Optimisation of the magnetic field configuration for the negative ion source of ITER neutral beam injectors

The negative ion source of the neutral beam injectors for ITER requires that the co-extracted electron current is not larger than the negative ion current. To this purpose a suitable magnetic field configuration was adopted, generated by a current flowing in the plasma grid and by two permanent magnets on either side of the source. In the present design of the system however the magnetic field lacks uniformity across the beam. This paper focuses on strategies aimed at optimising the magnetic field distribution and improving the beam optics, based on two-dimensional magnetic simulations, including permanent magnets, ferromagnetic materials and electrical currents. A careful distribution of the filter field current can provide a more efficient extraction of negative ions with respect to electrons. The use of ferromagnetic material can reduce the magnetic field downstream from the accelerator, resulting in lower beam deflection. The interference with the permanent magnets of ITER reference design is also discussed. It is proposed that the path of the plasma grid current is divided between several conductors to minimise the stray field. Moreover, ferromagnetic material should be inserted in the grounded grid. It is shown that the proposed modifications reduce the magnetic field in the region downstream from the grounded grid, with the advantage of a smaller deflection of the beam. The effect on electrons is discussed. However, a three-dimensional simulation will be necessary to address the issue of electrons as well as the vertical uniformity of the beam, taking into account the effects of finite extension of the permanent magnets, and optimising the return current path.