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3. PROPERTIES OF MICRO-DISCHARGES 3.1. INTRODUCTION positive ions (low drift velocity) separated by electrons (high drift velocities) in the ava- lanche head develops which is characterized by an electric field (Es). When Es attains a critical value, the avalanches transforms into a streamer, the second stage of a filament. According to the Meek criterion, the critical value for Es is equal to the applied external field (Eo). Meek criterion is a rough approximation that when the amplification of elec- trons due to the space charge exceeds approximately 108, the avalanche transforms into plasma channel [133]. The motion of streamer head (the space charge region) can be described as an ion- ization wave, which has a velocity far greater than the drift velocity of electrons (in the range of 106 −107cm/s); in 1-2 ns the streamer head reaches cathode head creating in its wake a plasma channel which is highly conductive . Also, the streamer head on reaching the surface establishes a cathode fall region [134]. Once this streamer head reaches the opposite electrode, it spreads on the surface [135, 136]. The extent of the spread on the surface is determined by the electric field set up due to the difference of charge between the cathode surface (location of the micro-discharge) with respect the surrounding [137]. As the plasma spreads on the surface it reduces the excess charge locally thus countervail- ing the applied electric field and hence finally choking the plasma. The plasma column, which has a very high density of charged (and other active species) slowly recombines, resulting in a decay of plasma. The time scales of each of these stages of filaments de- pends mainly on the type of the working gas, applied voltage and dielectric layer used and typical values are shown in the figure 3.1. Another interesting feature of a DBD is the memory effect. Memory effect can be described as the localization of filaments, in a DBD, in space and in time. The origin of the memory effect can be explained in terms of volume mechanism or more importantly in terms of charge accumulation on the dielectrics [138]. As per explanations proposed in terms of volume mechanisms, residual ions or metastables produced from one filament are present in higher concentrations in the location where the filament occurred in previ- ous voltage cycle. These active species can help reignite the filament again at the same location and for the same applied voltage (i.e time in a voltage cycle). For example, the residual plasma conductivity can help in re-igniting the plasma again at the same location [139]. However, in a steady state operation, in most cases the memory effect is attributed to surface charge accumulation. It is still an open question how the charge accumulated on the surface can cause memory effect. Three different explanations are proposed: First hypothesis claims that the uneven charge accumulated on the surface will enhance the electric field locally repeating the breakdown in the same location [140]; Second hy- pothesis involves the residual plasma channel depleting the charge locally, rather than 44PDF Image | Understanding CO2 containing non-equilibrium plasmas
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