Page 304 - Fundamentals of Magnetic Thermonuclear Reactor Design

284 Fundamentals of Magnetic Thermonuclear Reactor Design

FIGURE 9.1 Schematic of a neutral beam injector (NBI).

Reactor-scale tokamak injectors are unique electrophysical devices. For in-
stance, the ITER injector system consists of three modules, designed to inject
beams of a total energy of 50 MW. Each module generates a beam of protium
or deuterium atoms with a design energy of 1 MeV and a current of 16.5 A
(Table 9.3) [4].
The recharge target is the injector centrepiece. Recharge probability de-
pends on the ion energy and electric charge. As one can see from Fig. 9.2, it
is reasonable to use protons and positively charged deuterons up to the energy
limit of 150 keV, and to use negatively charged protium and deuterium ions at
higher energy.
Injectors based on positively charged ions neutralise initial rated-energy
ion fluxes as they pass through the recharge target. Injectors using negatively
charged ions form low-energy beams at the source. Then the injected ions are
additionally accelerated up to a nominal energy. After that, a special (scraping)
target is used to tear off the electron from the negative H ion. The scraping target
may be based on a gas, metallic vapour or laser technology.

TABLE 9.3 Design Parameters of ITER Neutral Beam Injector Module [4]
Parameters D o H o
Maximum atomic energy 1000 870
(keV)
Accelerated ions D ─ H ─
Maximum ion current (A) 40 46
Beam power (MW) 16.5 16.5
Pulse duration (s) 3600 ∼1300 a
a The pulse length is limited by the saturation of the activated charcoal used in cryosorption panels.