Mechanisms controlling the transition from glow silent discharge to streamer discharge in nitrogen

Low-energy dielectric-barrier controlled discharges in nitrogen are studied by undertaking electrical measurements to determine mechanisms controlling the transition from glow to streamer-like discharge, The highest and the lowest values of the frequency and the amplitude of power supply voltage leading to a glow discharge have been found dependent on the gas flow and the nature of the surface in contact with the discharge. These boundary values have been related to the criteria necessary for initiating a Townsend breakdown rather than a streamer breakdown commonly observed under such conditions. This implies two conditions: 1) that the seed electron density just before the breakdown is high enough to allow the development of numerous small avalanches under a low held avoiding the formation of only one large avalanche mechanism at the origin of the streamer formation; and 2) to let the time for ions issued from the first avalanches to reach the cathode before the electrical field becomes large enough to induce the formation of large avalanches. Practically, the transition from a Townsend breakdown to a streamer breakdown is analyzed from electrical measurements data coupled to the visual aspect of the discharge. Without any gas flow, the obtaining of an atmospheric pressure glow discharge (APGD) is mainly limited by the species etched from the surface in contact with the gas, Indeed, these species can be quenchers of the nitrogen metastable molecules, which are the species at the origin of the formation of seed electrons via the Penning effect, This limitation can be overcome by the use of a laminar gas flow. However,: this type of gas flow through the discharge induces a depletion of N-2 metastables and, consequently, influences the electron density at the entrance of the discharge, leading to a tendency on this part of the discharge to transit to a streamer-like one, A final limitation is the time variation of the voltage applied to the discharge during the breakdown. It has to be low enough to accord the time to the first ions to reach the cathode before the field across the gap becomes sufficiently high to induce a large avalanche and a consequent breakdown through photons.