The usual form of cathode for plasma jet apparatus is a graphite disk having a central hole through which the jet of plasma emerges. Burning of the graphite continuously enlarges the hole and necessitates frequent renewal of the disk. For small-scale work, this inconvenience can be avoided by replacing the disk with a metal orifice plate (electrically neutral), and by using an external graphite rod as cathode. With the 7 kW apparatus described, the jet of plasma produced is some 5 cm long, with a maximum core temperature believed to be about 10 000° C. The maximum rate of heat transfer to a surface of 6 mm diameter is approximately 300 cal cm -2 s -1 . By injecting various gases the atmosphere of the arc can be modified. Argon is notable for reducing the arc voltage by about 30%, and for suppressing the usual crater formation at the anode.
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A B Osborn (1959) studied this question.