Can one of you physicists tell me how 4.5
Kelvin is different from 2 Kelvin, operationally?
The magnets they use to shape and steer the beam require about 12,000 amps, so they use superconductors. Between 2K and 4.5K, the superconductor undergoes a phase change and becomes non-superconducting, and the resistance goes from zero to not zero all of a sudden. The 12,000 amps suddenly produces an incredible amount of heat (P=I^2R) which drastically increases the pressure from the liquid He. That much pressure means the He needs to escape (violently), causing all sorts of trouble. It's called a 'quench.'