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Flinders University of South Australia Leon Mitchell Nathan Prior University of Sydney Brian James

Flinders University of South Australia Leon Mitchell Nathan Prior University of Sydney Brian James Alex Samarian Felix Cheung. Program. Introduction Experimental setup Results and discussion on the rotation of various dust plasma crystal configurations: Large Small Annular

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Flinders University of South Australia Leon Mitchell Nathan Prior University of Sydney Brian James

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

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

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

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

  7. _  X

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

  9. _  X

  10. Experimental Setup _  X

  11. Experimental Setup _  X

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

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

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

  15. 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. 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. Rotation of Large Crystals _  X • Double layer crystal formed • Particles are stable • Brownian motion • Local position exchange No Magnetic Field • 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 B Magnetic Field On

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

  19. Trajectory of the Planar 2 Structure _  X Legend Particle 1 Particle 2 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

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

  21. Electrode Improvement Copper electrode  

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

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

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

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

  27. Ion Drag   Ion Drag Force Ion Drag Force z Neutral Drag Force Neighboring Particle Force Neighboring Particle Force Source: G. E. Morfill et al (Feb 2000) Rigid and differential crystal rotation induced by magnetic fields • The equation of motion of the particles in cylindrical coordinates is: FCent= FElec + FID + FID + FND + FNP + FNP • we can approximate the above in the azimuthal direction: FID ()+ FND 0 • However, the magnitude of both forces are out by an order of 102

  28. Change of potential? V r • Magnetic field might affect electric potential • Electrons confined by magnetic field more than ions because of smaller mass (Bq/m) • 2V = -/o •  ~ ni + ne A change in the shape of the potential might make particle to rotate

  29. Rotation of Small Crystals Planar 1 Planar 2 Planar 4 Planar 5 Planar 3 Planar 8 Planar 9 Planar 10 Planar 11 Planar 12 Planar 13 Planar 14 Mandeleev table of the periodic packing of N particles Planar 7 Planar 6 Planar 15 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. 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 Velectrode (again structure changes…) • Use Helium plasma • Provide information on importance of ion drag • Consideration of Neighboring particle forces

  31. 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. 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. Thank you for coming!

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