Choice of materials and problems of design of heavy-current toroidal discharge tubes

Abstract

One of the principal problems of heavy-current discharges is the contamination of the plasma by material eroded from the walls of the discharge tube. Erosion may occur by thermal evaporation, sputtering and---in the case of metal walls---the formation of arcs. These processes are examined in detail, with particular reference to the conditions which may exist in a thermonuclear reactor working on the pinched-discharge principle. It appears that, for stable discharges of long duration, thermal evaporation and sputtering should not be serious if materials of low atomic number are used. In the more severe transient initial conditions, sputtering may limit the duration of the first stage of the discharge. Metals are superior to insulators in thermal properties and ease of fab rication, but tend to form unipolar arcs when exposed to plasma. The mechanism of these arcs is discussed, and some experiments on the arcing properties of different materials are described. A critical review is given of a number of possible torus designs. (auth)