On 19 September 2008, during powering tests of the main dipole circuit in Sector 3-4 of the LHC, a fault occurred in the electrical bus connection in the region between a dipole and a quadrupole, resulting in mechanical damage and release of helium from the magnet cold mass into the tunnel. Proper safety procedures were in force, the safety systems performed as expected, and no-one was put at risk....
During the ramping-up of current in the main dipole circuit at the nominal rate of 10 A/s, a resistive zone developed leading in less than one second to a resistive voltage of 1 V at 9 kA. The power supply, unable to maintain the current ramp, tripped off and the energy discharge switch opened, inserting dump resistors into the circuit to produce a fast current decrease.... Within one second, an electrical arc developed, puncturing the helium enclosure and leading to a release of helium into the insulation vacuum of the cryostat.... The spring-loaded relief discs on the vacuum enclosure opened when the pressure exceeded atmospheric, thus releasing helium into the tunnel...
At the bottom of the press release, they define the term cold mass:
The magnets, equipped with their helium vessel and end covers, constitute the "cold masses".... The weight of the cold mass is transmitted to the vacuum enclosure via cold support posts and is further transmitted to the tunnel floor by adjustable support jacks, anchored in the concrete.
Here is an illustration of the electrical connection between the two magnets where the resistance developed:
The press release goes on to itemize the damage, which I've arranged into a list:
- "contamination by soot-like dust which propagated over some distance in the beam pipes"
- "damage to the multilayer insulation blankets of the cryostats" (the magnet "sleeves")
- "the cryostats housing [the] quadrupoles broke their anchors in the concrete floor of the tunnel and were moved away from their original positions"
- "the electric and fluid connections pull[ed] the dipole cold masses in the subsector from the cold internal supports inside their undisplaced cryostats"
- "the displacement of the quadrupoles cryostats damaged "jumper" connections to the cryogenic distribution line, but without rupturing its insulation vacuum"
The bottom line is that "at most 5 quadrupoles and 24 dipoles" [magnets] need to be fixed, and they need to be brought out of the tunnel and up to the surface for that to happen. They also say that more magnets might need to be cleaned (from that "soot-like dust") and get new "multilayer insulation" (new "sleeves"); these might need to come up to the surface, or they are considering trying to clean the magnets in place in the tunnel. The good news is that "Spare magnets and spare components appear to be available in adequate types and sufficient quantities to allow replacement of the damaged ones..." So let's be optimistic that everything will be fixed in time to start up again next spring!