Hendrik Jung

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.

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.

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.

Size and density evolution of a single microparticle embedded in a plasma

This article presents two measurement techniques to determine the diameter of a single dust particle during plasma operation. Using long-distance microscopy (LDM), the particle is imaged from outside the plasma chamber. In combination with phase-resolved resonance measurements the development of the volume-averaged particle mass density is measured over several hours. The measurements show a significant decrease of mass density for polymethyl methacrylate (PMMA) particles due to a plasma etching process on the surface. This is explained by a core-shell model and is supported by a surface roughness effect seen in the LDM images, an out-of-focus imaging of the angular Mie scattering pattern and ex-situ laser scattering microscopy measurements.

Experiments on wake structures behind a microparticle in a magnetized plasma flow

The wake behind a spherical microparticle in a magnetized ion flow is studied experimentally by analyzing the arrangement of a pair of particles. It is shown that there are two stable particle arrangements at intermediate magnetic inductions, whereas only oblique (horizontal) particle configurations are found at the highest magnetic field. Self-consistent collisional molecular dynamics simulations of the particle system show that the underlying mechanism of these arrangements is the weakening of attractive wake forces by the increasing magnetic field. Plasma instabilities provide a trigger for the onset of the transition between the two different arrangements. Furthermore, the course of the transition is qualitatively explained by the charge variation of the downstream particle in the wake of the upstream particle. In addition, a thorough analysis of the sheath by means of particle-in-cell simulations in combination with particle resonance measurements yields consistent values of the particle mass and charge, as well as the levitating electric field and ion flow velocity.The wake behind a spherical microparticle in a magnetized ion flow is studied experimentally by analyzing the arrangement of a pair of particles. It is shown that there are two stable particle arrangements at intermediate magnetic inductions, whereas only oblique (horizontal) particle configurations are found at the highest magnetic field. Self-consistent collisional molecular dynamics simulations of the particle system show that the underlying mechanism of these arrangements is the weakening of attractive wake forces by the increasing magnetic field. Plasma instabilities provide a trigger for the onset of the transition between the two different arrangements. Furthermore, the course of the transition is qualitatively explained by the charge variation of the downstream particle in the wake of the upstream particle. In addition, a thorough analysis of the sheath by means of particle-in-cell simulations in co...

Molecular dynamics simulations of wake structures behind a microparticle in a magnetized ion flow. I. Collisionless limit with cold ion beam

The potential and density structure behind a spherical microparticle in a magnetized ion flow are studied by means of molecular dynamics simulations. It is shown that, with increasing magnetization of the flow, the ion accumulation in the wake diminishes. Instead, ion depleted regions (shadows) form and ions accumulate at the edge of the shadows. The change of the ion density distribution also affects the force on other microparticles in the downstream region. For weak magnetization and a short distance, these interparticle forces can be attractive and non-reciprocal, as in the unmagnetized case. For large magnetization and further downstream, the force becomes repulsive. The mechanism of shadow formation is shown to involve a fast Coulomb scattering during a short fraction of the gyroperiod and subsequent trapping of the ions on large-radius gyro-orbits.

Molecular dynamics simulations of wake structures behind a microparticle in a magnetized ion flow. II. Effects of velocity spread and ion collisions

The influence of velocity spread and ion-neutral collisions on the wake of a microparticle in a collisional magnetized ion flow is explored by means of molecular dynamics simulations. The ion flow is described in the constant-mean-free-path limit. A constant electric field is superimposed that maintains the ion drift at the Bohm speed and approximates conditions in the plasma sheath. The contribution of ion Landau damping to the wake structure is separated by simulations with a collisionless drift distribution. It is found that ion Landau damping and collisions have a counteracting effect on the ion density in the focus region. The dynamic shadows that are a typical feature of collisionless magnetized wakes with cold ion beams are damped by the velocity spread and vanish by a collision-enhanced ion density in the wake. Dynamic shadows reappear only at very high magnetic fields, B ≈ 10 T. In two-particle arrangements, the full collisional model shows that horizontal attractive forces persist up to B = 4 T but become repulsive for higher magnetization.

Charging of an irregularly shaped particle in the sheath of an rf plasma

The charging process of micrometer-sized irregularly shaped particles in the sheath of a radio frequency discharge is measured using a combination of long-distance microscopy and the phase-resolved resonance method. The applicability of the method is shown for a cylindrical zinc oxide particle by measuring its mass density. A particle with more complex geometry is compared to a spherical polymethyl methacrylate particle to investigate the charging of irregularly shaped particles in detail. The results are similar to the charging of the smallest enclosing sphere suggesting that the charging process is independent of the shape of the particle. Furthermore, molecular dynamics simulations were performed, which support the experimental results.