Influence of the limiter material on the plasma characteristics in the PETULA Tokamak

In the PETULA Tokamak, three series of discharges were run, with, successively, limiters made of tungsten, alumina and carbon. The characteristics of the plasma in these three situations are presented and compared. It is found that the lowest oxygen impurity level and lowest effective ion charge were obtained with the alumina limiter. With this limiter, the plasma energy confinement time is 50 per cent higher than that found for the tungsten limiter. During a rather short period, the machine was operated with a carbon limiter which led to very resistive discharges and the appearance of disruptive instabilities related to a plasma-limiter interaction. From a technological point of view, the alumina limiter was appreciably damaged after about one thousand discharges whereas the carbon limiter appeared to withstand thermal stresses perfectly.