Nanosecond repetitively pulsed plasmas in preheated air at atmospheric pressure - The diffuse regime

Many applications for atmospheric-pressure air plasmas require non-thermal large-volume low-power plasmas with high chemical reactivity at low gas temperature. The Nanosecond Repetitively Pulsed (NRP) method can generate such non-thermal plasmas for power budgets lower than those of traditional generation methods by several orders of magnitude. A diffuse non-thermal plasma regime in air at atmospheric pressure from 300- 1000 K has been generated using the NRP method. This diffuse plasma develops through an initial cathode-directed streamer, followed by a return wave of potential redistribution. Furthermore, at a given gas temperature, there is a minimum gap distance required for the existence of the diffuse regime. The sequence of events observed in the formation of the diffuse plasma lead us to conclude that it is an "imminent" glow discharge. The applied electric field is maintained long enough (~10 ns) to initiate a streamer that interacts with the cathode to produce a return wave. However, the field is switched off before the development of significant ion-electron emission, which requires tens of nanoseconds. This limits the formation of the cathode fall, thus avoiding the imminent creation of a glow discharge. The non-uniform electric field generated by the pin-pin geometry can explain the minimum gap distance requirement. Using an approximate analytic expression for the Laplacian field, it can be shown that the gap distance primarily affects the field in the middle of the gap, whereas the radius of curvature of the electrodes primarily affects the field near the tips of the electrodes. Thus, the discharge gap can be divided into high-field regions near the electrodes where strong ionization occurs and a low-field weakly ionizing region between the electrodes, which can inhibit the spread of ionization instability and prevent the diffuse-tofilamentary regime transition. As the gas temperature is decreased, the field in strongly ionizing regions must be increased to maintain sufficient ionization, but the gap distance must be simultaneously increased for the weakly ionizing region to remain a buffer against the spread of the ionization instability.