EXPERIMENTS IN FTU WITH DIFFERENT LIMITER MATERIALS zyx M.L. APICELLA, G. APRUZZESE, M. BORRA*, G. BRACCO, M. CIOTTI, I. CONDREAa, F. CRISANTI, R. DE ANGELIS, C. FERRO, L. GABELLIERI, G. GATTI, H. KROEGLER, M. LEIGHEB, G. MADDALUNO, G. MARUGCIA*, R. ZAGORSKIb, F. ALLADIO, R. BARTIROMO, G. BUCETI, P. BURATTI, C. CENTIOLI, V. COCILOVO, B. ESPOSITO, A. FRATTOLILLO, E. GIOVANNOZZI, M. GROLLI, A. IMPARATO, L. LOVISETTO, P. MICOZZI, S. MIGLIORI, A. MOLETI, F. ORSITTO, L. PANACCIONE, M. PANNELLA, S. PODDA, G.B. RIGHETTI, E. STERNINI, A.A. TUCCILLO, 0. TUDISCO, F. VALENTE, V. VITALE, R. ZANINO", V. ZANZA, M. ZERBINI Associazione Euratom-ENEA sulla Fusione, Centro Ricerche Frascati, Frascati, Rome, Italy G. MAZZITELLI, D. PACELLA, V. PERICOLI-RIDOLFINI, L. PIERONI, ABSTRACT. Over the last few years, a great deal of effort has been devoted to solving the prob- lem of power and particle handling in divertors, which has been recognized zyx as a critical issue for the operation of a magnetic fusion reactor. In particular, the choice of materials for plasma facing components has been examined with a view to developing heat and erosion resistant materials for divertor target plates. A large database on the behaviour of low-2 (carbon or beryllium) materials in tokamaks is available, while for high-2 materials there is little experience in the present generation of magnetic fusion devices. Frascati Tokamak Upgrade (FTU), a high field compact tokamak, has devoted part of its experimental campaign to studying the plasma characteristics when its limiter material is changed from the usual Inconel (nickel)to molybdenum and tungsten. Siliconization of the machine has also allowed the comparison of plasma performance when a relatively low-2 (silicon) ion is the dominant impurity. In this article, results are reported concerning 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 experimental results, made with a self-consistent edge-core coupled model is presented, in order to provide evidence of the main physics mechanisms responsible for the observed behaviour. 1. INTRODUCTION During the International Thermonuclear Experi- mental Reactor (ITER) conceptual design activity and still at present, in the engineering design phase, it has been recognized that two main problems have to be solved for achieving success in the next step of fusion research: the problem of how to maintain a steady state ignited plasma avoiding pollution from the interaction of plasma with the first walls, and the problem of how to extend the lifetime of the various zyxwvu * ENEA Fellow. * zyxwvutsrqponm Present afiliation: INRS, Varennes, Quebec, Canada. Present afiliation: Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland. ' Dipartimento di Energetica, Politecnico di Torino, Turin, Italy. structures subjected to high particle fluxes, neutrons and high heat loads [l, 21. Thus, the main research effort is now addressed on the one hand to the best divertor configuration (slot divertor, gas bag, etc. [3]), to limit the heat load on the materials to techno- logically sustainable levels (< 5 MW.m-2), and, on the other hand, to the choice of suitable materials for the plasma facing components. Low-2 materi- als, such as carbon or beryllium, which are used at present on most operating tokamaks, present many problems in the application to ITER. Beryllium is satisfactory only if a sufficiently low plasma temper- ature (T, < 10 eV) and heat load can be achieved at the divertor plates. Graphite has the additional prob- lem of chemical sputtering. More important are the large erosion rates of low-2 materials, due to sput- tering and evaporation [4]. The large erosion rates would greatly reduce the lifetime of these first wall components. From a preliminary assessment both NUCLEAR FUSION, Vol. 37, No. 3 (1997) 381