Optical emission spectroscopy of dielectric barrier discharges with multiple current peaks

Abstract

Summary form only given. Dielectric barrier discharges (DBDs) at atmospheric pressure can be operated in either filamentary or homogenous regime. The former is characterized by many short and erratic current pulses while the latter has a broad and well-defined current peak per half-cycle of the applied sinusoidal voltage. In selected conditions, homogeneous discharges can exhibit more than one current peak. However, the physics driving such multiple current peak discharges remains unexplored. In this work, time-resolved optical emission spectroscopy (OES) is used analyze the time evolution of the plasma characteristics and discharge regime in DBD operated in nominally pure He (3 SLPM, UHP grade). As described previously, the discharge was sustained in a plane-to-plane configuration (1 mm gap between the two alumina dielectric plates), with a sinusoidal applied voltage of either 1.3 or 2.5 kV peak-to-peak, and a frequencies of either 6 or 12 kHz. In addition to the expected emission lines from the He I n=3 levels (triplet and singlet states), OES spectra recorded from He2 (bandhead at 640 nm) as well as from Ar I (750.4 nm, bottle impurity). All He n=3 lines sharply rose before the current peaks and then decreased before the discharge current reached maximum values. On the other hand, for Ar I, maximum line intensities were observed after the discharge current peaks maxima. Over the range of experimental conditions investigated, Ar I excited states are mostly populated by Penning reactions with He metastable atoms such that their time behaviors corresponds to the evolution of the population of He n=2 states (in relative units). This set of data along with all He n=3 line intensities were coupled to the predictions of a collisional-radiative model using the electron temperature Te (assuming Maxwellian electron energy distribution function) as the only adjustable parameter. For single current peak discharges, Te decreased from about 1 eV early in the discharge cycle to 0.2 eV at the current maximum (and during discharge extinction). Such decrease of Te with increasing discharge current is ascribed to the Townsend-to-glow transition. In presence of additional current peaks, only small rises in Te values were observed, indicating that the discharge remains in glow mode.