Jan Carstensen

Effect of neutral gas motion on the rotation of dust clusters in an axial magnetic field

Experiments are carried out to investigate the rotation of dust clusters in a radio-frequency plasma sheath with a vertical magnetic field. Our observations are in disagreement with the standard model, in which it was assumed that the neutral gas is at rest and that a steady rotation is attained when the ion-drag force is balanced by neutral friction. Here, we re-examine this basic assumption by carefully designed experiments. Our results suggest that the neutral gas is set into rotation by E×B induced ion flow through ion-neutral collisions and that the dust particles are advected by this flow. A hydrodynamic model is proposed to describe the rotation of the neutral gas and it can explain our observations.

Imaging Mie ellipsometry: dynamics of nanodust clouds in an argon–acetylene plasma

For the in situ analysis of nano-sized particles in a laboratory plasma, Mie ellipsometry is a well established technique. We present a simple setup with two CCD cameras to gain online spatiotempor ...

Mass changes of microparticles in a plasma observed by a phase-resolved resonance method

The influence of a plasma environment on melamine formaldehyde particles is studied. High-precision measurements of the vertical confinement frequency with a phase-resolved resonance method indicate that the particle mass is affected in two ways: the deposition of sputtered material at the particle leads to a mass gain, whereas the outgassing of water causes a mass loss.

Charging and coupling of a vertically aligned particle pair in the plasma sheath

The phenomenon of particle chain formation is studied in a two-particle system. A wake of positive ions leads to an alignment of the negatively charged particles parallel to the ion flow. The dynamic response of this dust system to a small external perturbation is evaluated. It is shown that the eigenfrequency of the downstream particle is reduced compared to an isolated particle. This effect can be identified as a decharging of the particle by the focused ion flow in the wake of the upstream particle. Furthermore, a strong asymmetry of the particle interaction parallel to the ion flow is found. This asymmetry may not be attributed entirely to the interaction forces mediated by the ion wake.

Exploring the wake of a dust particle by a continuously approaching test grain

The structure of the ion wake behind a dust particle in the plasma sheath of an rf discharge is studied in a two-particle system. The wake formation leads to attractive forces between the negatively charged dust and can cause a reduction of the charge of a particle. By evaluating the dynamic response of the particle system to small external perturbations, these quantities can be measured. Plasma inherent etching processes are used to achieve a continuous mass loss and hence an increasing levitation height of the lower particle, so that the structure of the wake of the upper particle, which is nearly unaffected by etching, can be probed. The results show a significant modification of the wake structure in the plasma sheath to one long potential tail.

Magnetizing a complex plasma without a magnetic field.

We propose and demonstrate a concept that mimics the magnetization of the heavy dust particles in a complex plasma while leaving the properties of the light species practically unaffected. It makes use of the frictional coupling between a complex plasma and the neutral gas, which allows us to transfer angular momentum from a rotating gas column to a well-controlled rotation of the dust cloud. This induces a Coriolis force that acts exactly as the Lorentz force in a magnetic field. Experimental normal mode measurements for a small dust cluster with four particles show excellent agreement with theoretical predictions for a magnetized plasma.

Determination of dust grain charge and screening lengths in the plasma sheath by means of a controlled cluster rotation

Dusty plasma experiments with flat dust clusters are often performed in the boundary sheath of radio frequency discharges at typical gas pressures of 1–100 Pa. The interaction of the dust grains is usually assumed to be of the Yukawa type, which is determined by the particle charge and the screening length. For the experimental determination of these quantities we present a method that does not require prior knowledge of the plasma parameters. The method is based on the application of centrifugal forces by means of a rotating electrode method (REM). The results are critically compared with an analysis of thermally excited normal modes, which can be studied at pressures below 10 Pa. The REM has a wider range of applicability that can be extended to 100 Pa.

Probing the Plasma Sheath by the Continuous Mass Loss of Microparticles

A novel approach of using microparticles as probes for the sheath structure of radio-frequency discharges is presented. Starting with a heavy (large) particle confined deep in the plasma sheath, the ambient plasma causes a continuous mass loss, which increases the levitation height of the particle. It is shown that this process can be precisely monitored with the phase-resolved resonance method, which allows probing the force balance of the particle with high spatial resolution. The resulting force profile is in reasonable agreement with recent sheath models.

Resonance methods for the characterization of dust particles in plasmas

The fundamentals of the ‘resonance method’ are presented. The method relies on evaluating the dynamic response of one or more dust particles in the sheath of a laboratory plasma to small external perturbations. It allows one to make in situ high-precision measurements of particle properties. It is shown that the particle mass and charge and the strength of the interaction between two particles can be measured. Technical requirements, limitations and application examples are presented and discussed.

Sheared and unsheared rotation of driven dust clusters

Finite size plasma crystals confined in an anisotropic potential well were studied under a rotating and radially unsheared drive in experiment and simulation at moderate rotational frequencies. A radially sheared rotation of these strongly coupled systems is observed for most cluster configurations with a low symmetry. The results show that a differential rotation can be effected by a non-sheared driving force.