3. Pulse length

All neutral beam injection systems that have been designed or built have been conceived for a pulsed mode operation, whereas fusion reactors, and any NBI system to be used on the reactor, are expected to operate continuously for 1 year or more. Continuous operation will require fundamental changes in the design of the injectors, such as:

#### 3.1 Pumping and gas flow

All NBI systems so far designed and/or built require very high pumping speed at the exit of the accelerator and downstream of the residual ion dump in order to minimise beam loss. The former is needed in order to reduce the gas density, and hence the stripping losses, in the accelerator and the latter to reduce the pressure in, and downstream of, the residual ion dump and hence the re-ionisation of the neutral fraction of the beam in that region and in the duct leading to the reactor vessel. That is important as re-ionised particles will be deflected into the walls of the duct by the stray magnetic field from the reactor, reducing the injected power as well as heating the duct walls. The latter is important as removing heat deposited in the duct leads to water-cooled components in the duct, and those may occasionally require maintenance. As those components will become highly active during operation of the reactor, such maintenance will only be possible by remote means, which is inherently complicated and difficult. Reducing the power load means that the components can be designed with a high safety factor, leading to less, perhaps no, maintenance in the lifetime of a reactor.

Cryopumps that "wallpaper" each side of the injector beamline vessel can provide a sufficient pumping speed for D2, which in the ITER heating injectors is <sup>≈</sup><sup>3</sup> � <sup>10</sup><sup>6</sup> l/s [12]. Gas is not removed from an injector by cryopumps, it is simply frozen on the cryogenically cooled panels of the pumps, and the quantity of D2 that can be stored on the pumps is limited because of the possible explosion; if the D2 is released from the pumps, there is sufficient oxygen inside the injector, and a source of ignition is present. Although the risk is low, it is not acceptable, especially in the case of a reactor where the injector is part of the confinement barrier. Consequently, when the gas storage limit is reached, it is necessary to regenerate the pumps, i.e. to release the stored D2, and pump it away with an external pumping system. As several injectors are likely to be installed on a reactor in order to provide the required total heating and current drive power, continuous provision of the required power can be assured by installing one "excess" injector. Then if the n injectors are installed, n 1 injectors provide the required power, and only n 1 are usually operating. That allows the non-operating injector to be isolated vacuum wise from the reactor and to be regenerated, with each injector being regenerated in turn. That only works if the regeneration time<sup>1</sup> is <sup>≤</sup>τ/(n 1) hours, where <sup>τ</sup> is the time for which each injector can operate before regeneration is needed. As an example, assume that the injectors can be regenerated in 1.5 h and that each injector can operate for >3 h before needing to be regenerated. Then with three injectors installed on the reactor, one is regenerated each hour, so that the cryopump of each injector is regenerated after 3 h of operation; all three injectors are regenerated after 4.5 h, and the cycle is then repeated, as shown in Figure 1. Having an "extra" injector is expensive, both because of the cost of the injector and the additional cost of operating the extra injector and because it reduces the efficiency of the reactor as the wall space needed for the extra injector cannot be used for generating power.<sup>2</sup>

In the example given above, the injectors would have to operate >3 times longer than permitted for the ITER injectors, so that, all other things being unchanged, the total gas flow into the injectors on the reactor would have to be reduced by a factor 3 compared to the gas flow into the ITER injectors, and that must be achieved without degrading the injector performance or the global injection efficiency.

An alternative to having an extra injector is to instal a higher pumping speed than is required for the efficient operation of the injector and to be able to regenerate the "unnecessary" part of the cryopumps in situ. For example, if the installed pumping speed were twice that required for the efficient operation of the injector, then half the pump could be shut off from the injector, whilst the other half continues to operate; once the first half has been regenerated, that can be opened up to the injector and the second half closed off for regeneration and so on.

#### Figure 1.

Schematic of the regeneration cycle for a neutral beam system with three injectors installed, with each injector capable of operating for >3 h before regeneration of the cryopumps being necessary. The yellow areas indicate that the injector is operational and the green ones that it is being regenerated. Two injectors are always operational.

<sup>1</sup> The regeneration time consists of the time to isolate the injector from the fusion device, warming up the cryopumps to the release the gas, pumping down the injector and cooling down the cryopumps, plus the time to recondition the injector.

<sup>2</sup> If a reactor costs 2 <sup>10</sup><sup>9</sup> €, and the apertures in the blanket for the injectors take up 1% of the wall space, the cost of not producing power from that fraction of the wall is 2 <sup>10</sup><sup>7</sup> €, i.e. 66 M€ per injector.

Research, Design, and Development Needed to Realise a Neutral Beam Injection System… DOI: http://dx.doi.org/10.5772/intechopen.88724

Unfortunately no system for closing off part of a cryopump sufficiently to allow it to be regenerated whilst the rest of the pump continues operation has yet been developed.

In the above discussion, it is assumed that cryopumps will be used in the NB injectors. It has been suggested that non-evaporable getters (NEGs) could be used instead of cryopumps. NEGs have an advantage that no sudden release of the gas captured by the getters is possible; thus there is no safety hazard associated with the storage of large quantities of D2 in the getters. If NEGs are to be considered, a viable assembly of NEGs needs to be designed, using NEGs that will not be poisoned by any impurities in the NB injector and, obviously, that the assembly needs to provide the required pumping speed. However the problem of regeneration of the pumps remains as NEGs do not actually pump the D2 out if the injector, but they trap it within the getter material, and regeneration is needed once the NEGs become saturated with D2. The possible ways to overcome the lack of pumping during regeneration are, in essence, the same as suggested for cryopumps above. However the regeneration time for NEGs is expected to be several hours, so that with three injectors installed on the reactor, as in the above example with cryopumps, the NEGS would have to operate for 10 h if the regeneration time is 5 h. No design of such a system has yet been done.

Because of the quasi-continuous operation of the injectors, all the gas used for the injectors will be gas recovered from the gas recycled through the reactor "tritium plant". That must include the gas released from the injector cryopumps as that will be contaminated with T2 that has flowed to the injectors from the reactor. However, the gas flowing into the ion sources must contain only a small fraction of T2 to make sure that the neutral beams will have a negligible fraction of T<sup>0</sup> as the lower velocity of T<sup>0</sup> would lead to deposition in the plasma of the reactor nearer the outside of the plasma which is undesirable.<sup>3</sup> The requirement to have fairly pure D2 for the ion source operation impacts directly on the design of the tritium plant, which could lead to substantial cost increase for the tritium plant, and therefore any reduction in that gas flow is highly desirable.

It is to be noted that some of the He produced in the fusion reactions will flow into the neutral beam duct and the injector, adding to the gas density in the duct. That will increase the fraction of the neutral beam that is re-ionised by collisions with the gas in the duct and thus the power to the duct walls. The density of He in the duct and the injector must be kept at a level that the He in the ion source does not compromise its performance and that the He density in the beamline and the neutral beam duct does not significantly enhance re-ionisation losses. An estimate of the density in the ion source and the increase in re-ionisation loss due to the presence of He in the injector and duct is given below:

Assume that a 1 MeV D beam is being injected into the reactor.

If the pumping speed for He in the injectors is zero, the He density in the duct between the injector and the reactor will rise until the flow out of the duct into the reactor is equal to the flow from the reactor into the duct. Assuming that the He temperature in the duct is 100°C (due to collisions with a 100°C duct wall), that the gas flow out of the reactor into the injector duct is ≈1020 atoms/s (a value calculated for the plasma in ITER), and that 10% of the outflow is He (the rest being D and T), the density of He in the duct will be <sup>≈</sup>2.5 <sup>10</sup><sup>16</sup> <sup>m</sup><sup>3</sup> .

<sup>3</sup> T2 in the source may also impact on the source performance. Operating in D2 is known to result in a higher fraction of co-extracted electrons compared to operation in H2, and it is possible that the electron fraction with T2 is even higher.

Now the He flow out of the injector into the duct must be equal to the flow into the injector from the duct. Assuming that the He temperature in the injector is 20°C (due to collisions with water-cooled components in the injector), then the gas density in the injector will be <sup>≈</sup>2.8 1016 <sup>m</sup><sup>3</sup> .

Re-ionisation loss occurs between the entrance to the residual ion dump and the entrance into the reactor. The cross section for re-ionisation of D on He is ≈3 10<sup>21</sup> m<sup>2</sup> . Assume that the length of the duct between the injector and the reactor is 10 m and that the length of the residual ion dump plus that of the beamline calorimeter is 3.5 m. Then the extra re-ionisation loss due to the presence of He in the system is calculated to be ≈0.1%.

If the He temperature in the ion source is 1200 K (the gas temperature in a negative ion source of the type to be used in the ITER injectors has been measured to be 1200 K), then as the He flow into the ion source from the beamline must equal the He flow out of the source, the He density in the ion source will be ≈1.6 10<sup>16</sup> m<sup>3</sup> . To put that in perspective, the D2 density in the ion source should be of the order of 6 <sup>10</sup><sup>18</sup> <sup>m</sup><sup>3</sup> , that is, the He will represent <0.25% of the gas particles in the ion source.

In conclusion, the small increase in the re-ionisation loss due to the presence of He in the injector and the duct of ≈0.1% is almost certainly acceptable. The presence of an He density of significantly above <sup>≈</sup>1.6 <sup>10</sup><sup>16</sup> <sup>m</sup><sup>3</sup> has been shown experimentally not to have any significant effect on the source performance [13].
