Kurt D. Sieber

Analysis of two-dimensional microdischarge distribution in dielectric-barrier discharges

The two-dimensional spatial distribution of microdischarges in atmospheric pressure dielectric-barrier discharges (DBDs) in air was studied. Experimental images of DBDs (Lichtenberg figures) were obtained using photostimulable phosphors. The storage phosphor imaging method takes advantage of the linear response of the phosphor for characterization of microdischarge intensity and position. A microdischarge interaction model in DBDs is proposed and a Monte Carlo simulation of microdischarge interactions in the discharge is presented. Comparison of modelled and experimental images indicates interactions and short-range structuring of microdischarge channels.

Experimental and numerical investigation of a capacitively coupled low-radio frequency nitrogen plasma

Abstract A capacitively coupled nitrogen discharge driven at a frequency of 40 kHz was analyzed using a particle-in-cell (PIC) code, electrical probe measurements and optical emission spectra (OES). The configuration studied is used to generate plasmas for surface modification of polymer webs and consists of a pair of coplanar electrodes spaced several centimeters from the web plane and housed in a grounded shield. Both the probe measurements and the simulations indicate the presence of a group of high-energy electrons in concentrations of order 0.1% of the bulk electron concentration. Furthermore, bulk electron temperatures from the simulations are less than 1 eV. The energetic electrons and the low temperature of the bulk electrons are both characteristics of discharges operating in the gamma regime, where secondary electron emission from ion bombardment of the cathode sustains the ionization in the discharge. Because ions can respond to the instantaneous potential at the low-driving frequency used, half of the current at the electrode location is ion current. (In contrast, displacement current from the electron motion dominates at significantly higher driving frequencies.) The energetic electrons can provide a valuable source of N + ions through dissociative ionization. The formation of the N + ion was not included in the simulation, but was detected by the OES measurements. The atomic nitrogen ions and neutrals, together with the high-energy electrons, may be responsible for the formation of nitrogen-containing species in the surface region of polymer films treated with nitrogen plasmas using the configuration studied in this work.