# Current Filaments in Asymmetric Surface Dielectric Barrier Discharge
## 4 Plasma Diagnostics
### Photomultiplier Tube  
As is well known, during the development of an electrical discharge, part of the energy is lost into the excitation of internal states of atoms and molecules. From their de-excitation, plasma emissions of electromagnetic radiation are produced, and part of this radiation lies in the optical range. This could be exploited for diagnostics purposes by using optical emission spectroscopy (OES) [46]. In addition to providing information on the emitting species, the time of emission could give some hints about the mechanisms of the discharge itself. This is all the more true for SDBD, where the region of emission is concentrated and discharge happens mainly through quick, independent events of ionization and subsequent electric current flow, when most of the excitations happens [6; 23]. The spreading and development of the discharge in the air gap can then be traced by registering the delays in the emissions. Here, we use a multi-anode PMT (Hamamatsu H8711), built upon 16 channels arranged in a \(4\times 4\) matrix form. Each channel can collect electrons coming from a \(4.3\,\mathrm{mm}\) square photocathode. The rise time of the PMT signal, \(0.83\,\mathrm{ns}\), is short enough to retain the relevant information of the emitting times. Their high sensitivity is optimal to obtain light even from single discharge events. In our experiments, we kept the gain of the PMT constant, by supplying it with a regulated high-voltage generator (Matsusada AU-5R20-LCl), biased to \(-850\,\mathrm{V}\). Besides having suitable rise time and multichannels, the choice of the PMT was motivated also by the radiant sensitivity of the photocathode, a bialkali one, in order to match with the plasma emission spectrum. The model we use has a maximum sensitivity in the \(300\)-\(500\,\mathrm{nm}\) range. This fits well the reported average discharge emission spectra, measured in the range \(200\)-\(850\,\mathrm{nm}\) by means of a wide-band and low-resolution spectrometer (PS2000 by Ocean Optics), similar to those already discussed in [23]. They are consistent with typical spectra of dielectric barrier discharges in air at atmospheric pressure, with the brightest feature being the second positive system of molecular nitrogen \(N_{2}\), approximately lying in the interval \(300\)-\(500\,\mathrm{nm}\), and showing minor evidence of the first negative system of the nitrogen molecular ion \(N_{2}^{+}\), extending to the \(380\)-\(480\,\mathrm{nm}\) range. One such spectrum is shown in Figure 5, together with the sensitivity curve of the PMT. Figure 5: Light emission spectra of a typical discharge recorded by a low resolution spectrometer, compared to the sensitivity range of the photomultiplier tube (PMT) (continuous line). The radiative lifetime of the excited levels involved in the emission of the second positive system is around 40 ns. However, quenching in atmospheric air, enhanced also by impurities and excited states produced in the plasma gas-phase, reduces the lifetime of such excited levels to below 1 ns. Thus, the sensitivity of the excitation timing is retained and OES could be used to measure the spatio-temporal development of the discharges [47]. This was the main goal of our OES measurements. The outputs of the PMT were registered on the low impedance input (50 \(\Omega\)) of a fast digital scope (Agilent MSO-8104 AI). This retains the timing information of the emission, while producing pulses which could easily be recorded and separated from the noise. Usually, the light emitted from each microdischarge appears in the recorded temporal series as a \((3\pm 1)\) ns negative voltage pulse, which is due to the temporal response of our PMT. Indeed, the signals were absolutely similar to those detected when collecting light emitted by a lamp. The shape of the signal could also be used to safely remove electromagnetic disturbances occasionally coming from the discharge. The light emitted from plasma microdischarges was collected by UV-enhanced optical fibres by Avantes (FCB-UV800-2).