Modelling of voids in complex radio frequency plasmas

\u3cp\u3eIn this paper hydrodynamic and kinetic approaches to model low pressure capacitively coupled complex radiofrequency discharges are discussed and applied to discharges under microgravity. Experiments in the PKE-Nefedov reactor on board the International Space Station, as well as discharges in which gravity is compensated by means of thermophoresis are simulated with a 2-D cylindrically symmetric hydrodynamic model. Kinetic effects are studied with a 1-D Particle-In-Cell plus Monte Carlo model in which capture and scattering by dust grains is included. Simulations with this model address non-local effects and modulated discharges.\u3c/p\u3

How to make large, void-free dust clusters in dusty plasma under micro-gravity

\u3cp\u3eCollections of micrometer-sized solid particles immersed in plasma are used to mimic many systems from solid state and fluid physics, due to their strong electrostatic interaction, their large inertia, and the fact that they are large enough to be visualized with ordinary optics. On Earth, gravity restricts the so-called dusty plasma systems to thin, two-dimensional (2D) layers, unless special experimental geometries are used, involving heated or cooled electrons, and/or the use of dielectric materials. In micro-gravity experiments, the formation of a dust-free void breaks the isotropy of 3D dusty plasma systems. In order to do real 3D experiments, this void has somehow to be closed. In this paper, we use a fully self-consistent fluid model to study the closure of a void in a micro-gravity experiment, by lowering the driving potential. The analysis goes beyond the simple description of the 'virtual void', which describes the formation of a void without taking the dust into account. We show that self-organization plays an important role in void formation and void closure, which also allows a reversed scheme, where a discharge is run at low driving potentials and small batches of dust are added. No hysteresis is found this way. Finally, we compare our results with recent experiments and find good agreement, but only when we do not take charge-exchange collisions into account.\u3c/p\u3

Effect of large-angle scattering, ion flow speed and ion-neutral collisions on dust transport under microgravity conditions

\u3cp\u3eThe transport of dust particles through a plasma depends mostly on the ion drag force, the neutral drag force and the electrostatic force. The standard expressions for these forces were originally derived for a single dust particle placed in a collisionless plasma, with negligible flow speeds of the ions. Recent theories show deviations from the standard expressions for the charging and the ion drag force acting on dust particles in a plasma, when there are collisions and a significant ion flow. Experiments show only a small deviation from the standard expressions for the ion drag. We have extended a self-consistent dusty plasma model for a radio-frequency discharge with recent theories regarding the calculation of the ion drag force, including the effect of ion scattering beyond the screening length, ion flow and ion-neutral collisions. A change in the dust charge due to these collisions is also considered. Inside the dust-free void, that is generated by the ion drag force, scattering beyond the screening length is very important. Inside the dust cloud however, the effect is only moderate. Ion flow speeds under typical discharge parameters are low, except near the electrodes. Therefore, the effect of the ion flow speed on the ion drag force is very small. Collisions only increase the ion drag force near the outer walls. Only there does the screening length become much larger than the ion meanfree path. The dust charge however, is strongly reduced inside the void, and near the edge of the dust cloud, which is due to the low ion flow in both regions. When we compare our model with experiments, we conclude that in the bulk of the discharge and at the void edge, large angle scattering is important and the velocity-dependent linearized Debye length is the appropriate screening length. Using small angle scattering with the electron Debye length actually overestimates the ion drag, resulting in inconsistent values of the electric field and the ion drift speed.\u3c/p\u3

The plasma inside a dust free void:hotter, denser, or both?

\u3cp\u3eThe existence of a dust-free void, as often observed in dusty plasmas with small particles, or in dusty plasmas under micro-gravity conditions, requires a maximum of the ionization inside the void. Enhanced optical emission inside the void has indeed been observed. The extra losses of plasma on the dust has to be compensated for by extra ionization, which means that the electron temperature must rise. Inside the void there is no depletion of electrons, so a rise in electron temperature is not immediately obvious. It was therefore proposed that the relatively high electron density in the void with respect to the surrounding dusty region, where the electrons are depleted, causes the higher ionization inside the void. Different observations and models have until now not given a decisive answer, however. Using a global model, we predict that a homogeneous dusty plasma without a void should have an increased electron temperature, but at a reduced electron density. A dusty plasma with a fully formed void should have both an increased electron temperature and density in the void. A fully self-consistent two-dimensional model agrees with these conclusions and also shows that the void is a complex system, which is heated by the dust on the outside, but has most of the ionization on the inside.\u3c/p\u3

Numerical modelling for Pilot-PSI and Magnum-PSI, the num'' of Magnum

Abstract only

Chemical sputtering by H\u3csub\u3e2\u3c/sub\u3e \u3csup\u3e+\u3c/sup\u3e and H\u3csub\u3e3\u3c/sub\u3e \u3csup\u3e+\u3c/sup\u3e ions during silicon deposition

\u3cp\u3eWe investigated chemical sputtering of silicon films by H\u3csub\u3ey\u3c/sub\u3e \u3csup\u3e+\u3c/sup\u3e ions (with y being 2 and 3) in an asymmetric VHF Plasma Enhanced Chemical Vapor Deposition (PECVD) discharge in detail. In experiments with discharges created with pure H\u3csub\u3e2\u3c/sub\u3e inlet flows, we observed that more Si was etched from the powered than from the grounded electrode, and this resulted in a net deposition on the grounded electrode. With experimental input data from a power density series of discharges with pure H\u3csub\u3e2\u3c/sub\u3e inlet flows, we were able to model this process with a chemical sputtering mechanism. The obtained chemical sputtering yields were (0.3-0.4) ± 0.1 Si atom per bombarding H\u3csub\u3ey\u3c/sub\u3e \u3csup\u3e+\u3c/sup\u3e ion at the grounded electrode and at the powered electrode the yield ranged from (0.4 to 0.65) ± 0.1. Subsequently, we investigated the role of chemical sputtering during PECVD deposition with a series of silane fractions S\u3csub\u3eF\u3c/sub\u3e (S\u3csub\u3eF\u3c/sub\u3e(%) = [SiH\u3csub\u3e4\u3c/sub\u3e]/[H\u3csub\u3e2\u3c/sub\u3e]∗100) ranging from S\u3csub\u3eF\u3c/sub\u3e = 0% to 20%. We experimentally observed that the SiH\u3csub\u3ey\u3c/sub\u3e \u3csup\u3e+\u3c/sup\u3e flux is not proportional to S\u3csub\u3eF\u3c/sub\u3e but decreasing from S\u3csub\u3eF\u3c/sub\u3e = 3.4% to 20%. This counterintuitive SiH\u3csub\u3ey\u3c/sub\u3e \u3csup\u3e+\u3c/sup\u3e flux trend was partly explained by an increasing chemical sputtering rate with decreasing S\u3csub\u3eF\u3c/sub\u3e and partly by the reaction between H\u3csub\u3e3\u3c/sub\u3e \u3csup\u3e+\u3c/sup\u3e and SiH\u3csub\u3e4\u3c/sub\u3e that forms SiH\u3csub\u3e3\u3c/sub\u3e \u3csup\u3e+\u3c/sup\u3e.\u3c/p\u3

Integral simulation of the creation and expansion of a transonic argon plasma

A transonic argon plasma is studied in an integral simulation where both the plasma creation and expansion are incorporated in the same model. This integral approach allows for simulation of expanding plasmas where the Mach number is not known a priori. Results of this integral simulation are validated with semi-analytical models. Inside the creation region the results for the electron temperature, the heavy particle temperature and the electron density are compared with a global model of the creation region. In the expansion region, the simulation results of the compressible flow field are compared with predictions for the shock position. Both the results inside the creation region as well as in the expansion region are in good agreement with the semi-analytical models

Particle-in-cell Monte Carlo simulations of an extreme ultraviolet radiation driven plasma

A self-consistent kinetic particle-in-cell model has been developed to describe a radiation driven plasma. Collisions between charged species and the neutral background are represented statistically by Monte Carlo collisions. The weakly ionized plasma is formed when extreme UV radiation coming from a pulsed discharge photoionizes a low pressure argon gas. The presence of a plasma close to optical components is potentially dangerous in case the ions that are accelerated in the plasma sheath gain enough energy to sputter the optics. The simulations predict the plasma parameters and notably the energy at which ions impact on the plasma boundaries. Finally, sputter rates are estd. on the basis of two sputtering models. [on SciFinder (R)

Residual gas entering high density hydrogen plasma:rarefaction due to rapid heating

\u3cp\u3eThe interaction of background molecular hydrogen with magnetized (0.4 T) high density (1-5 × 10\u3csup\u3e20\u3c/sup\u3e m\u3csup\u3e-3\u3c/sup\u3e) low temperature (∼3 eV) hydrogen plasma was inferred from the Fulcher band emission in the linear plasma generator Pilot-PSI. In the plasma center, vibrational temperatures reached 1 eV. Rotational temperatures obtained from the Q(v = 1) branch were systematically ∼0.1 eV lower than the Q(v = 0) branch temperatures, which were in the range of 0.4-0.8 eV, typically 60% of the translational temperature (determined from the width of the same spectral lines). The latter is attributed to preferential excitation of translational degrees of freedom in collisions with ions on the timescale of their in-plasma residence time. Doppler shifts revealed co-rotation of the molecules with the plasma at an angular velocity an order of magnitude lower, confirming that the Fulcher emission connects to background molecules. A simple model estimated a factor of 90 rarefaction of the molecular density at the center of the plasma column compared to the residual gas density. Temperature and density information was combined to conclude that ion-conversion molecular assisted recombination dominates plasma recombination at a rate of 1 × 10\u3csup\u3e-15\u3c/sup\u3e m\u3csup\u3e3\u3c/sup\u3e s\u3csup\u3e-1\u3c/sup\u3e. The observations illustrate the general significance of rapid molecule heating in high density hydrogen plasma for estimating molecular processes and how this affects Fulcher spectroscopy.\u3c/p\u3