2.1.1.4 Electrons exiting the accelerator

Some electrons are co-extracted from the ion source, and, as noted in Section 2.1.1.3, electrons are created in the accelerator via reactions (1)–(13). Electrons in the extractor and accelerator will be co-accelerated along with the negative ions until they are deflected onto the extraction grid or one of the accelerator grids, or they exit the accelerator. The power loss in the extractor is given in Table 1, line 2, and line 5 gives the power in the electrons exiting the accelerator. That power is deposited on downstream beamline components and the beamline vessel. The reduction in the gas density in the extractor and accelerator, as discussed above, results in a reduction in the exiting electron power by a factor similar to the reduction factor for the back-streaming ions.

### 2.1.1.5 Total accelerated power

Line 6 of Table 1 gives the total accelerated D power for the injector to be used on a fusion reactor as 22.3 MW compared to 40 MW for a heating neutral beam injector for ITER. The main reason for the reduced accelerated D power is to keep the power density on the calorimeter similar to that of the ITER injector.

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

#### 2.1.1.6 Beamlet halo

Early measurements of negative ion beamlet profiles suggested that the optics of the beamlets are not well described by a simple Gaussian divergence, that is,

$$P(o) = P\_0 \* \exp\left(-\left\{\frac{o^2}{o\nu\_0^2}\right\}\right) \tag{14}$$

where P(ω) is the power density at a radial position that subtends an angle with respect to the beamlet axis of ω and ω<sup>0</sup> is the beamlet divergence. The measured profiles were found to be better fitted by a bi-Gaussian profile. For the ITER design, the latter was chosen, that is,

$$P(\boldsymbol{\alpha}) = (1 - f) \ast P\_0 \ast \exp\left(-\left\{\frac{\boldsymbol{\alpha}^2}{\boldsymbol{\alpha}\boldsymbol{\alpha}^2}\right\}\right) + f \ast P\_0 \ast \exp\left(-\left\{\frac{\boldsymbol{\alpha}^2}{\boldsymbol{\alpha}\boldsymbol{\alpha}^2}\right\}\right) \tag{15}$$

where f is the fraction of the beamlet power carried by the "halo", which has a divergence of ω0h. For the ITER design, f was assumed to be 15%. It is assumed that R&D from the ITER neutral beam test bed and other negative ion-based systems will allow an improved optics with a halo carrying only 5% of the beamlet power.
