Modelling the ion source for ITER NBI: from the generation of negative hydrogen ions to their extraction

The neutral beam injection (NBI) system for ITER is based on a large (A_source = 1.9 × 0.9 m²) negative hydrogen or deuterium ion source. In this source negative ions are produced in a low-pressure (p_fill ≈ 0.3 Pa) plasma by conversion of atoms and protons on a caesiated molybdenum surface with low work function. Then the negative ions are transported through the plasma to the extraction system where extraction of these ions and co-extraction of electrons also take place. This paper describes the status of the modelling activities connected with the negative ion test facilities of IPP Garching. It is illustrated that these modelling activities constitute a strong support of the experimental activities connected with the development of the negative ion source for ITER NBI. Several numerical codes developed in the past years—in close collaboration with the experiment—and their results are introduced. Focus is laid on the production, transport and extraction of negative hydrogen ions and on the inevitable co-extraction of electrons. (paper)