After the successful demonstration of the high current negative ion production of 10 A with reasonable current density and negative ion extraction/
acceleration with good beam optics, the first negative-ion-based NBI was designed and applied in JT-60 in 1996 [5.75]. The good beam optics of the negative ion beam made it possible to install the ion source far from the tokamak and guide the neutral beam through a narrow neutralizer cell [5.121]. Figure 5.51 shows the cross-sectional view of the JT-60 negative ion source. Deuterium negative ions are produced in a Cs seeded volume production source. The source called “KAMABOKO source” has a semi-cylindrical chamber that has a large volume to surface ratio and the surface of the chamber is covered by strong magnetic line cusps. Because of the good confinement of the source plasma, the operating gas pressure is as low as 0.3 Pa. This low operating gas pressure is convenient to accelerate the negative ions at energies up to 500 keV without significant stripping loss of negative ions in the accelerator.
FIG. 5.51. Negative ion source developed for JT-60 N-NBI [5.75].
5.2.7.2. N-NBI for LHD
Figure 5.52 shows the negative ion source for the negative-ion-based NBI for LHD [5.122]. The plasma generator has a dimension of 35 cm × 145 cm in width and 21 cm in depth and is covered by magnetic line cusps. A pair of strong line cusps are installed outside the chamber to create a transverse magnetic filter.
Three Cs ovens are installed on the top of the chamber to inject a small amount of Cs to enhance the negative ion production. The negative ions are extracted by a multi-aperture three-grid extractor and accelerated by a single acceleration system. There are 770 apertures over the extraction area of 25 cm × 125 cm.
Hydrogen negative ion beams of 30 A and 180 keV were achieved with a good beam divergence of less than 10 mrad.
FIG. 5.52. Negative ion source developed for LHD N-NBI [5.122].
5.2.7.3. N-NBI for ITER
ITER will use two 1 MeV heating NBIs and one 100 keV diagnostic NBI.
A third NBI for heating may be added later to increase the total neutral beam power from 33 MW to 50 MW.
Figure 5.53 shows the schematics of the 1 MeV negative-ion-based NBI for ITER [5.78, 5.79]. It is necessary to develop a negative ion source producing 1 MeV, 40 A (40 MW) D-ion beams. According to the reference design [5.79], D-ions are produced in an RF negative source which has a long life and no
filaments and simplifies the electric power supplies. The RF negative ion source developed in IPP Garching has been demonstrated to have a capability to generate the D-ions required for ITER. The D-ion beams produced are extracted from a multi-aperture three-grid extractor having 1300 extraction apertures of 14 mm in diameter over an area of 60 cm × 164 cm and accelerated by an accelerator with a current density of 20 mA.cm–2 at a gas pressure as low as 0.3 Pa.
FIG. 5.53. Isometric illustration of the ITER NBI [5.79].
Since the H-/D-ions have an electron affinity of only 0.75 eV, they are easily neutralized by collisions with the residual gas in the accelerator before having their full energy. This is called “stripping” to distinguish it from the desired neutralization in the neutralizer. To minimize the stripping, the gas pressure in the accelerator has to be minimized, which leads to the requirement that the ion source, which is directly connected to the accelerator, operate at a low pressure of less than 0.3 Pa.
Two types of accelerator concept have been developed for ITER, namely SINGAP [5.123] by CEA Cadarache, and MAMuG [5.124] by JAEA in Naka.
In the SINGAP accelerator the extracted and pre-accelerated negative ions are accelerated in one single step to 1 MeV, while the negative ions are accelerated in five stages (200 keV in each stage) in the MAMuG accelerator. The SINGAP accelerator has the advantage that the accelerator and the power supply are highly simplified because it does not require intermediate potential electrodes. However, it turned out that the voltage holding of the SINGAP accelerator is worse than that of the MAMuG accelerator, and that the power carried by electrons that exit the SINGAP accelerator is much higher than the power of electrons from MAMuG [5.124, 5.125]. For this reason the MAMuG accelerator is chosen in the reference design of ITER.
The ITER NBI is operated in a radiation environment. Neutrons from the tokamak will stream through the NB injection port, which has a large
opening about 1.4 m in height and 1.3 m in width, and activate all the beam line components, including the ion source/accelerator. A three-dimensional nuclear analysis has clarified that the neutron flux at the position of the negative ion source is as high as 1010 neutrons/cm2 [5.126] and a conventional insulation gas such as SF6 is not applicable because of the radiation induced conductivity (RIC) [5.127]. The insulation gas is ionized by the radiation and the dark current produced by the ionization becomes huge and degrades the power efficiency.
Therefore, the design of ITER NBI adopts vacuum insulation for the ion source with a high voltage bushing as a vacuum boundary between the ion source and insulation gas in the high voltage power supply.