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Correction coils

First of 18 lowered

25 Oct 2021

In all tokamak devices, ITER included, small deviations in the shape or position of the magnets cause unwanted field perturbations that can affect plasma stability and confinement. Correcting these "magnetic field errors" in ITER is the job of the correction coils—18 small superconducting coils inserted between the major toroidal and poloidal field systems. The first correction coil was lowered into place on 21 October.

BCC4_5.jpg
A 4-tonne bottom correction coil is inserted into the Tokamak pit on 21 October 2021. Five others will complete the set at the bottom of the machine.
Despite extreme precision in the manufacturing and assembly of the ITER magnet system, the smallest deviation from nominal in the magnets themselves—or in the system's joints, current leads or electrical busbars—perturbs the magnetic trap that confines and shapes the plasma.

Eighteen correction coils—six top, six side and six bottom—will act in pairs to correct, or reduce, the most troublesome field errors. Based on a build-to-print design developed by the ITER Organization, the Chinese Domestic Agency and its contractor ASIPP (Institute of Plasma Physics, Chinese Academy of Sciences) are procuring the coils. The first two bottom correction coils arrived on site at ITER in December 2020; since then, four other bottom coils have been delivered.

On site, each coil undergoes site acceptance tests including electrical tests, dimensional surveys, leak tests and weld inspections. For bottom coils BCC/4 and BCC/5, these tests were completed this month.

BCC4_3.jpg
Planarity is maintained for the duration of the lift (within the maximum planarity error tolerance of 3 mm). The large structure on the side of the coil is a "termination box," which hosts piping and insulating breaks.
In parallel to the acceptance tests, the team has to prepare the very tight environment of the Tokamak pit for the installation of the each coil.

"We know the as-built position of the first two poloidal field coils in the pit; we needed to work back from that to ready the temporary supports and plan the lift," says Josep Pallisa, assembly engineer in the Magnet Section. "In the crowded environment of the lower Tokamak pit, the bottom correction coils and their supports have only a small sliver of space between poloidal field coil #6, poloidal field coil #5, lower magnet feeders, and staging platforms."

The lift of BCC/4 on Thursday 21 October went according to plan. The second correction coil, BCC/5, will be positioned in the pit on 26 October, followed in the coming months by two other bottom correction coil pairs. In the final machine geometry, the correction coils will be attached to the toroidal field coil superstructure.  

The lowering of the BCC/4 coil onto its temporary supports goes smoothly, thanks to detailed preparation that included metrology, reverse engineering and accurate positioning. In the final machine geometry, the correction coils will be attached to the toroidal field coil superstructure.
Final touch down is the most critical part of the lift. The "sliver" of space between poloidal field coil #5, poloidal field coil #6, lower magnet feeders, and staging platforms does not exceed 25 mm for BCC/4 and 15 mm for BCC/5.
Correction coils are manufactured in unusual shapes to fit into the space between the poloidal and toroidal field magnets. At the bottom and the top of the machine, the coils are rectangles that curve inward to espouse the torus shape of the vacuum vessel.
The lift operation gets off to a start at 10:30 a.m. on 21 October. The total load under the crane is 5.3 tonnes including lift accessories.
Each correction coil is equipped with a large coil termination box that houses piping and electrical breaks. Coil BCC/4 was installed in the pit on 21 October; coil BCC/5 will follow on 26 October.
Verifying that the load is balanced is an important step before the lift can start. Chain blocks were installed to allow perfect alignment between the load's centre of gravity and the crane hook.
TAC1 assembly contractors (for Tokamak Assembly Contract 1) carry out the final preparatory activities before the lift operation. In the ITER machine, 18 of these (relatively) small coils will correct or reduce magnetic field errors caused by small deviations in the shape or position of the larger magnets.
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