On 2 September the cool down of LHC sector 6-7 got underway, following some minor repair work. As the last sector to be cooled down, this marked a major milestone towards the restart of the collider later this year.
The cool down of sector 6-7 began two weeks earlier but it was interrupted by the detection of a short-circuit on the main dipole circuit. The cause of the short-circuit was later tracked down to poor insulation in a magnet busbar, which had been degraded by friction against a screw as the structure contracted during the cool down. After repair work followed by electrical and vacuum validation, the sector was once again ready to cool down.
In sector 8-1, the flexible hose that caused the helium leak into the insulation vacuum has been replaced and the sector is now being cooled down again. The cool down of sector 3-4, the one affected by the incident on 19 September 2008, also began at the end of August, thus marking the end of a complex phase of repair work.
Meanwhile consolidation work on the LHC has continued. On 26 August, the first two fully tested crates for the new quench-protection system (QPS) were installed in sector 1-2. These are the first of a total of 436 crates to be installed around the ring. The two crates include detectors for both the enhanced busbar protection and the symmetric quench protection.
Training quenches on magnets in sector 5-6 in June 2008 revealed that heat transfer to a neighbouring magnet can cause a quench that develops identically in two magnet coils. The original detection system compared voltage signals from two coils to detect a resistive build-up in either one, but if the signals develop in the same way, the quench would go unnoticed. The new protection system monitors the voltage across four adjacent dipoles (or two adjacent quadrupoles), allowing a symmetric quench to be detected, as well as providing a back-up detection method for normal, asymmetric quenches.
To test the crates before installation, a dedicated test bed has been created, capable of simulating all of the conditions in the LHC, from a symmetric quench to an increase in busbar resistance. The teams are working two shifts a day, including weekends, to test the new crates. Two more test benches are also being built to increase the production rate. The whole task is on target for completion in mid-October.
Another important new task for the QPS team is to speed up the energy extraction from the magnets. The quicker the energy can be extracted, the lower the risk of dangerously high temperatures in the event of a quench. The time constant for the dipoles will be halved to about 50 s. The decision to run at 3.5 TeV, and therefore with lower current in the magnets, has made this task relatively straightforward. Switching two of the three “dump” resistors into a series circuit, instead of having all three resistors in parallel, allows the energy to be converted into heat much faster. In the quadrupole circuits, the task is more complex. Reducing the time constant to the desired 10 s, from a previous 35 s, requires adding extra, newly designed resistors.
The new QPS system will also allow accurate resistance measurements to be taken remotely. This will save a huge amount of time and effort for the next rounds of interventions – for example when the energy of the LHC is increased.
• CERN is publishing regular updates on the LHC in its internal Bulletin, available at www.cern.ch/bulletin, as well as via twitter and YouTube at www.twitter.com/cern and www.youtube.com/cern.
