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4.4. RESULTS AND DISCUSSION 4. FLUID MODELLING OF CO2 DISSOCIATION instantaneous anode and reaches a maximum at a certain distance from the instantan- eous cathode during the discharge event. This distance at which the maximum electron density occurs during positive and negative half-cycles is denoted as d0 and corresponds to the thickness of the cathode fall region. It is approximately 200 μm for the standard condition. After breakdown the electrons drift towards the anode where they are partly absorbed. Note that in negative half-cycles not all electrons reach the anode before the polarity changes. The remaining electrons drift back to the former cathode as it becomes the anode in the following discharge. Figures 4.3c,d show that the mean electron energy as well as the electric field have their maxima in the cathode fall region. However, the electron density in front of the cathode is small and thus the highest electron induced CO2 dissociation is expected to occur in the cathode sheath to plasma transition region. Plasma electrons (electrons not in the sheath region) have a low mean energy of less than 1 eV and thus the CO2 dissociation in collisions with plasma electrons is negligible (cf. figure 4.2). As it can be noticed from all the discharge characteristics, there is a difference in positive and negative half-cycles. Discharges occurring in negative half-cycles are char- acterized by, i.a., a higher breakdown voltage, stronger current peak and longer period with higher electron density compared to discharges in positive half-cycles. This asym- metry is caused by the relatively high frequency of 130 kHz, which doesn’t allow the charge carriers to recombine completely between subsequent discharge events. The res- ulting volume memory effect leads to a mutual influence of the discharges occurring in positive and negative half-cycles. Golubovskii et al. [196] have observed a similar dif- ference between positive and negative half-cycles in a helium discharge. In their model, they attributed this difference to the slow recombination of He+2 ions. They could reduce the memory effect and thus the differences between positive and negative half-cycles by using an artificially increased recombination rate coefficient for electron-ion recom- bination. Note that even the slow decay of surface charges could possibly induce the asymmetry between subsequent half-cycles. To understand the influence of the charged particles on the observed asymmetry in the discharge evolution, the spatiotemporal behaviour of the charged particle densities is plotted in figure 4.4. Instead of 3 cycles like in the figure 4.3, only one representative cycle (18.45–19.45) is chosen so as to capture the important features of both half-cycles. Obviously, there is a clear difference in the spatiotemporal evolution of the charged particles between positive and negative half-cycles. The density of the CO+2 ions shown in figure 4.4a decays slowly after ignition of discharges in the negative half-cycle due to electron-ion recombination accordant with process E54 in table 4.2. Note that the dens- 93PDF Image | Understanding CO2 containing non-equilibrium plasmas
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