2.1.1 Detailed discussion

This section discusses the items of Table 1 that are considered not to be self-explanatory.


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


Table 1.

Global efficiency of the ITER HNBs and possible injectors for a fusion reactor, based on reduced gas flow into the ion source, improved RF power supplies and a photon neutraliser.

#### 2.1.1.1 RF power to the ion source

The requirement to limit the neutral power to the calorimeter combined with efficiency increases elsewhere in the injector for a fusion reactor leads to a reduction in the accelerated negative ion current of about a factor 2 (see Section 2.1.1.5.), which leads to a similar reduction in the maximum power into the RF source. The reduction in the accelerated negative ion current leads to a more easily realised extracted negative ion current, lower power to the extraction and acceleration grids, lower back-streaming ion power, and lower electron power exiting the accelerator, all of which are very desirable.

### 2.1.1.2 Electrical power for the ion source

The current design of the RF power supply for the ITER neutral beam injectors uses a high power tetrode oscillator, which results in an efficiency of RF power production of about 50%. More modern RF power supplies which use solid-state technology have a power efficiency of about 85%. The use of such solid-state RF power supplies with an ITER-relevant type of RF-driven ion source has recently been successfully demonstrated at the ELISE facility in IPP, Garching, Germany [8].

### 2.1.1.3 Stripping loss and back-streaming positive ions

The negative ions extracted from the ion source can, and do, undergo diverse charge changing reactions with the background gas:

$$
\underline{D}^- + D\_2 \Rightarrow \underline{D}^0 + D\_2 + e
\tag{1}
$$

$$
\underline{D}^- + D\_2 \Rightarrow \underline{D}^0 + 2D + e \tag{2}
$$

$$
\underline{D}^- + D\_2 \Rightarrow \underline{D}^0 + D\_2^+ + 2\varepsilon\tag{3}
$$

$$
\underline{D}^- + D\_2 \Rightarrow \underline{D}^0 + D + D^+ + 2e
\tag{4}
$$

$$
\underline{D}^- + D\_2 \Rightarrow \underline{D}^0 + 2\underline{D}^+ + 3\underline{e} \tag{5}
$$

$$
\underline{D}^- + D\_2 \Rightarrow \underline{D}^+ + D\_2 + 2\varepsilon\tag{6}
$$

$$
\underline{D}^- + D\_2 \Rightarrow \underline{D}^+ + 2\underline{D} + 2\underline{e} \tag{7}
$$

$$
\underline{D}^- + D\_2 \Rightarrow \underline{D}^+ + D\_2^+ + 2\varepsilon \tag{8}
$$

$$
\underline{D}^- + D\_2 \Rightarrow \underline{D}^+ + D^+ + D + 2\varepsilon \tag{9}
$$

$$\underline{D}^- + D\_2 \Rightarrow \underline{D}^+ + 2\underline{D}^+ + \mathfrak{B} \tag{10}$$

where the underlined species are high-energy particles. Because the above reactions occur inside the accelerator, the produced D<sup>0</sup> will not have the full acceleration energy, and the precursor D� will not have experienced the full electrostatic optics of the extractor and accelerator, and therefore the D<sup>0</sup> will in general have a higher divergence than the fully accelerated D�. After it is created, the D<sup>þ</sup> will be decelerated, and it will either exit the accelerator with reduced energy, or they will be reflected, and they return to the ion source, and they will impinge on the rear of the ion source.

In addition to the above reactions, the accelerated D� can simply ionise the background gas, that is:

$$
\underline{D}^- + D\_2 \Rightarrow \underline{D}^- + D\_2^+ + e
\tag{11}
$$

$$D^{-} + D\_{2} \Rightarrow \underline{D}^{-} + D^{+} + D + e \tag{12}$$

$$D^{-} + D\_{2} \Rightarrow \underline{D}^{-} + 2\underline{D}^{+} + 2\varepsilon\tag{13}$$

The D<sup>þ</sup> and D<sup>þ</sup> <sup>2</sup> created in reactions (11)–(13) will be back-accelerated, and they return to the ion source, and they will impinge on the rear of the ion source.

The background gas in the extractor and accelerator of a negative ion-based injector comes overwhelmingly from the ion source, and the background gas density decreases with the distance from the plasma grid. That, combined with the fact that the cross sections of reactions (1)–(13) decrease with the energy of the precursor D� at energies above ≈10 keV, means that essentially all the D<sup>0</sup> produced by reactions (1)–(5) is not useful for heating a fusion reactor and the precursor D� is considered as lost in the accelerator. Although the D<sup>þ</sup> created by reactions (6)–(10) can be neutralised to produce D0, that reaction is negligible for D<sup>þ</sup> at energies close to that required of the D<sup>0</sup> needed to heat the fusing plasma, so the D� that undergoes reactions (6)–(10) is also considered to be completely lost. The losses via reactions (1)–(10) are commonly referred to as stripping loss.

In the case of the ITER heating neutral beam injectors, the stripping loss is calculated to be ≈8 MW, and the power in the back-streaming positive ions (mainly from reactions (11)–(13)) is calculated to be ≈1 MW. It is obvious that losing ≈9 MW in an injector that is designed to deliver ≈17 MW to the plasma has a major impact on the global efficiency of the injector, and it must be reduced if the target of 60% global efficiency is to be met. As noted above, most of the background gas in the accelerator comes from the ion source. Hence to achieve a global efficiency of 60%, that gas flow must be reduced. Fortunately, it has been demonstrated with a filamented ion source that extracted current densities higher than those needed in the injectors for a fusion reactor assumed in Table 1 can be achieved with a filling pressure (the pressure in the in source without source operation) of 0.1 Pa [9], a factor 3 lower than the target value for the ITER injectors. Thus that value is chosen in Table 1 for the injector of a fusion reactor. It must be noted that any filamented source, including the source type where operation at the low gas flow has been demonstrated, is not considered suitable for use on an injector to be used on a fusion reactor because of the limited lifetime of the filaments (<200 h). Operation at such a low gas flow has not yet been achieved in the type of source to be used on the ITER injectors, an RF-driven source, and significant R&D is needed to develop an RF-driven source that can operate at such low gas flows.

Back-streaming ions are positive ions that are created inside the accelerator, which are then accelerated back to the ion source by the electrical fields in the accelerator. There are three reasons the power in the back-streaming ions must be reduced in an injector on a reactor:

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


It has been suggested that if the plasma grid were made from a low work function material, there would be no need to inject caesium into the source, with the accompanying problems discussed in Section 4. However, as mentioned above, if the PG is coated by several monolayers of metal sputtered from the backplates, the work function will increase leading to a reduction in the negative ion production and hence in the extracted current. The sputtering rate for the ITER beam source is by back-streaming D2 <sup>+</sup> which is calculated to be 5 <sup>10</sup><sup>16</sup> atoms/s, and the rate for the injector on a reactor may be a factor ≈6 lower if the extracted current density is lower (see Section 1.2) by a factor 2 and the gas density in the accelerator is reduced by a factor 3. As a monolayer corresponds to about 10<sup>17</sup> m<sup>2</sup> , so it is obvious that several monolayers of the sputtered material will be deposited on the PG in a time that is short compared to the reactor lifetime.

Potential solutions to the problems discussed above are:

a. Easily replaceable backplates plus a reduction in the back-streaming ion flux. Since the back-streaming D2 <sup>+</sup> is directly proportional to the of D2 in the

extractor and accelerator and that of Cs+ ion intensity is directly proportional to the density of Cs in the extractor, reducing by a factor 3 the D2 flow out of the ion source and the Cs density in front of the PG would reduce the sputtering of the ion source backplates enough to avoid the erosion causing a water leak in <1 year of reactor operation, allowing the backplates to be replaced during an annual maintenance period.

