1. B. Liu, J. Goree, V. Nosenko, K. Avinash
Radiation pressure and gas drag forces on a single particle and wave excitation in a dusty plasma
B. Liu, J. Goree, V. Nosenko, K. Avinash

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

3. Forces Acting on a Particle
Coulomb QE
Gravity mg
Other forces:
Gas drag
Ion drag
Thermophoresis
Radiation Pressure

4. Particles polymer microspheres 8 mm diameter separation a » 0.5 mm
charge Q » e

5. Confinement of 2D monolayer
Interparticle interaction is repulsive Coulomb (Yukawa)
External confinement by curved electric sheath above lower electrode

6. 2D lattice triangular lattice with hexagonal symmetry
Yukawa inter-particle
potential

7. Radiation Pressure Force
incident laser intensity I
momentum imparted to microsphere
transparent microsphere
Force = I  rp2

8. Argon laser pushes particles in the monolayer
Setup
Argon laser pushes particles in the monolayer

9. Chopping chopped beam beam dump scanning mirror chops the beam Ar
laser
mirror

10. Single-particle laser acceleration
laser beam
radiation
pressure
Accelerated by laser radiation pressure
Coulomb
drag
Restored by confining potential
Damped by gas drag

11. Movie of particle accelerated by laser beam
2 mm
Ar laser
sheet

12. Equation of motion Assumption: The dominant forces are
Gravity
Vertical sheath electric field
Radiation pressure force
Drag force
Horizontal confining potential
One dimensional motion

13. Calculation: radiation pressure, gas drag, confining potential
record particle’s orbit
Calculation: radiation pressure, gas drag, confining potential
Gas drag coefficient R is an adjustable parameter to minimize the discrepancy between and
R  R

14. Horizontal confining potential energy

15. Radiation pressure force

16. Gas drag force

17. Coefficients for radiation pressure and gas drag
q result:
measurment 0.94  0.11
ray optic theory 0.97
Gas drag
 result:
measurment  0.13
Epstein theory ~ 1.44
Epstein, Phys. Rev. 1924

18. Application of radiation pressure force
Laser sheet

19. Dispersion relations in 2D triangular lattice
Q=0,
 / 0
Dispersion relations in 2D triangular lattice
Wang et al. PRL 2001

20. Waves in one-dimensional dusty plasma chain
Longitudinal (along the chain) : acoustic
laser
beam y x z
Transverse (perpendicular to the chain) : optical
The oscillation in
y direction ( horizontal confining potential)
z direction ( potential well formed by gravity and sheath )

21. Optical mode in solid (two atom in primitive cell)
acoustic

22. Optical mode in one-dimensional chain
Assumptions:
One dimension, infinite in x direction
Parabolic confinement in y direction
Yukuwa interaction potential
Nearest neighbor interaction
No gas damping
Optical:
Acoustic:

23. “Optical” branch
Acoustic branch
Dispersion relation

24. Formation of one-dimensional chain
22-particle chain
Ashtray electrode x y z

25. Bifurcation of chain y x Potential gradient in x direction
Minimum potential energy requirement
Particle-particle interaction energy
Confining potential energy

26. Bifurcation conditionUy y 1 2 Ux x
No bifurcation condition
Case 1
Case 2

27. Resonance frequency: x
x = 0.07 Hz
Single-particle
laser acceleration

28. Resonance frequency: y
laser-excited
resonance vibration
laser sheet

29. Resonance frequency: y
Velocity autocorrelation function of random motion

30. Excitation of optical mode
Laser beam

31. Excitation of optical mode
Laser beam

32. dusty.physics.uiowa.edu