Characteristics of an arc discharge in sulphur hexafluoride

Voltage gradients and electrode-fall voltages have been measured for an SF6 arc burning in a 6.35mm-diameter tube. The voltage gradients are lower than those in a nitrogen arc, the reason being that the thermal conductivity of the SF6-arc plasma is lower, than that of a nitrogen plasma. Using previously calculated electrical and thermal conductivities the theoretical voltage gradients agree very closely with those measured. Also, the radial temperature distributions for various SF6 arcs have been calculated, the agreement with the measured distributions obtained by Maecker and Motschmann being very good. For low currents, the calculated temperature distributions show that the SF6 arc is very narrow and filamentary compared to an arc in nitrogen or air; this is in agreement with visual and photographic observations. The main reason for this is the dissociation of the SF6 molecule at a temperature of 21000°K, resulting in a large peak in the thermal conductivity. Thus the time constant of the SF6 arc is much lower than that of the nitrogen arc. This, coupled with the dastic reduction of the electron concentration below 5000°K by the formation of negative fluorine ions, results in the excellent arc-quenching properties of the gas.