Via US LHC Blog -
This message was sent from Director General Rolf Heuer to the CERN community today:
The foreseen shutdown work on the LHC is proceeding well, including the powering tests with the new quench protection system. However, during the past week vacuum leaks have been found in two “cold” sectors of the LHC. The leaks were found in sectors 8-1 and 2-3 while they were being prepared for the electrical tests on the copper stabilizers at around 80 K. In both cases the leak is at one end of the sector, where the electrical feedbox, DFBA, joins Q7, the final magnet in the sector.
Unfortunately, the repair necessitates a partial warm-up of both sectors. This involves the end sub-sector being warmed to room temperature, while the adjacent sub-sector “floats” in temperature and the remainder of the sector is kept at 80 K. As the leak is from the helium circuit to the insulating vacuum, the repair work will have no impact on the vacuum in the beam pipe. However the intervention will have an impact on the schedule for the restart. It is now foreseen that the LHC will be closed up and ready for beam injection by mid-November.
Liquid nitrogen is used to cool 37,000 tonnes of equipment for the Large Hadron Collider (LHC) down to 80 K. Then liquid helium is used to chill some parts of the accelerator to temperatures as low as 1.8 K.
But liquid helium isn't created directly....
The cryoplants produce high-pressure supercritical helium gas at 4.6 K, which will be distributed along the sector to a number of local cooling loops. There, the supercritical helium will be expanded into a lower-pressure environment, which causes it to liquefy at either 4.5 K or 1.8 K. This liquid will then be used to cool the superconducting magnets.