Simulating the charging of a particle on a surface in a plasma

It is common knowledge that a floating surface will charge negative when a plasma is applied over it. One can imagine that any macroscopic dust particle on the surface will also get charged. The theory that describes this charging is, however, underdeveloped. It has been suggested that the particle will share its charge with the surface, leading to roughly the same surface charge density. This is, however, only valid when both the surface and the particle are electrically conductive. In this contribution, we show a novel model to simulate the charge on a non-conducting particle on a surface in a plasma. It is based on balancing the ion and electron fluxes through the plasma sheath towards the particle. With this, we show that the charge on a particle on a surface can be five orders of magnitude higher than what was previously assumed. Knowledge of the charge on a particle on a surface is important, because it, combined with the plasma sheath electric field, will lead to an electric force on the particle. It has been proposed that this force is important in the lofting of dust from the surface of extra-terrestrial bodies. Additionally, it has been suggested, that it can be used for cleaning in high-tech applications, such as lithography machines and spacecrafts

Plasma particle lofting

In plasma particle lofting, macroscopic particles are picked up from a surface by an electric force. This force originates from a plasma that charges both the surface and any particle on it, leading to an electric force that pushes particles off the surface. This process has been suggested as a novel cleaning technique in modern high-tech applications, because it has intrinsic advantages over more traditional methods. Its development is, however, limited by a lack of knowledge of the underlying physics. Although the lofting has been demonstrated before, there are neither numerical nor experimental quantitative measures of it. Especially determining the charge deposited by a plasma on a particle on a surface proves difficult. We have developed a novel experimental method using a "probe force.'' This allows us to, for the first time, quantitatively measure the plasma lofting force. By applying this method to different plasma conditions we can identify the important plasma parameters, allowing us to tailor a plasma for specific cleaning applications. Additionally, the quantitative result can help in the development of new models for the electron and ion currents through a plasma sheath

Streamers in air splitting into three branches

We investigate the branching of positive streamers in air and present the first systematic investigation of splitting into more than two branches. We study discharges in 100 mbar artificial air that is exposed to voltage pulses of 10 kV applied to a needle electrode 160 mm above a grounded plate. By imaging the discharge with two cameras from three angles, we establish that about every 200th branching event is a branching into three. Branching into three occurs more frequently for the relatively thicker streamers. In fact, we find that the surface of the total streamer cross-sections before and after a branching event is roughly the same.Comment: 6 pages, 7 figure

A Mathematical Morphology Approach to Cell Shape Analysis

This contribution aims to apply mathematical morphology operators to quantify the shape of round-objects which present irregularities from an ideal circular pattern. More specifically we illustrate, on the one hand, the application of morphological granulometries for size/shape multi-scale description and on the other hand, the radial/angular decompo- sitions using skeletons in polar-logarithmic representation. We discuss also the aspects related to the properties of invariance of these tools, which is important to describe cell shapes acquired under different magnifications, orientations, etc

Streamer knotwilg branching; sudden transition in morphology of positive streamers in nitrogen

We describe a peculiar branching phenomenon in positive repetitive streamer discharges in high purity nitrogen. We name it knotwilg branching after the Dutch word for a pollard willow tree. In a knotwilg branching a thick streamer suddenly splits into many thin streamers. Under some conditions this happens for all streamers in a discharge at about the same distance from the high-voltage electrode tip. At this distance, the thick streamers suddenly bend sharply and appear to propagate over a virtual surface surrounding the high-voltage electrode, rather than following the background electric field lines. From these bent thick streamers many, much thinner, streamers emerge that roughly follow the background electric field lines, creating the characteristic knotwilg branching. We have only found this particular morphology in high purity nitrogen at pressures in the range 50 to 200 mbar and for pulse repetition rates above 1 Hz; the experiments were performed for an electrode distance of 16 cm and for fast voltage pulses of 20 or 30 kV. These observations clearly disagree with common knowledge on streamer propagation. We have analyzed the data of several tens of thousands of discharges to clarify the phenomena. We also present some thoughts on how the ionization of the previous discharges could concentrate into some pre-ionization region near the needle electrode and create the knotwilg morphology, but we present no final explanation

Dust on a surface in a plasma : a charge simulation

An electrically isolated particle on a substrate surface will be electrically charged when a plasma is applied above it. We show that the magnitude of this charge is determined by a balance of the impeding ion and electron fluxes that are strongly influenced by the nearby substrate. By simulating this process we find that the charge density of the particle can be much higher than that of the substrate. This is due to the height of the particle, which causes additional electron collection