Effects of wall boron coating on FTU, an all metallic and carbon free medium size tokamak

Boronization of FTU tokamak strongly affects its gas recycling, impurity concentration and its temporal evolution. Two main phases are distinguished. In the first one, lasting about 60 discharges, with the machine fully boronized, the radiation losses decrease from 80% to 40% and the impurity content drops from Zeff = 6 to Zeff ∼ 1.5 at nebar ∼0.35x1020 m-3 in ohmic conditions. In the second phase, lasting >500 discharges, with the B film eroded from the TZM toroidal limiter surface but not from the SS walls, the reduction of radiated power (∼60%) and impurities (Zeff ∼ 2.0) though smaller, is still important with respect to pre-boronization, allowing for optimization of plasma characteristics up to the maximum RF power so far injected (2.6 MW ∼ 1.6 MW m-3). The recycling properties of the B film are fully stabilized, making the plasma operations much more reliable, while the gettering properties are preserved and they maintain the oxygen below nO ≤ 0.5%