# Current Filaments in Asymmetric Surface Dielectric Barrier Discharge
## 5 Experimental Results
### Photomultiplier Results  
The fast time response of PMT allows us to employ optical diagnostics to study the temporal evolution of the discharge pattern and evaluating the differences between FS and BS. In particular, the use of a multi-anode PMT allows us to measure the correlations and the delays in the plasma light emission between different regions above the active dielectric surface. In the following, we report results from the analysis of the emitting times of light measured with photomultiplier tubes (PMT) [48]. In particular, we focus on the measurements of the direction and velocity of propagation of light emission along the insulating barrier surface. This was achieved by simultaneously collecting light coming from different points of the discharge air gap. Figure 18: Internal structure of BS. (**a**) The second BS (red vertical lines), compared with the first one (\(i=1\)) (blue vertical lines), the latter multiplied by \(-1\) in order to directly visualize their differences, for the applied voltage HV = 5.48 kV. The black vertical lines represent the mean backward stroke (shown as a reference), obtained by averaging BS over the whole record (in this case 34 half-cycles). The time origin coincides with the start of the BS half-cycle, and therefore it is the same for all strokes. (**b**) Mean current differences, Equation (6), between consecutive BS (blue vertical lines). The accumulated mean differences, \(S(n)\), with \(n=t/\Delta t\) Equation (7), behave as a random walk time profile (red line). First, we locate the position of the bulk of the plasma emission. The discharges are well confined within a thin air sheet of one\(-\)two millimeters above the dielectric surface, next to the exposed electrode edge (see Figure 20). Discharges occur where the electric field, determined by the applied external high voltage signal, is maximal. Plasma emissivity is concentrated on such edge and expands for a few millimeters above the dielectric surface, whereas its extension above the exposed electrode is much more limited [23]. The actual size of the active optical region does depend, although not strongly, on the applied high voltage, also increasing in the late phases of the strokes. This is of use to look for correlations and delays in the light emission along the dielectric surface. It should be noted that the plasma emissivity is quite uniform, on average, in the direction parallel to the exposed electrode edge, until the lateral edges (see Figure 20). Indeed, we did not observe significant differences between different portions of the exposed electrode edge. Moreover, we did not observe any significant correlation in the light emission times from regions even a few millimeters apart along the electrode edge direction. This implies that each discharge event develops mainly in the direction perpendicular to the electrode edge and its lateral extension is limited, as well as its height on the dielectric surface. This  
Figure 19: Analysis of time correlations between BS. (**a**) Accumulated mean current differences (random walk profiles) between successive BS for different HV. (**b**) Hurst exponent analysis of fluctuations for the profiles shown in (**a**). The values \(H=1\), for times \(t<6\,\mathrm{ns}\), and \(H=0.67\), for intermediate times \(6\,\mathrm{ns}<t<200\,\mathrm{ns}\), represent the slopes of the black and green straight lines, respectively. confirms also that each discharge event, identified by current bursts, evolves independently from the others, even when partial overlapping in time happens. The most interesting findings were, however, observed when the PMT channels collected light from regions aligned along a direction perpendicular to the exposed electrode edge. Here, correlation in light emission could be observed, and also delays between the light emission timing could be measured.