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The Formation of Sparks of Minimum Ignition Energy

A study has been made of incendiary discharges with minimum energy and between metal electrodes. The circuit used has a characteristic relaxation time smaller than the time required to distinquish several successive stages in spark formation. Examples are provided to show that the character and duration of each stage as well as the possibility to produce a hot gas channel depend both on the gas and electrodes used. Experimental evidence is presented to illustrate that in both uniform and divergent field configurations heating of the gas is always preceded by a long time compared to that required to produce either streamers or avalances in the discharge gap. These long times are associated with the formation and maintenance of a non-equipartition electron gas (Te /> Tn) in the discharge channel. Heating of the neutrals takes place either by continuous Joule heating once a critical electron density of 1017 cm-3 is procured or by nonlinear waves of potential gradient associated with sudden changes at the electrodes. In both cases, final heating is produced by thermal electrons and collective Coulomb collisions and not directly by the effect of the applied field on individual electrons as it is normally assumed.