2. Issues related to the design of a neutral beam injector operating on a fusion reactor, some possible ways to resolve those issues and suggested R&D

### 2.1 Global efficiency

The global efficiency of a heating system is simply the ratio of the electrical power required to operate the heating system divided by the power absorbed by the device being heated, the fusing plasma in the case of a fusion reactor, and it is of overriding importance for a fusion reactor. This can easily be understood with an example: suppose that the heating power required to heat the fusing plasma to the temperature required to ensure that the rate of fusion reactions in the plasma is that required to achieve the electrical output from the reactor is 100 MW. Then, if the global efficiency of the heating system were similar to that achieved by the systems operating today, of the order of 25%, about 400 MW of electrical power would be required simply to operate the heating system, that is, the output of a typical power station. There have been several studies aimed at defining an acceptable global efficiency for the heating systems of a fusion reactor, and the typical result is ≈60% or higher [7].

The specification of the neutral beam injectors designed for ITER is the closest of any design to that which would be suitable for use on a fusion reactor.
