The BEBC was a giant cryostat, full of liquid hydrogen: http://en.wikipedia.org/wiki/Big_European_Bubble_Chamber http://en.wikipedia.org/wiki/Big_European_Bubb

The BEBC was a giant cryostat, full of liquid hydrogen: http://en.wikipedia.org/wiki/Big_European_Bubble_Chamber http://en.wikipedia.org/wiki/Big_European_Bubble_Chamber My guess is that the cooling wasn't for heat related to the power itself (though perhaps), but to act as a sort of "temperature lock" for the entry point of an outside line. If heat can get in through the openings in the cryostat, you risk boiling off the contents. So you surround the line with something colder than liquid hydrogen, and ensure that the line inside the cryostat isn't warmer than the contents.