spark discharge[′spärk ′dis‚chärj]
one of the types of electric discharge in gases. It usually occurs at near-atmospheric pressures and is accompanied by a characteristic acoustic phenomenon, the “crackling” of the spark. Under natural conditions spark discharge is most frequently observed in the form of lightning.
Spark discharge in the proper sense of the term occurs if the power of the energy source feeding it is insufficient to maintain a stationary arc discharge or a glow discharge. In this case the discharge current increases sharply; simultaneously, the voltage across the discharge gap drops below extinction voltage within a very short time (a few microseconds to several hundred microseconds), thus ending the discharge. Subsequently, the potential difference between electrodes again increases until the firing voltage of spark discharge is attained, and the process repeats itself. In other cases, where the power supplied is sufficiently high, all the phenomena characteristic of spark discharge are also observed, but only as a transient process leading to the establishment of another type of discharge, most frequently arc discharge.
A spark discharge is a bundle of bright filament-like strips (spark channels), which are often strongly branched and disappear or alternate rapidly. The channels are filled with plasma, which in a high-power spark discharge consists of ions not only of the initial gas but also of the electrode material, which is evaporated intensively under the action of the discharge.
The mechanism of formation of spark channels (and consequently, the mechanism of generation of a spark discharge) can be explained by the streamer theory of electric breakdown in gases. According to this theory, under certain conditions streamers will form from the electron avalanches developing in the electric field of the spark gap. The streamers consist of thin, dimly glowing branched channels containing ionized gas atoms and free electrons split from the atoms. As the streamers become elongated, they bridge the discharge gap and develop continuous conductive filaments connecting the electrodes. The subsequent transformation of streamers into spark channels is accompanied by a sharp increase in current and the amount of energy released in the channels. Each channel widens rapidly, and the pressure in it increases abruptly, as a result of which a shock wave is generated at the boundaries of the channel. The aggregate of shock waves at the widening spark channels generates a sound perceived as the crackling of sparks (or thunder in the case of lightning).
The quantities that characterize spark discharge—firing voltage, extinction voltage, maximum current strength, and duration —may vary within wide limits, depending on the parameters of the discharge circuit, the size of the spark gap, the electrode geometry, and gas pressure. As a rule, the firing voltage of a spark gap is rather high. The voltage gradient within a spark falls off from several dozen kilovolts per centimeter at the moment of breakdown to about 100 V/cm a few microseconds later. The maximum current intensity in a high-power spark discharge may be as high as several hundred kiloamperes.
Spark creepage is a special type of discharge, which originates along the interface between the gas and a solid dielectric placed between the electrodes. Regions of spark creepage in which the charges are predominantly of one sign induce charges of opposite sign on the surface of the dielectric, as a result of which the spark channels spread over the surface of the dielectric (Lichtenberg figures). Processes similar to those of spark discharge are also typical of brush discharge.
Spark discharge has found various uses in technology. It is used to initiate explosions and combustion processes and in the measurement of high voltage, in spectroscopic analysis, in switching of electric circuits, and in high-precision metalworking.