The particle and energy flux from the plasma core on the wall can extract impurity atoms which penetrate into the plasma. The impurity ions can radiate energy and can replace deuterium or tritium ions inside the plasma, reducing the total amount of fusion power. Thus, impurity penetration must be avoided by a proper choice of the wall materials.
FTU is testing several materials for the wall. Due to the reduced dimension, the power per unit area is comparable with that of ITER. Even though the configuration is different (on ITER the particle escaping from the plasma column are diverted in regions far from the plasma and accelerated towards a plate, whereas in FTU they hit the inner wall) the obtained results are relevant for the choice of the plate material on ITER.
The results obtained so far on FTU, with a limited amount of heating power, show that the use of high atomic number material such as Molibdenum and Tungsten can significantly reduce the plasma contamination as shown in the figure, where the impurity concentration is quantified in terms of the so called effective charge Zeff, defined as an appropriate average of the charges of the ions in the plasma (Zeff=1 for a pure Deuterium plasma).
Such a behaviour is due to the fact that, in order to estract high-Z impurities, a plasma ion must have a kinetic energy significantly larger than the energy required to estract low-Z impurities.
Indeed, with low-Z materials such as Carbon, the impurity concentration in FTU is significantly larger. It is important to note the beneficial effect of high-particle density operation: as the density increases the plasma contamination decreases for all the considered materials.
Since the FTU programme requires reliable operation also at low density, in order to limit the inflow of low Z impurities (O or C), which are detrimental for the plasma purity in these conditions, two conditioning systems have been implemented on the machine. Titanium gettering and more recently a boronization systems have indeed provided a strong decrease of oxygen concentration, allowing plasma operation at low Zeff at all densities for a large number of shots. Beside the decrease of Zeff and the impurity radiation losses, these systems also decrease the time to obtain a good plasma performance, for instance after a long shutdown , and a rapid recovery after disruptive discharges.