6.2 Low work function PG surface

Sputtering from the ion source backplates by the back-streaming ions means that the PG surface will be coated in the sputtered material in a time that is very short compared to the operational time between the reactor maintenance periods. That will result in an increase of the work function and a significant reduction in the extracted ion current. Thus either the PG needs to be replaced or the surface renewed. Replacing the PG is not reasonably possible with the current type of accelerator design. Therefore R&D into low work function PG surfaces should not be prioritised. However the continued use of Cs injection means that:

<sup>5</sup> In principle other gases could be used in the plasma neutraliser, such as argon (Ar), and it is easier to obtain a higher ionisation degree with Ar [18]. However the potential problems that might occur with Ar such as contamination of the tritium plant and Ar+ back-streaming into the ion source need to be evaluated, which has not been done.

