The final magnet – a quadrupole short straight section – to refit sector 3-4 of the LHC was lowered into the tunnel and transported to its location on 30 April, two weeks after the 39th and final, repaired dipole magnet was lowered and installed. This magnet system was the last of the spares to be prepared for use in the refurbished sector .
With all of the necessary magnets now underground, work in the tunnel will continue to connect them together. In total 53 magnets were removed from sector 3-4 following the incident on 19 September 2008. Of these, 16 magnets had sustained minimal damage and so were refurbished and put back into the tunnel; the remaining 37 were replaced by spares, depleting the number of reserve magnets to nearly zero. Work will continue on the surface to repair the remaining damaged magnets to replenish the pool of spares.
Since the start of the repair work in sector 3-4, the Vacuum Group has been cleaning the beam pipes to remove metallic debris and soot created by the electrical arc at the root of the incident. All 4800 m of the beam pipes in sector 3-4 were first surveyed centimetre by centimetre to document the damage before the cleaning work began. The cleaning process itself involves passing a brush through the pipe to clean the surface mechanically, followed by vacuuming to remove any debris both inside and outside the beam pipe. This procedure is repeated ten times, followed by a final check with an endoscopic camera. By the end of April some 70% of the affected zone had been cleaned.
Work meanwhile continues on the installation of new pressure release ports to allow a greater rate of helium escape in the event of an incident similar to that of 19 September. This is now proceeding in the areas outside the arc sections – in particular on the inner triplets (the focusing magnets either side of the collision point). The ports have been slightly modified to fit the geometry of these magnets.
The root of the incident on 19 September was a splice failure interconnection between two magnets and since then CERN has developed highly sensitive methods to detect resistances of splices at the nano-ohm level. These have revealed a small number of splices with abnormally high resistance, which are being investigated, understood and dealt with. Now a new test has been developed to measure the electrical resistance of the connection joining the busbars of the superconducting magnets together. Each busbar consists of a superconducting cable surrounded by a larger copper block. Although the copper cannot carry the same level of current as the superconducting cable for sustained periods, it plays the essential role of providing a low resistance path to the current when a magnet or a busbar quenches: the copper gives time to the protection system to discharge the stored energy. The new test allows the electrical continuity of the copper part to be checked and so provides another important quality control safety check for the electrical connections.
Careful tests have revealed that in some cases, the process of soldering the superconductor in the interconnecting high-current splice can melt the solder joining the superconducting cable to the copper of the busbar, and thereby impede its ability to do its job if a quench occurs. As a result, the teams at work on the consolidation process are improving the soldering process, and checking the whole of the LHC for similar faults. A test has been done for sectors at room temperature and studies are now going on to allow the same procedure at cryogenic, but non-superconducting temperatures. By mid-May, three sectors had been tested at room temperature, and five potentially faulty interconnections found. These are being repaired accordingly.
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