Gas discharge physics. – In this field, we have carried out measurements of electron transport parameters (drift velocity, longitudinal diffusion coefficient, and Townsend ionization coefficient) in different gases: argon, synthetic air, methane, deuterium as well as carbon dioxide. The experiments have been accompanied by kinetic computations based on Monte Carlo simulation of the electron transport, and solutions of the Boltzmann kinetic equation. The research on radiofrequency plasmas has focused on electronegative gases in which we have investigated the frequency dependence of the asymmetry effects, the electron power dynamics and the formation of spatial structures (“striations”). We have also addressed the origin of resonance effects in radiofrequency plasmas via particle-based simulations and developed a new, in-situ technique for the determination of secondary electron emission yields of discharge electrodes in radiofrequency plasma sources.
Strongly coupled plasmas. – In a vertically confined quasi-two-dimensional dusty plasma composed of superparamagnetic, charged dust grains and immersed in an external magnetic field B, the grains interact via both Yukawa and magnetic dipole-dipole potentials. We have analyzed the effect of the strength of the confining potential on the in-plane correlations and on the wave propagation. In addition to the in-plane compressional and transverse waves, there appears an out-of-plane transverse wave generated by the oscillation of the grains in the confining potential. The theoretical approach used the quasi-localized charge approximation paralleled by molecular dynamics simulations. The influence of an external homogeneous magnetic field on the quasilocalization of the particles in strongly coupled three-dimensional Yukawa systems was investigated via molecular dynamics computer simulations. The caging time is found to be enhanced by the magnetic field B. The anisotropic migration of the particles in the presence of magnetic field indicate a more significant increase of localization in the direction perpendicular to B, while a moderate increase is also found along the B field lines. Associating the particles’ escapes from the cages with jumps of a characteristic length, a connection is found with the diffusion process: the diffusion coefficients derived from the decay time of the directional correlation functions in both the directions perpendicular to and parallel with B are in very good agreement with respective diffusion coefficient values obtained from their usual computation based on the mean-square displacement of the particles.
Furthermore, in this field the transport (thermal conductivity) effects in strongly coupled plasmas have been investigated and quantified by molecular dynamics simulations.
High-frequency discharge systems for surface treatment. – We have determined the characteristics of the empty and loaded afterglow systems based on a flowing surface wave microwave discharge. In this system, the discharge is generated in a 5 mm diameter tube, where due to the relatively high gas flow rate used, a pressure drop occurs along the tube. Since the surfatron used for coupling the microwave power into discharge can be moved along the tube, discharges at different pressure conditions can be generated under the same flow condition and system configuration, as illustrated in Fig. 1. Meanwhile, the length of the early-afterglow region varies with the surfatron's position, which further defines the composition of the gas entering the reactor. We have determined the effect of the surfatron position on the afterglow plasma composition entering the reactor in the case of binary and ternary mixtures. In the case of loaded systems, we determined how the pressure varies along the system when a small-diameter tube is placed into reactor and the afterglow is guided through it, and further on, how the plasma composition changes due to the pressure drops developed along the system at different flow conditions.
Figure 1. Images of ternary and binary mixture discharges realized along the quartz tube at different pressures when using 900 sccm and 400 sccm gas flow rates, which yields 2 mbar and 1 mbar, respectively, in the reactor connected to the discharge tube.