Behavior of Heavy Metal Ions in FTU Plasmas

The high field, high density tokamak FTU closed its 30-years of operation at the end of 2019. FTU is a circular machine (R0=0.93 m, a=0.29 m) with an Inconel Vacuum Vessel, Ni and Fe being its dominant elements, and Mo poloidal and toroidal limiters. The relatively high plasma densities, in combination with baking and boronization conditioning techniques, have ensured the possibility of producing plasmas characterized by an extremely low level of impurities of any kind, thus making FTU especially well-suited for investigating non-intrinsic impurities and the performances of liquid metal limiters under high thermal loads (up to 18 MW/m2). Initial tests were performed with a Lithium Liquid Limiter, while the more recent experiments have explored the plasma behavior with a Tin Liquid Limiter (TLL). Both are based on the innovative Capillary Porous System [1]. Lithium contamination was considerable, and traces can occasionally still be seen on various spectroscopic diagnostics. Oxygen is hardly present, and C is also low; N is detected at times, while He, Ne and Argon are detected when injected for diagnostic purposes.