Study of two-dimensional Debye clusters using Brownian motion

Abstract

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-Hückel 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, and q decreases slightly with n. The two center-of-mass modes and the breathing mode are found to have the same temperature, indicating that the clusters are in thermal equilibrium with the neutral gas. The average cluster temperature is 399±5K.

Dusty plasma (Yukawa) rings

“…Forκ = 1.32 the critical value of the lattice parameter α = a/r 0 for the zigzag instability is α c = 1.13. For the ring studied here, α = 1.41 is significantly above [7,[18][19][20][21].…”

“…1. Monodisperse microscopic (8.94 ± 0.09 µm diameter) melamine dust particles are levitated in an rf plasma discharge above the capacitively-coupled powered electrode in the DONUT (Dusty ONU experimenT) apparatus [7,[18][19][20][21]. Dust confinement is due to a 2D annular potential well created by cutting an annular rectangular groove in the surface of the 89-mm powered electrode.…”

“…The particle positions undergo small amplitude thermal fluctuations, mostly due to their interaction with the neutral gas component [18]. For the 1-ring [ Fig.…”

Power law behavior of the zigzag transition in Yukawa clusters

“…Monodisperse microspheres may then form strongly-coupled one-dimensional (1D) [3][4][5], two-dimensional (2D) [6][7][8] or even threedimensional systems [9]. The general 2D potential well is biharmonic for small displacements from the potential energy minimum [10][11][12].…”

“…1) [7,8,12,16,18,19]. Argon gas was leaked into the vacuum chamber and the pressure was stabilized at 15.4 mtorr.…”