Jan Voráč

Measurement of hydroxyl radical (OH) concentration in an argon RF plasma jet by laser-induced fluorescence

The concentration of hydroxyl (OH) radicals in a plasma pencil, an atmospheric RF plasma jet ignited in argon, was measured by laser-induced fluorescence calibrated by Rayleigh scattering on ambient air. A suitable excitation scheme for this discharge was suggested based on laser excitation to the lowest vibrational level of the A 2Σ+ state. Effects of spectral overlap between the laser and absorption line, fluorescence saturation, temporal evolution of fluorescence radiation, rotational energy transfer and rotational distribution on the diagnostic method were analysed. The maximum OH concentration was approximately 5 × 1020 m−3. The maximum was reached when the measurement point localized in the effluent of the plasma pencil was inside but close to the tip of the visible active discharge.

Analysis of the electric field development and the relaxation of electron velocity distribution function for nanosecond breakdown in air

Using theoretical and experimental methods, the electric field and the electron multiplication in direct vicinity of a sharp cathode is analysed. The development of the electric field in the pre-breakdown phase of the atmospheric pressure air negative DC corona discharge in the Trichel pulse regime is determined. During the following ultra-fast electrical breakdown, the emission of selected spectral bands of the nitrogen molecule is recorded with high spatiotemporal resolution using the time-correlated single photon counting method. The emission of a Townsend discharge is used to calibrate the setup for the quantitative determination of electric field. Therefore, the Trichel pulse corona and Townsend discharge cell are arranged in the same single-table setup. This direct calibration procedure is described step-by-step including the discussion of known limitations. Finally, the electric field development of the positive streamer passing the 160 microns distance in less than two nanoseconds is determined. Due to the high spatiotemporal gradients of the electric field strength within the streamer breakdown, the local field approximation of the electron component is analysed by investigating numerically the temporal and spatial electron relaxation by means of the solution of the electron Boltzmann equation and Monte Carlo simulation. Results of these computations are given for several reduced electric field values and prove that the electrons are in a hydrodynamic equilibrium state for experimentally given space and time scales for reduced elds above 100 Td.

Gas mixing enhanced by power modulations in atmospheric pressure microwave plasma jet

Microwave plasma jet operating in atmospheric pressure argon was power modulated by audio frequency sine envelope in the 10^2 W power range. Its effluent was imaged using interference filters and ICCD camera for several different phases of the modulating signal. The combination of this fast imaging with spatially resolved optical emission spectroscopy provides useful insights into the plasmachemical processes involved. Phase-resolved schlieren photography was performed to visualize the gas dynamics. The results show that for higher modulation frequencies the plasma chemistry is strongly influenced by formation of transient flow perturbation resembling a vortex during each period. The perturbation formation and speed are strongly influenced by the frequency and power variations while they depend only weakly on the working gas flow rate. From application point of view, the perturbation presence significantly broadened lateral distribution of active species, effectively increasing cross-sectional area suitable for applications.

Spatially resolved measurement of hydroxyl (OH) radical concentration in a microwave plasma jet by planar laser-induced fluorescence

Abstract The spatially resolved concentration of OH radicals in the effluent of a microwave (MW) surfatron plasma jet was measured by planar laser-induced fluorescence. Two cases were compared – constant MW power and MW power modulated by 80 Hz. In both cases the maximal concentration was at the tip of the visible discharge, but for constant MW power the OH was spread over a larger volume. The maximum concentration in both cases was on the order of 1022 m-3. Graphical Abstract

Using Diffuse Coplanar Surface Barrier Discharge for improvement of Felting Properties of Animal Fibres

Summary form only given. The Diffuse Coplanar Surface Barrier Discharge (DCSBD) has shown its great potential for applications in textile industry recently1. A rather interesting synthesis of the latest technologies and a very traditional industry is currently being developed in cooperation of our physics department and TONAK - a company with more than 200 years tradition in the hat making from natural felt. This is also covered by international patent WO2011044859: An Apparatus and Method for Improving Felting Properties of Animal Fibres by plasma Treatment.

Measurement of absolute density of OH radicals in an atmospheric plasma pencil by laser-induced fluorescence

Summary form only given. OH radical has been in the scope of scientists for a long time, because it allows to track important chemical processes. It is also expected to play an important role in the plasma material processing and the interaction of plasma with living cells.