The largest fusion device ever to be built looks like a mere appendage to the much larger neutral beam system whose injectors are sized like steam locomotives. In yellow, the diagnostics neutral beam; in light brown a third possible heating neutral beam. The green and blue structures to the right belong to the other auxiliary heating systems, the electron cyclotron resonance heating (ECRH) and ion cyclotron resonance heating (ICRH).
Construction work underway in the Tokamak Building already gives a sense of how big the equipment for the neutral beam system will be. Giant circular cut-outs in the rebar at level 3 (L3) of the building—more than 3 metres in diameter each—will provide the passageway for high-voltage “bushings,” which allow electrical power, cooling, and other services such as diagnostics to reach the neutral beam injectors hosted below.
Just below the bushings, a vast, cavernous space has been reserved for the neutral beam cell where the beam injectors will be located. The largest devices (the heating neutral beam injectors) are sized like steam locomotives—25 metres long, 5 metres high and 5 metres wide—with a chimney-like bushing reaching up 9 metres to connect to the openings on the third floor. The injectors will be connected to the Tokamak at L1 level—exactly across from the Tokamak’s mid-plane and the equatorial port openings.
Giant power outlets for a giant appliance. These circular openings in the rebar at L3 level of the Tokamak Building are more than 3 metres in diameter each. They will allow the high-voltage bushings of the neutral beam system to deliver electrical power, cooling, and other services such as diagnostics to the injectors below.
The heating neutral beam injectors will each contribute 16.5 MW of heating power to the plasma; the diagnostics neutral beam will provide information on the helium ash density produced by the D-T fusion reactions in the fusion plasma.
At the entry end of the heating neutral beam, a beam source generates the electrically charged deuterium ions that are accelerated through a succession of five grids (each separated by a 200 kV electrical potential) to the required energy of 1 MV at the exit end of the beam source, a “neutralizer” rips them of their electrical charges to become “neutrals,” allowing them to penetrate the Tokamak’s magnetic cage and, by way of multiple collisions with the particles inside the plasma, raise plasma temperature to the point where fusion reactions can occur. The heating neutral beams are designed to be able to operate during the entire plasma duration, up to 3,600 seconds.
Neutral beams are routinely used in tokamak devices as the workhorses of auxiliary heating. In ITER however they will be considerably larger and more powerful than in any previous fusion device.