The formation of a small incendiary spark at atmospheric pressure is identified with the transition from a weakly to a strongly ionized plasma. It is shown that initial gaseous ionization produced by avalanches and/or streamers always creates a high-temperature ideal electron gas that can shield the applied voltage difference and reduce ionization in the volume of the gas. The electron gas is collision dominated but able to maintain its high temperature, for times long compared to discharge events, through long-range Coulomb forces. In fact, electrons in the weakly ionized plasma constitute a collisionless independent fluid with a thermodynamic state that can be affected directly by field or density changes. Accordingly, with metal electrodes, cathode spot emission is always associated with the transition to a strongly ionized plasma. Neutral heating can be accomplished in two different ways. Effective dispersal of the electrons from the cathode leads to electron heating dominated by diffusion effects. Conversely, a fast rate of emission or rapid field changes can produce nonlinear wave propagation. It is shown that solitary waves are possible, and it is suggested that some spark transitions are associated with shock waves in the collisionless electron gas. In either the diffuse or nonlinear regime, neutral gas heating is controlled by collisions of ions with isotropic thermal electrons. This interaction is always subsequent to changes in state of the electron gas population. The basic results obtained should apply to all sparks.

Charged aerosol clouds emerging from supersonic nozzles have been used to induce electrical breakdown from large falling drops of water. As expected, it is shown that only positive streamers are produced independently of the polarity of the aerosol. These streamers are not suppressed by spray from the drop, and their propagation is enhanced through negative aerosols. Two interacting jets have been used to produce very high concentrations of aerosol space charge in a region far removed from any electrodes. For positive charge this region behaves like a positive corona discharge. For negative charge the local negative streamer breakdown produced invokes either positive streamers or actual sparks from nearby regions depending on the charge density of the aerosol. The physical mechanism required for the explanation of the results obtained completely agrees with, and supports, Loeb's recent theory for the formation of stepped leaders in lightning discharges. It is shown that breakdown between positive and negative aerosols can also be obtained by means of interacting jets.

Significant space-charge accumulations caused by thermodynamic changes are produced whenever an electrical corona discharge of the streamer type is maintained in a region of rapid and sustained mass density variation. The experimential investigation uses a highly charged supersonic aerosol jet (0.03 C/m3) at temperatures below 300°K and pressures below 3 atm. The aerosol emerges from an underexpanded nozzle producing a characteristic series of expansion and compression regions. The charge density of the aerosol is sufficiently high so that electrical breakdown is induced at a grounded needle placed near or within the jet. It is shown that the ionization produced within the supersonic region, where rapid mass density changes occur, liberates charge in excess of the charge carried by the aerosol and that the current to the induced corona needle reproduces density variations in the underexpanded jet. A model is presented whereby changes in electrical conductivity, produced by the streamers propagating through the jet, result in space charge accumulations. Computation shows that the charge accumulation on shocks and compression and expansion waves is of sufficient magnitude to be responsible for the excess ionization and the visual appearance of the corona discharge.

The fast luminous pulses experimentally observed to propagate in electrical discharge channels, and believed to have a significant role in the process of electrical breakdown, are investigated, using a plasma fluid approach. These pulses fall in two regimes characterized by different average velocities of 106 and 108 m/s. We concentrate on the lower velocity pulses. A nondimensional analysis is used to show that these waves can be considered as longitudinal electron fluid waves driven by electron pressure gradients in weakly ionized plasmas. Model equations are derived considering each additional term in the momentum equation as a small but finite perturbation. In all cases, standard nonlinear fluid model equations are obtained as possible solutions. Electric forces lead to the Korteweg-deVries equation, viscosity to Burgers equation, and electron-neutral collisions to a damped wave equation. The propagation and attenuation of fluid waves along preionized channels is naturally associated with the formation of steps. It is suggested that thermalization must be associated with the injection of electrons from a source external to those in the preionized channel.

Using propane-air mixtures, it is shown that the minimum ignition energy corresponds precisely to the amount that produces, in the discharge channel, the electron density (1017 cm−3) that changes the discharge from a cold streamer to a hot incipient arc. This electron density together with the minimum quenching distance are suggested as a physically meaningful criterion for ignition. The procurement of the critical electron density is also studied using a polyester film and water as examples of solid and deformable dielectric electrodes. It is found that thermalization is controlled by the rate of supply of charge into the discharge channel and that a stored minimum energy value applies only when two metal electrodes are involved. Ignition capability can be determined by the characteristics seen in the oscilloscope recordings of the current in the discharge. Electrode conduction effects are studied by varying the conductivity of a saltwater solution. It is found that either surface-charge accumulation or resistive effects may prevent ignitions depending on the conductivity of the solution. Also, that except for metals, ignitions are always invariably associated with nonuniform electric fields and the procurement of electrons by cold-streamer discharges. In the better dielectrics, surface charges prevent ignition independently of the energy supply. However, these same charges may be stored for a long time (days) and produce ignitions at very low voltages with polarity reversals. Finally, local thermalization is suggested as a necessary requirement for the propagation of long sparks characterized by stepping.