potential of a dust grain in a plasma with ion flow

15. X_ Crystal Rotation in an Axial B Field First reported by Sato et al. (1998) When an axial magnetic field is applied, the particles will rotate collectively Weakly ionized argon dc plasma, 120G magnetic field was used. Rotational behavior of particles is right-handed in direction dependent on the magnetic field strength and number density

16. X_ Our Project Aim To rotate large crystals (~1000 particles) An axial B field (~130G) was used To determines whether rotation exists within our system To rotate small crystals (1-20 particles) An axial B field (~23G) was used Provide much simpler model for analysis Never been done before To rotate annular crystals (with void at center) To study the collective effect of the Argon ions Provide insights to the properties of void

17. Double layer crystal formed Particles are stable Brownian motion Local position exchange Collective rotational motion of particles Uniform angular velocity Left-handed direction !! [Lin I et al. (1999)] One rpm approximately No radial variance No shear velocity X_ Rotation of Large Crystals

18. X_ Rotation of Small Crystals 1 only 1 particle 2 string Planar 2 2 particles 3 string Planar 3 2-on-1 3 particles Tetrahedral 4 string Planar 4 3-on-1 4 particles 4-on-1 5 particles Planar 5 5-on-1 1-on-2-on-3 6 particles Planar 6 Can Spin Not Confirmable Partially Spins

19. Trajectory of the Planar 2 Structure Trajectory of the 2 particles in rotational motion is circular In other cases, the trajectory of the particles in a particular layer is circular The angular velocities of these crystals are in the order of few rpm Particle 1 Particle 2 Legend X_

20. Angular Position vs. Time (Planar 2) Planar-2 & 2-on-1structures exhibit periodic pausing in their motion In general, small crystals rotate collectively in left-handed direction X_ Particle 1 Particle 2 Legend

21. X_  Copper electrode Electrode Improvement

22. In other crystal structures, angular velocity is constant with time X_ Angular Velocity vs. Time of 5 Particles on 1 at Different Magnetic Field Strength 10G 14G 18.5G 23G

23. X_ Angular Velocity vs. Magnetic Field Strength of 5 Particles on 1 Increase in magnetic field strength => Increase in angular velocity

24. X_ Angular Velocity vs. Magnetic Field Strength of 5 Particles on 1 Increase in electrode voltage => Decrease in angular velocity

25. X_ Collective effect of Ar + vortices Ar+ exhibit horizontal velocity component vxB drift will make Ar+ orbit about the base of the dust particles, creating Ar+ vortices Summation of all the Ar+ vortices will cause the crystal to rotate as a whole

26. Void was induced at the center of crystal Left-handed rotation observed No shear velocity between the boundaries X_ Rotation of Annular Crystals Another model explaining the rotation is needed The idea behind most explanation is also along the idea of ion drag

27. X_ Source: G. E. Morfill et al (Feb 2000) Rigid and differential crystal rotation induced by magnetic fields Ion Drag Force Neighboring Particle Force Neutral Drag Force Ion Drag Force   z Ion Drag The equation of motion of the particles in cylindrical coordinates is: F Cent  = F Elec  + F ID  + F ID  + F ND  + F NP  + F NP  we can approximate the above in the azimuthal direction: F ID  (  )+ F ND   0 However, the magnitude of both forces are out by an order of 10 2

28. X_ Change of potential? Magnetic field might affect electric potential Electrons confined by magnetic field more than ions because of smaller mass (Bq/m)  2 V = -  /  o  ~ n i + n e A change in the shape of the potential might make particle to rotateVr

29. X_ Rotation of Small Crystals Planar 6 Planar 4 Planar 5 Planar 3 Planar 2 Planar 1 Planar 7 Planar 8 Planar 9 Planar 10 Planar 11 Planar 12 Planar 13 Planar 14 Planar 15 Mandeleev table of the periodic packing of N particles Source: Lin I et al (Apr 1999) Structures and motions of strongly coupled quasi-2d dust coulomb clusters in plasmas: from small N to large N

30. X_ Future Research Examination of the relationship between  and N (with different shells?)  and B (single layer/ multi-layer)  and p (hard to do as structure changes…)  and V electrode (again structure changes…) Use Helium plasma Provide information on importance of ion drag Consideration of Neighboring particle forces

31. X_ Summary Large, small and annular dust crystals rotate collectively under the influence of an applied axial magnetic field Direction of rotation is left-handed to the magnetic field Increase in magnetic field strength corresponds to an increase in the angular velocity Angular velocity is in the order of few rpm Increase in electrode confinement voltage corresponds to a decrease in angular velocity

32. X_ Conclusion Models using ion drag to explain rotation of crystal do not agree with experimental results Current models are insufficient to explain the nature of rotation in general Need to develop a new theoretical model Need to study the effect from magnetic field on the electric potential

33. X_ Thank you for coming!