Speaker
Description
The new baseline of ITER foresees tungsten (W) as the sole plasma-facing material in the reactor vessel. To ensure proper conditioning of the all-W device, in particular for controlling oxygen levels in the plasma and enabling reliable start-up of the experimental operations, regular boronizations have been proposed. An unavoidable consequence of boronizations, however, will be the formation of (thick) boron (B)-containing co-deposited layers on the plasma-facing components, possibly as an outcome of several material migration and erosion steps in the reactor vessel. To obtain insights on the properties and formation mechanisms of such layers, an extensive programme has been set up by the EUROfusion Consortium to produce and characterize various B-containing reference layers with pre-defined specifications. In addition, the physics behind their erosion and deposition behaviour will be investigated via dedicated experiments in laboratory conditions and in linear plasma devices. In this contribution, we will report on advances made for the topic during the last two years.
Several reference layers with varying thicknesses (from ~50 nm to several micrometers) and compositions, ranging from pure B to mixed W-B deposits and with inclusions of plasma fuel (here, deuterium (D)), oxygen (O), and gaseous impurity elements have been prepared, exposed to plasmas or controlled particle beams, and analysed in the participating laboratories. Pure B layers can easily oxidize at ambient conditions, however typically no significant compositional changes are observed after extended exposures to air. The layers are also relatively brittle whereas inclusion of D seems to strengthen and stabilize them but only at low enough temperatures; once the deposits are treated at temperatures >200-250°C, cracks start to occur and the fuel content to decrease. Comparison against films originating after boronizations on different tokamaks is ongoing. Several similarities have already been observed such as a common tendency for layer oxidization and the fact that the composition does not notably changed with ageing. Ongoing studies include assessing the role of porosity on oxygen gettering, permeation of fuel into the substrate, and the dependence of the layer properties on their thickness and substrate. Finally, exposures to plasmas in linear machines have highlighted the role of chemical erosion in sputtering of the films at low energies as well as a consistent picture of erosion in varying flux regimes.