How to install ITER’s diagnostic cables with the highest precision

Despite the colossal size of ITER, the assembly of some of its component is a matter of millimetres. Achieving the required level of confinement in the vacuum vessel, crucial to the operation and safety of the device, requires an impressive degree of accuracy during the installation of equipment.

This is particularly critical for the diagnostics systems, which will monitor what happens inside the device and send lots of data outside. These remote 'eyes' have their special nerves: more than 3000 mineral-insulated cables that must cross the vessel wall without breaking the vacuum. To this end, Fusion for Energy (F4E) will supply ITER with 75 feedthroughs, the small cylindrical gates welded into different ports of the machine. They were designed to let the cables through whilst hermetically sealing the vessel, by using two glass-to-metal barriers. The feedthroughs were designed in collaboration with IDOM (Spain) and are currently under fabrication by Alsymex (France), after successfully passing a series of resistance tests.

However, once manufactured the engineering puzzle will not be over. Inserting the feedthroughs in the port plugs is a delicate task. It's like threading a needle - only with long rigid wires that have fragile terminations. For this reason, in 2022 the teams of F4E and IDOM decided to develop two assembly and alignment tools, tailored for the upper and lower ports. Two years later, both systems have been fabricated and delivered to ITER.

"It was an unforeseen spin-off within our collaboration with IDOM. Despite the limited budget allocated and the time pressure, the contractor presented a solution that complied perfectly with the strict assembly protocols and metrology challenges of ITER", explains Miguel Perez Lasala, F4E Plasma Systems Project Manager. IDOM built a real-scale environment of the ports and tested the alignment tools with different feedthrough variants. This gave the team the confidence that the equipment is readily usable for the operators at ITER.

IDOM tested the alignment procedure with an ITER port mock-up. ©F4E

In Cadarache, ITER experts will now be able to run trials of the assembly and alignment procedure, using a prototype of a feedthrough. It's a stepped choreography, adapted to the different architecture of the upper and lower ports. "The tools were devised to fit in a corridor busy with other equipment like the cryogenic systems. We used a modular design, so they can easily be assembled and disassembled when needed", notes Nevenka Beslic, F4E external support officer.

In the upper port, the tool carrying the feedthrough will be mounted on a two-level scaffolding. From there, a set of mechanisms will pivot it and adjust it to make the feedthrough slide neatly into the plugs in an arc shape. In the lower port, however, the system will first need to unwind and straighten the cable. Then, a set of pulley carriages and trolleys will lift the feedthrough and thread it in.

Once welded into their exact place, the feedthroughs will act as the doorkeeper of all ITER diagnostics systems. The data travelling across will give scientists insights into the behaviour and properties of the plasma, through a wide range of sensors measuring temperature, irradiation, and other variables. Europe is responsible for roughly 25% of all diagnostics in ITER, involving more than 60 companies and research laboratories.

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