A 1 MeV-neutral beam injection system planned for ITER has to operate on the basis of  “negative ion technology”, i.e. the acceleration and neutralisation of negative rather than positive hydrogen or deuterium ions as on most present NB systems today. R&D is in progress in Europe and in Japan on the two key items: the ion source and the accelerator. In both cases two alternatives are being considered. Such R&D activity should be completed by the middle of 2005.

In the ion source, D- ions can be generated either in an arc discharge or in an RF discharge. The arc discharge is produced between tungsten filaments and the source body that acts as the anode. R&D is in progress in Naka, Japan, and in Cadarache, France, where the so called Kamaboko III source, built in Naka, is tested on the MANTIS test bed. The RF discharge is produced with an RF generator; R&D is in progress in Garching, Germany. In both cases small quantities of caesium are introduced into the discharge in order to enhance the production of negative ions. The R&D activity on the RF discharge source is justified, since the presently favoured arc source does not meet all requirements, which are: a negative ion current of 40 A at a current density of 20 mA/cm2 and a pulse duration up to 1 hour. Further obstacles are the rather limited life time of the arc filaments, resulting in major maintenance effort, and high costs.

The negative ions can be accelerated with two different accelerators. The multi-aperture multi-grid (MAMuG) accelerator, under development in Naka, consists of 5 gaps (four intermediate acceleration grids). In the single aperture, single gap accelerator (SINGAP), under development in Cadarache, a pre-acceleration stage (~40 keV) is followed by a single 1 MeV acceleration stage.

The arc discharge source and the MAMuG accelerator, already developed in Naka for the 500 keV injectors of JT-60 U, were considered the more conservative extrapolations from existing devices, and therefore they are the basis for the ITER NB injector reference design. Both alternatives (the RF source and the SINGAP accelerator) offer significant advantages. The RF source would not need filaments and therefore their replacement (routine maintenance) could be eliminated. The SINGAP accelerator could offer substantial simplification of the accelerator and of the high voltage transmission lines and power supplies. Such design simplifications would also lead to cost saving.

There is a general appreciation within the international NB community, that an ITER-like NB system should be built and tested before the ITER NB system is installed at the ITER site. Such an “ITER-like NB system”, should demonstrate high voltage acceleration at ITER relevant currents, to ensure that reliable NB operation is achieved in ITER and to minimise the time required for on-site commissioning. Many elements of the ITER NB system should be tested at essentially full scale and at pulse lengths commensurate with those required for ITER (~1 hour).

A possible approach to achieve this aim would be to consider the required equipment and all associated auxiliaries as parts of the “first ITER NB injector” to be installed initially at an ITER NB Test Facility and later at the ITER site. A formal agreement on ITER construction and site are necessary preconditions to start procurement. The construction of the first ITER NB injector should be shared between the ITER Partners; the European Union, Japan and the Russian Federation are likely to be involved and discussions are underway with the ITER International Team to prepare for such collaborative development. In the frame of EFDA design activities, the European laboratories are close to starting the preparation for the test facility and to optimise the ITER NB system design.

The ITER Neutral Beam (NB) system consists of two heating and current drive (H&CD) injectors and of one diagnostic neutral beam; space, auxiliaries and services have been foreseen for a third H&CD NB injector.

The energy (1MeV deuterium) and power (16.5 MW per injector) specifications of the ITER NB system significantly exceed those of any NB system currently operating.