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Measurement of the Charge of a Particle in a Dusty Plasma

Measurement of the Charge of a Particle in a Dusty Plasma. Jerome Fung, Swarthmore College July 30, 2004. Introduction. What is a dusty plasma? Why do we care about dusty plasmas? Making dusty plasma crystals The importance of electric charge Theory: Vertical resonance methods

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Measurement of the Charge of a Particle in a Dusty Plasma

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  1. Measurement of the Charge of a Particle in a Dusty Plasma Jerome Fung, Swarthmore College July 30, 2004

  2. Introduction • What is a dusty plasma? • Why do we care about dusty plasmas? • Making dusty plasma crystals • The importance of electric charge • Theory: Vertical resonance methods • Preliminary results • Sound speed methods

  3. Low PressureGas Anode Cathode P L A S M A High Voltage What is a plasma? • Plasma: ionized gas • Contains positive ions, negative electrons, and neutral particles • 4th state of matter: hotter than gases • Most abundant state of matter in the universe: found in stars, fluorescent light bulbs!

  4. plasma = electrons + ions small particle of solid matter • absorbs electrons and ions • becomes negatively charged What is a dusty plasma? & neutral gas

  5. Solar system • Rings of Saturn • Comet tails • Manufacturing • Particle contamination • (Si wafer processing) • Fundamental science • Coulomb crystals • Waves Who cares about dusty plasmas?

  6. Dusty Plasma Crystals • Small (micron-sized) particles in plasma disperse into 2-D lattice • Exhibits properties of solid crystal • Order of crystal lattice

  7. Argon RF plasma 20 mTorr 8 - 20 W Polymer microspheres diameter 8.09 ± 0.18 m Making Dusty Plasma Crystals

  8. Voilà!

  9. qE ∑F = 0 mg Particle Interactions and Forces • Electrostatic ( Fe = q E ) • Levitating sheath electric field • Horizontal particle confinement • Interparticle interactions • Gravitational ( Fg = m g ) • Ion drag force, gas drag, thermophoresis

  10. Charge matters! • Electrostatic force is the most significant • Many interactions, all depend on q • Most experiments/theory require knowledge of q • Measurement techniques • Vertical Resonance (Melzer et al., Phys. Lett. A 191,1994) • Variation of vertical resonance (Goree) • Sound speed methods • Natural phonons • Laser-induced longitudinal / transverse waves

  11. Vertical Resonance Method • Key idea: modulate levitating RF electric field to “shake” crystal up and down, measure amplitude of oscillation • In practice, modulate voltage on electrodes • View oscillations via side view video camera • Observe resonance ⇒ measure resonance frequency ⇒ determine particle charge!

  12. Vertical Resonance: Theory Damped, driven oscillator equation: Resonance frequency: ni = plasma ion density (ions/unit volume)

  13. Vertical Resonance: Issues • Original method: requires measurement of ion density • Must be measured with a Langmuir probe in the bulk plasma, above the sheath • Problem: method requires extrapolation of ion density in bulk plasma to sheath • Large uncertainties in q, ~50% in original papers • Modified method • Does not require ion density measurement • Makes assumption about the variation of the sheath electric field, which has been tested experimentally • Should result in smaller uncertainties

  14. Preliminary Results: Resonance Curve ni ≈ 2 × 1015 m-3 ωo/2π = 10.08 ± 0.01 Hz m ≈ 4.2 × 10-13 kg q ≈ 3000 e

  15. Vertical Resonance: Variation • Assumes linear dependence on height for the electric field in the sheath • Uses more easily measured quantities (e.g. plasma potential) instead of ion density q ≈ 9000 e

  16. Sound Speed Methods • Charge determined from material properties of plasma crystal • Natural phonons • Laser-induced pulses • Longitudinal • Transverse

  17. Longitudinal Pulse

  18. Conclusions • Knowing charge necessary for lots of interesting experiments / theory with dusty plasma crystals • Charge measured with 2 vertical resonance methods • Further analysis of data from these methods and from sound speed methods is ongoing

  19. Acknowledgements This project would not have been possible without the advice and assistance of Dr. Bin Liu and my advisor, Prof. John A. Goree. Several useful discussions with V. Nosenko and K. Pacha were also had. Work supported by an NSF REU grant.

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