The electronic stopping cross sections (SCS) of Ta and Gd for slow protons have been investigated experimentally. The data are compared to the results for Pt and Au, to learn how electronic stopping in transition and rare earth metals correlates with features of the electronic band structures. The extraordinarily high SCS observed for protons in Ta and Gd cannot be understood in terms of a free electron gas model, but are related to the high densities of both occupied and unoccupied electronic states in these metals.

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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% 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.

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