1. Basic considerations for a neutral beam injection system on a reactor

A neutral beam system on a fusion reactor will have to meet specifications that are significantly beyond those of any system so far designed. The injectors will be directly connected to the reactor vessel, and therefore they will both form a part of the nuclear confinement barrier and be subjected to high levels of neutron and gamma radiation. Consequently, the injector design must include a radiation barrier around the injectors; the choice of materials that can be used must be acceptable to the vacuum environment, be radiation tolerant, and, where possible, be lowactivation materials. In addition, the design will have to satisfy the nuclear regulator, which, typically, limits the engineering design codes that can be used.

It is clear that the main factors that will influence the design of the injectors, and require R&D, are the pulse length, the global efficiency, operation and maintenance in a nuclear environment, and component lifetime. In the following sections of this chapter, each of the aforementioned aspects is discussed more in detail and some

suggestions given as to how problems arising from each aspect may be resolved and the parameters of the future injectors achieved. However it is important to understand that although various basic conceptual designs of an injector to be used on a fusion reactor have been considered [1–3], no concept has been chosen, and no serious engineering design of any concept has been carried out. Experience with the design of the neutral beam systems has shown that many aspects of the conceptual design are changed significantly during the engineering design phase. For example:


It is clear from the above examples that the resolution of problems arising from operation in the fusion reactor environment, for example, maintenance of the injector components, can depend strongly on details of the injector design and that the methods to achieve the required parameters of the injectors may also depend strongly on the details of the design. Therefore the following sections of this chapter do not consider any design in detail, but, against the background of the design of the injectors for ITER, they try to describe the problems that will arise and to suggest ways in which they might be resolved. Also it is important to understand that the components of a neutral beam injector are interdependent and that in the following sections the assumptions made about the design and/or performance of the injector are self-consistent.
