William Theisen

Study of two-dimensional Debye clusters using Brownian motion

A two-dimensional Debye cluster is a system of n identical particles confined in a parabolic well and interacting through a screened Coulomb (i.e., a Debye-Huckel or Yukawa) potential with a Debye length λ. Experiments were performed for 27 clusters with n=3–63 particles (9μmdiam) in a capacitively coupled 9 W rf discharge at a neutral argon pressure of 13.6mTorr. In the strong-coupling regime each particle exhibits small amplitude Brownian motion about its equilibrium position. These motions were projected onto the center-of-mass and breathing modes and Fourier analyzed to give resonance curves from which the mode frequencies, amplitudes, and damping rates were determined. The ratio of the breathing frequency to the center-of-mass frequency was compared with theory to self-consistently determine the Debye shielding parameter κ, Debye length λ, particle charge q, and mode temperatures. It is found that 1≲κ≲2, and κ decreases weakly with n. The particle charge averaged over all measurements is −14200±200e,...

Transition to chaos in a driven dusty plasma

Dynamical chaos has previously been observed experimentally in a driven dusty plasma with three particles [T. E. Sheridan, Phys. Plasmas 12, 080701 (2005)]. In the present work, the transition to chaos in this system is studied as a function of the amplitude of a periodic driving force for two different driving frequencies fd. It is found that the system follows a quasiperiodic route to chaos. The dusty plasma’s center-of-mass modes are driven by the first harmonic of fd and lock to the driving force for small driving amplitudes. The breathing mode is driven by the second harmonic of fd and shows asymmetric spectral features indicating quasiperiodic dynamics for intermediate driving amplitudes. For large driving forces both the center-of-mass and breathing modes are entrained and a region of low-dimensional chaotic dynamics due to a resonance overlap is observed. In the chaotic regime the correlation dimension and Lyapunov exponent are found to increase with the driving force.