EXPERIMENTS IN FTU WITH DIFFERENT LIMITER MATERIALS

Over the last few years, a great deal of effort has been devoted to so lving the problem of power and particle handling in diverters, which h as been recognized as a critical issue for the operation of a magnetic fusion reactor. In particular, the choice of materials for plasma fac ing components has been examined with a view to developing heat and er osion resistant materials for divertor target plates. A large database on the behaviour of low-Z (carbon or beryllium) materials in tokamaks is available, while for high-Z materials there is little experience i n the present generation of magnetic fusion devices. Frascati Tokamak Upgrade (FTU), a high held compact tokamak, has devoted part of its ex perimental campaign to studying the plasma characteristics when its li miter material is changed from the usual Inconel (nickel) to molybdenu m and tungsten. Siliconization of the machine has also allowed the com parison of plasma performance when a relatively low-Z (silicon) ion is the dominant impurity. In this article, results are reported concerni ng the plasma operation, the differences in plasma characteristics and radiation losses, the impurity generation mechanisms and the relative impurity concentrations in the core plasma. A simulation of the exper imental results, made with a self-consistent edge-core coupled model i s presented, in order to provide evidence of the main physics mechanis ms responsible for the observed behaviour.