**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

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

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

**polymer microspheres** 8 mm diameter Particles • separation a» 0.5 mm • charge Q» - 104e

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

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

**momentum imparted to microsphere** Radiation Pressure Force incident laser intensity I transparent microsphere Force =0.97I rp2

**Setup** Argon laser pushes particles in the monolayer

**Chopping** chopped beam beam dump scanning mirror chops the beam Ar laser mirror

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

**2 mm** Ar laser sheet Movie of particle accelerated by laser beam

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

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

**Horizontal confining potential energy**

**Radiationpressureforce**

**Gas drag force **

**Coefficients for radiation pressure and gas drag** Radiation pressure q result: measurment0.94 0.11 ray optic theory0.97 Gas drag result: measurment1.26 0.13 Epstein theory 1 ~ 1.44 Epstein, Phys. Rev. 1924

**Laser sheet** Application of radiation pressure force

**Q=0,** / 0 Dispersion relationsin 2D triangular lattice Wang et al. PRL 2001

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

**optical** acoustic Optical mode in solid(two atom in primitive cell)

**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:

**“Optical” branch** Acoustic branch Dispersionrelation

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

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

**1** 2 Case 1 No bifurcation condition Case 2 Ux Uy x y Bifurcation condition

**Resonance frequency:x** x = 0.07 Hz Single-particle laser acceleration

**Resonance frequency:y** laser-excited resonance vibration laser sheet

**Resonance frequency:y** Velocity autocorrelation function of random motion

**Excitation of optical mode** Laser beam

**Excitation of optical mode** Laser beam

**dusty.physics.uiowa.edu**