acta physica slovaca

Abstract: The storage of matter and energy in confined plasma configurations is limited by an intermittent loss of particles and energy, usually referred to as anomalous transport. For tentatively explaining this phenomenon, we present in this paper experimental evidence of intermittent shelling off of electrical double layers generated at the border of confined plasma following a self-organization scenario. Moving against the electrons kinetic energy gradient, these structures enable the anomalous transport of matter and energy from the plasma to the surrounding. Our study can be related to the flicker noise observed in fusion devices, phenomenon associated with the loss of particles and energy from confined plasma.

Abstract: A spectral-domain interferometric technique is applied for measuring mirror distances and displacements in a dispersive Michelson interferometer when the effect of transparent thin films coated onto the interferometer beam splitter and compensator is known. We employ a low-resolution spectrometer in two experiments with different amounts of dispersion in a Michelson interferometer that includes fused-silica optical sample. Knowing the thickness of the optical sample and the nonlinear phase function of the thin films, the positions of the interferometer mirror are determined precisely by a least-squares fitting of the theoretical spectral interferograms to the recorded ones. We compare the results of the processing that include and do not include the effect of transparent thin films.

Abstract: After presenting Clifford algebra and quaternions, the symmetry operations with Clifford algebra and quaternions are defined. This symmetry operations are applied to a Platonic solid, which is called as dodecahedron. Also, the vertices of a dodecahedron presented in the Cartesian coordinates are calculated.

Abstract: We have obtained the stable geometrical structures, energetics, melting behaviors and phase changes of Pd7 clusters by using molecular dynamics and thermal quenching simulations. Cohesion of the clusters is modeled by an embedded-atom potential, which contains many-body atomic interactions. Isomers' energy-spectrum-width and their sampling probabilities are calculated. Melting phenomenon of a cluster is described in terms of relative root-mean-square bond-length fluctuations, and of behavior of the specific heats. The transition region from solid- to liquid-like phases is investigated, and the melting temperature of the cluster is estimated. Minimum energy pathways for the phase changes among the isomers of the Pd7 are determined.

Abstract: We demonstrated error-free operation of 4 simultaneous users in a fast frequency-hopping time-spreading optical code division multiple access system operating at 2.5 Gbit/s in a Star architecture. Effective power penalty was <0.5dB. Novel optical code division multiple access receiver based on Terahertz Optical Asymmetric Demultiplexer was demonstrated to eliminate multiple access interference.

Abstract: The beam of a ruby laser was focused onto two targets, 12Mg and 13Al, the irradiance being 140 GW/cm^2. In both cases similar processes developed. Time resolved spectra in the region 9-23 nm showed highly ionized atoms. The electron temperature (about 50 eV) and the ion velocities (about 10^6 cm s^{-1}) were investigated. Due to plasma explosion, deep craters with radial jetting are shown on the target surfaces. Ripples around the craters are explained by spreading of sound waves, laterally deposited plasmas were mixed with expanding target particles.

Abstract: Decay studies were carried out in the afterglow of low temperature plasma generated by pulsed microwave discharge in He with small admixture of Ar and H2. In such a mixture all ions formed during a microwave discharge are by ion-molecule reactions converted to H3+ ions. The decay of the H3+ dominated afterglow plasma was monitored by means of infrared cavity ring-down spectrometer (CRDS) employing a cw-diode laser. Measurements were carried out at temperature of ~350 K and total pressure of 0.66 - 2.66 kPa. The CRDS signal on the n2=3 <-- 0 transition of H3+ (observed at around 1.4 mm) was detected at different moments during the discharge afterglow. Knowing the absorption cross-section the evolution of the absolute number density of H3+ n=0) during the afterglow was determined. The recombination rate coefficient calculated from the decay curves (at hydrogen number density of [H2] = 3 - 8 x 10^{20} m^{- 3}) is a = (1.8 +- 0.8) x 10^{-13} m^3 s^{-1}. The absorption spectra provided us the kinetic temperature of H3+ n=0) ions during discharge and the afterglow. Detailed description of the experimental set up is also given here.

Abstract: In this paper results of electrical and optical diagnostics of dielectric barrier discharges operating in different gas mixtures are presented. Discharges burning in pure nitrogen, in mixtures of nitrogen and argon, nitrogen and carbon dioxide, nitrogen and 1,3-butadiene, argon and 1,3-butadiene, argon and carbon dioxide were studied. The investigations were focused on the influence of different admixtures and on the finding of the conditions for the generation of diffuse dielectric barrier discharges.

Abstract: The ablation efficiency in polytetrafluoroethylene of single soft x-ray pulses emitted by laser-produced plasmas was investigated. Placement of samples at different distances from the x-ray source made it possible to investigate dose and dose rate effects on the ablation processes for constant spectral properties of the x-ray source. The high-temperature plasma was created by focusing a 1315 nm laser beam from the PALS iodine laser system (Institute of Physics, ASCR, Prague) on the surface of a metallic target placed in a vacuum interaction chamber. A gas fill in the chamber was used to reduce charged particle emission from the plasma before it arrived at the sample surface. Nonthermal processes in ablation induced by x-ray radiation are discussed. An ablation mechanism based on radiation-chemical scissions of polymer chains followed by blow-off of low-molecular fragments into the vacuum is proposed.

Abstract: In this paper we report on the morphology and field emission from nanotubes prepared by AC CVD (Alcohol Catalytic Chemical Vapour Deposition) technique. Samples were prepared by growing of carbon nanotubes (CNTs) on Ni and Fe supporting wires. The vapours of alcohol were used as a source of carbon atoms. A heat from halogen bulbs boosted the growth up. Raman spectroscopy was used for the detection of the presence of nanotubes. The morphology of created nanotubes was investigated by high resolution scanning electron microscopy. Cold electron emission properties (emission current density vs. electric field characteristics) in dependence on the material of wires and the alcohol vapour pressure have been determined from I-V measurements.