EFFECT OF BERYLLIUM ON PLASMA PERFORMANCE IN JET

JET is investigating beryllium as material for walls and limiters. Studies were carried out with initially a thin layer of beryllium evaporated onto the walls of the machine, later in addition the graphite material of the belt limiter was exchanged against beryllium. The use of this material was generally beneficial for the plasma behaviour. Combined with a reduction in the oxygen content, strong pumping of hydrogen isotopes was found which allowed JET to widen considerably the operational space with respect to ion temperatures, densities and plasma purity. In this paper a comparison of the plasma performance with graphite and beryllium will be presented. We will discuss especially the impurity behaviour with respect to fluxes, concentrations, effective charge and dilution, we will report on density limits, disruption behaviour, wall pumping, hydrogen isotope retention, and the power handling capability of the beryllium limiter in the present design. Examples of improved plasma performance will be given.