{'fraction_non_alphanumeric': 0.026490066225165563, 'fraction_numerical': 0.0, 'mean_word_length': 5.535135135135135, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Role of excited species in dielectric barrier discharge mechanisms observed in helium at atmospheric pressure

There exists, at atmospheric pressure, three different dielectric barrier discharges (DBD) resulting from three different breakdown mechanisms: (i) glow DBD (GD), which results from Townsend breakdown (ii) filamentary discharge (FD) due to streamer breakdown and (iii) homogeneous discharge (HD) formed by streamer overlapping. The transition between these breakdown mechanisms in helium has been studied. The analysis is based on electrical characteristics, short exposure time photographs and emission spectroscopy of the discharge. The last technique, used during and after the discharge, permits to separate the role of electrons from that of long life time excited species thanks to the discharge's pulsating nature. The study has pointed out the importance of seed electrons present in the gas at the moment of breakdown. Their density depends on the electric field distribution in the gap and the density of metastables. The field can maintain electrons in the gap from one breakdown to the following one, and the metastables, by Penning ionisation, create electrons between two breakdowns.