# Current Filaments in Asymmetric Surface Dielectric Barrier Discharge
## 5 Experimental Results
### Photomultiplier Results
When light is detected by the PMT on one of its channels, we observe a burst in the corresponding time series, as in Figure 6. Occasionally, we detect several bursts, sometimes partially overlapping, with a global duration which generally never exceeds 10 ns. These events, although interesting, were excluded from the analysis. Light detected in the first channel arrives from a region just across the edge of the exposed electrode, whereas the second channel samples a spot (each about 800 \(\upmu\)m in diameter) 2.5 mm away on the dielectric surface. We find that the detected bursts appear to be very similar in the two channels, with average amplitudes, respectively, of 22.9 \(\pm\) 0.9 and 21.8 \(\pm\) 0.9 mV and durations, respectively, of 3.71 \(\pm\) 0.08 and 3.62 \(\pm\) 0.09 ns. Their average numbers are different, unveiling the lower emissivity of the regions above the dielectric surface as one moves away from the electrode edge. Over a sample of about one thousand oscillations, the counting rate of bursts is, for the two channels, 130 \(\pm\) 11 and 74 \(\pm\) 8 kHz, of which about 65% was within the BS. Another interesting behaviour was found by looking at the times, and the HV phases, when light emission occurred. Although emissivity is somewhat stable over different oscillations, it shows a detectable and characteristic distribution during each HV cycle [48]. In both space regions, emissivity in the BS phase concentrates around the breakdown time and shows a second maximum later on. On the contrary, emission within a FS is more uniformly distributed with the different phases of the HV oscillation. Another interesting feature of the optical emission pattern is linked to its persistency. Although about half of the events are accompanied by at least another during the same half-cycle, but only a slight minority (less than 10%); this second one is nearer than 250 ns, and in the other case the next one happens in the following half-cycle (about 40%) or in the subsequent cycle. In only 0.5% or 3% of the events, respectively, above the electrode edge and on the dielectric surface, no light was detected in the two subsequent half-cycles. This means that the spatial pattern of the discharges stays more or less stable over a relevant number of subsequent strokes and that the discharges belonging to each half-cycle are spatially related. Moreover, as a path of discharge is established, this repeats over the same spatial location, even if the timing (and so the external electric field strength) could be quite different from one event to the subsequent one. This memory effect is probably related to the dynamics of ions after the microdischarge quenching and  
Figure 20: Plasma discharges observed in our SDBD for \(V=5.48\) kV. in particular to the charging of the dielectric surface, which could explain the persistance of the discharge pattern between subsequent HV cycles. Finally, we compare the results of the emitting times between the two PMT channels. Now, all PMT bursts are ordered in time and the minimum delay between bursts belonging to a couple of different channels are calculated. A histogram of estimated delays can thus be obtained, and it is shown in Figure 21[48]. A quite evident maximum appears in BS, corresponding to an average delay of \(8.4\pm 1.6\) ns. This implies a propagation of the ionizaing wave from the electrode edge to the dielectric surface, with an average rate of about \(300\,\mathrm{km/s}\). Such propagation occurs in the direction opposite to that of the electron drift, dictated by the external electrode voltage and compatible with a cathode directed streamer mechanism of discharge [2,48]. Nothing comparable could be revealed by the analysis of the FS delays. In this case, the distribution is quite symmetrical, with a slight prevalence of the positive delays. It is also significantly larger and peaking at almost zero delay. It looks like as if propagation happens on a faster time scale, that cannot be revealed by the emission timing alone and with a significant spread.