Neutral beam
The system that makes the tokamak feel small
8 Apr 2019
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R.A.
ITER is a big machine—by far the largest fusion device ever built. But there is a system just a few metres away that makes it look like a mere appendage to something much larger. The neutral beam system, with its three, possibly four, massive injectors is the real beast at the heart of the ITER installation.
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 left 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.
A neutral beam injector is essentially a particle accelerator. Its function is to deliver high-energy particles to the heart of the plasma. ITER is planning two one-million-volt (MV), 40A heating neutral beam injectors (and is making a space reservation for a possible third) as well as a smaller neutral beam line (100 kV, 60A) for diagnostic purpose.
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.
Generating a 1 MV beam that will deliver 16.5 MW to the plasma requires a unique power infrastructure. Located just outside the Tokamak Complex, two large buildings will host the transformers, the AC/DC converters and the vast high-voltage hall that will feed power to the neutral beam system by way of transmission lines entering the Tokamak Building through the "north wall" at the L3 level.
There is only one example of a high-voltage installation more powerful than ITER's. In China, where high-voltage DC current is used to deliver electrical power to populations far away from the productions sites, a 1.2 MV system was recently established to push power from Xinjiang, in the northeast corner of the country, to the megacities in the east—1.2 MV in China for a 3,000-kilometre distance; 1 MV in ITER for slightly more than one hundred metres ...
Located in the vast, double-level neutral beam cell at the "ground" level of the Tokamak Building, the 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.
Large space reservations
The neutral beam injection system occupies three levels of the Tokamak Building: L1 and L2 for the cavernous neutral beam cell, and L3 for the high-voltage bushings. Building contractors are working now on the large openings in the L3 slab that will allow electrical power, cooling, and other services such as diagnostics to reach the neutral beam injectors below.
Dwarfing the vacuum vessel
Taken together, the elements of the neutral beam system—heating neutral beam injectors, diagnostic neutral beam, high voltage bushings—dwarf the ITER vacuum vessel, making it seem like a mere appendage.
Generating a one million-volt (1 MV) high-voltage beam requires a unique power infrastructure. Situated along the western edge of the ITER platform, right in front of the Tokamak Complex, the infrastructure will convert the AC current from the grid into DC current, and distribute power to the neutral beam power sources, accelerating grids and ion sources. One of the most spectacular devices in this set of installations is the high voltage deck (centre) whose prototype is being tested at the ITER Neutral Beam Test Facility in Padua, Italy. Standing 6 metres high and looking like a science-fiction contraption, the device is an electrically insulated box that hosts the 1MV power supplies for the neutral beam injectors' ion sources and extraction grids.