Dr Zoltan Donko
DSc, Scientific advisor
Department of Complex Fluids
Institute for Solid State Physics and Optics,
Wigner Research Center for Physics

Flag Counter ZDElectrical discharges in gases

The plasma state of matter is most widespread in the Universe, more than 99% of the visible material is in the plasma state. The gases can change from an insulating state to a conduncting (plasma) state due to heat or interaction with energetic particles, resulting in the creation of charged particles via ionization processes. The plasma states cover an extremely wide range of particle densties and temperatures (as shown in the figure below). Besides the plasmas occurring in Nature, laboratory plasmas play an importat role in science and technology. Our research is related no non-thermal, or "low-temperature plasmas" of electrical discharges.

Plasma states in Nature and in the Laboratory

Laboratory and industrial plasmas utilize the light emission (in light sources), the processes leading to light amplification (in gas lasers), as well as the presence of charged  and/or reactive particles and molecules (in plasma processing of surfaces).

Electrical discharge plasma in low pressure helium gas.

The in-depth understanding of the complex processes in plasma discharges, as well the high-tech applications of electrical discharges have been aided by computational tools. Using numerical computational techniques the importance of elementary processes and reaction channels can be revealed with unpreceeded accuracy. Particle simulation methods (e.g. the Monte Carlo technique) provide full information about the kinetics of charged particles. As an illustration, the following figure shows the development of an electron avalanche between plane-parallel electrodes placed at 4 cm distance in argon gas, at 41.4 Pa pressure and 200 V voltage. The avalanche starts with the emission of an electron (from the left electrode). A movie of the process can be seen at this page.

Monte Carlo simulation of the development of an electron avalanche

Computer simulations make as well possible to describe the operation of more advanced plasma sources, e.g. of radiofrequency-excited discharges which have been used in hgh-tech application including chip and solar cell manufacturing.

Results of a "Particle in Cell" simulation of a radiofrequency discharge.

Another line of research is represented by the experimental investgation of plasmachemical processes, especialy charge transfer reaction between nobe gas ions and ground-state metal atoms. We have developed a unique experimental apparatus for the measurement of the rate coefficient of these processes.

Experimetal apparatus developed to determine the rate coefficients of
asymmetric charge transfer collisions between noble gas ions and metal atoms.
Development of the system has been supported by the EU FP6 GLADNET (MRTN CT 035459) project.

Our publications related to these activities can be found here.

Our research has been supported by the Hungarian Fund for Scientific Research (OTKA) and by international collaborations.

Complex (dusty) plasmas

Laboratory complex (dusty) plasmas can be created by dispersing micron-sized particles into gas discharges. The (typically noble gas) glow discharge can be direct current (d.c.) or radio frequency (r.f.) driven and serves primarily as a charging medium for the (typically spherical, dielectric) particles. The dust particles are exposed to electron and ion currents from the discharge plasma, a dynamic equilibrium is rapidly reached, where their net electric charge can be in the order of 10^4 electron charges. The interaction of these particles with their environment can be manifold: gravitation, Lorenz force, ion drag force, neutral drag force, thermophoretic effects, etc.  The dominance of the different force contributions can be tuned by adjusting the experimental conditions (microgravity, flowing gas, etc.). In the case when the particles are levitated in a horizontal plane-parallel electrode configuration r.f. discharge, the gravity is compensated by the vertical electric field of the plasma sheath, and the particles settle in a single quasi-2D layer near the lower electrode. Other effects, like ion- and neutral drag forces act also mainly perpendicular to the particle plane, therefore they influence only the equilibrium position of this layer. The remaining in-plane forces can be well approximated by a simple Yukawa type interaction, originating from the Coulomb repulsion of the charged dust particles and the polarizability (screening property) of the surrounding discharge plasma.

For more information see our review paper on strongly coupled plasmas:
Z. Donkó, G. J. Kalman and P. Hartmann:
"Dynamical correlations and collective excitations of Yukawa liquids",
J. Phys. Condensed Matter 20, 413101 (2008)

Complex (dusty) plasma experiment in our Laboratory.

Last modified : 3 March 2013 [Zoltán Donkó]