The Petula tokamak experiment: effects of low-Z materials on plasma characteristics

Alumina and carbon limiters have been used in Petula. Compared to discharges produced with a tungsten limiter, discharges with alumina limiter show better plasma characteristics: the concentration of oxygen, which is the dominant impurity in both cases, decreases by a large factor, as does the plasma resistivity at the same electron temperature. As a result, the gross confinement time increases significantly, which is thought to be due to smaller line radiation losses. The important part played by the limiter in oxygen contamination is clearly due to these results and, from this point of view, alumina is much better than tungsten. On the other hand, the poor ability of alumina to resist thermal stresses is a severe limitation to its practical use. In the cae of a carbon limiter, which does not suffer from this drawback, the amount of carbon released into the plasma is too high to provide stable discharges. The strong correlations observed between displacement, voltage fluctuation and C IV line emission suggests that feedback control stabilization and better-quality pyrocarbon could improve the plasma characteristics. (author)