Zhuanhao Zhang

One-dimensional vertical dust strings in a glass box

The oscillation spectrum of a one-dimensional vertical dust string formed inside a glass box on top of the lower electrode in a gaseous electronics conference (GEC) reference cell was studied. A mechanism for creating a single vertical dust string is described. It is shown that the oscillation amplitudes, resonance frequencies, damping coefficients, and oscillation phases of the dust particles separate into two distinct groups. One group exhibits low damping coefficients, increasing amplitudes, and decreasing resonance frequencies for dust particles closer to the lower electrode. The other group shows high damping coefficients but anomalous resonance frequencies and amplitudes. At low oscillation frequencies, the two groups are also separated by a π phase difference. One possible cause for the difference in behavior between the two groups is discussed.

Simple method to measure the interaction potential of dielectric grains in a dusty plasma.

A simple minimally perturbative method is introduced which provides the ability to experimentally measure both the radial confining potential and the interaction potential between two individual dust particles, levitated in the sheath of a radio-frequency (RF) argon discharge. In this technique, a single dust particle is dropped into the plasma sheath to interact with a second individual dust particle already situated at the systems equilibrium point, without introducing any external perturbation. The resulting data are analyzed using a method employing a polynomial fit to the particle displacement(s), X(t) , to reduce uncertainty in calculation. Employing this technique, the horizontal confinement is shown to be parabolic over a wide range of pressures and displacements from the equilibrium point. The interaction potential is also measured and shown to be well described by a screened Coulomb potential and to decrease with increasing pressure. Finally, the charge on the particle and the effective dust screening distance are calculated. It is shown for the first time experimentally that the charge on a particle in the sheath of an RF plasma decreases with increasing pressure, in agreement with theoretical predictions. The screening distance also decreases with increasing pressure as expected. This technique can be used for rapid determination of particle parameters in dusty plasma.

Vertical Interaction Between Dust Particles Confined in a Glass Box in a Complex Plasma

In this experiment, falling particle trajectories within and without a glass box placed on the lower electrode in a Gaseous Electronics Conference reference cell are recorded and analyzed, and the electrostatic forces exerted on the dust particles are measured and compared. Experimental results show that, for particles falling in a complex plasma with no glass box, only a single force balance point (i.e., the position where the gravitational force is balanced by the electrostatic force) exists in the vertical direction while, for particles falling inside a glass box, this force balance spans an extended vertical range.

Glow and Dust in Plasma Boundaries

The sheath region is probed in different complex plasma experiments using dust particles in addition to the measurement of the optical emission originating from the plasma. The local maximum in the optical emission coincides with the breaking of quasi-neutrality at the sheath boundary, as indicated by the vertical-force profile reconstructed from dust-particle trajectories as well as by the local onset of dust-density waves in high-density dust clouds suspended in a dielectric box.

Mode Couplings and Conversions for Horizontal Dust Particle Pairs in Complex Plasmas

The normal modes for horizontal dust particle pairs in a complex plasma are investigated using two methods, a numerical calculation and a molecular dynamics simulation. The ion wakefield downstream of each particle and the variation of charge with particle levitation height are taken into account independently. It is shown that both mechanisms can create mode couplings or hybrid modes. As the modes hybridize, their frequencies are altered. Mode conversions and resonance instabilities are also observed and examined near the resonance points. The resulting power spectrum clearly resembles the experimentally observed high energy density signature for large crystals. The criteria for the occurrence of both the mode conversion and resonance instability are found to be accurately calculated by the double derivative of the interparticle potential, for both the ion wakefield and charge variation cases.