A plasma generating system comprises a plasma gun including a hollow cylindrical anode member, a hollow cylindrical intermediate member electrically isolated from and juxtaposed coaxially with the anode member to form a plasma-forming gas passage through the intermediate member and the anode member, and an axially movable cathode member. The intermediate member comprises tubular segments separated by resilient insulating spacing rings held in compression. Arc radiation is blocked from the spacer rings by meanders in the inter-segment slots and further by ceramic barrier rings. An electric motor or pneumatic piston responsive to a measurement of arc voltage continually adjusts the axial position of the cathode tip relative to the anode nozzle so as to maintain a predetermined arc voltage.