5.1 Component lifetime

The lifetime of fusion reactors must be very long, and similar to that of a fission reactor, for example, >40 years, and, like fission reactors, they will operate continuously between outages for maintenance, which will occur not more frequently than yearly, and the outage time has to be as short as possible, for example, <1 month, see, for example [14]. The time that the NBI system on a reactor must operate, both in total and between maintenance periods, is orders of magnitude beyond any NBI system designed so far, and that means that many new considerations enter into play, such as the lifetime of components which are subjected to sputtering by the beams and thermal fatigue. It is also evident that as the reactor and the injectors will operate for 1 year or more between maintenance periods, any component that is designed to be replaced routinely must be able to operate for more than 1 year before the replacement becomes necessary.

#### 5.2 Lifetime and fatigue

The injector components should be designed to have a fatigue life that is greater than the life of the fusion reactor, that is, about 40 years with essentially continuous operation. When there is a breakdown in the accelerator, the beam will be reestablished in ≈180 ms, which is short compared to the thermal response time of the components, and the components "see" only a small part of a thermal cycle. Therefore fatigue failure will arise from the on-off cycles of the beam, when the components will experience the complete thermal cycle. The number of on-off cycles will probably be dominated by the regeneration cycle of the cryopumps. With a photon neutraliser and a gas flow into the ion source that is reduced by a factor 3, the cryopumps should be regenerated after ≈3 h of operation (see Figure 2). As, with continuous operation of the reactor, ≈3000 regenerations will be needed per year, there will be <sup>≈</sup>1.2 <sup>10</sup><sup>5</sup> cycles in the reactor lifetime, 40 years. The injector will almost certainly require conditioning pulses after a regeneration in order to regain full performance. The low power operation used at the start of the reconditioning does not contribute significantly to fatigue, and about five full power pulses should suffice to complete the conditioning. Thus the injector components will "see" <sup>≈</sup><sup>6</sup> <sup>10</sup><sup>5</sup> full thermal cycles in the reactor lifetime.

The relatively low extracted current assumed above, together with the reduction in the stripping losses by a factor 3 (due to the reduced source gas outflow), means that the power load to the grids should be easily handled and fatigue should not be a problem. Similarly, the power to the residual ion dump will be quite low because of the high neutralisation expected with a photon neutraliser and designing the residual ion dump to have the required fatigue life not be a problem. The reduced gas flow into the injector will significantly reduce the re-ionisation loss, hence the loads to the panels in the duct leading to the reactor, and fatigue should not be a problem. However, even in the reduced current density design considered in Section 1.2, the beamline calorimeter, which is essential for the commissioning and recommissioning of the beam source, will receive a similar power density to that received by the calorimeter of the heating neutral beams of ITER, and the fatigue life of the optimised design of that component is calculated to be <sup>≈</sup>7.5 <sup>10</sup><sup>4</sup> cycles, so either a non-negligible improvement of the design is needed or the calorimeter will have to be replaced after about 20 years, i.e. once in the lifetime of the reactor.

It is worth emphasising that, as assumed above, it is reasonable to conclude that the power "seen" by the beamline calorimeter of an injector on a reactor cannot be higher than that "seen" by the calorimeter of the heating injectors of ITER, which precludes increasing the accelerated D current density to >100 A/m<sup>2</sup> when a high efficiency neutraliser is used. Another consequence is that any increase in the beam energy would necessitate a decrease in the accelerated current density in order to keep the power to the calorimeter at the acceptable level.
