I've seen those copper busbars -- they drill holes for water throughout them. The whole design of high-B field environments is fascinating; you end up with things like Bitte designs with split rings and a whole lot of engineering to stop the copper vapourising. Highly recommended if you're ever in the area – they do two sorts of tours (or did, prior to covid), the "general public" tour and the "scientist" tour. I went on the latter. The sight of two giant robotic arms playing Jenga to train their operators is not one to forget.

I should visit JET sometime. Of all the systems on the machine I worked on (https://hsx.wisc.edu/), the coil current feeds were the most difficult. Trying to cram that much current through a small piece of copper with a discontinuity takes years of effort.