LS1 Report: testing Plan B

A team from the TE Department is currently testing the secondary electrical network for the LHC’s main dipoles – that is, the power circuit used in the event of a quench (loss of superconductivity). This secondary network is essential for the safety of the machine and has been strengthened as part of the SMACC project (see here).


In event of a quench, the current travels via a secondary circuit (in yellow).

In order to reach an energy of 6.5 TeV per beam, the LHC will need to be supplied with an electrical current of 11 kA. While the machine’s dozens of kilometres of superconducting cables usually transport the current without any problems (i.e. with no electrical resistance), quenches can sometimes occur as a result of instabilities that cause a loss of superconductivity. In this case, the current travels via a secondary circuit, a short back-up network:  diodes that divert the current if the quench occurs in a magnet and copper bars if it occurs in an interconnection (see picture). Of course, it’s important to make sure that this plan B is foolproof.

It’s for this reason that CSCM (copper stabiliser continuity measurement) tests are being carried out. “The principle of these tests is to inject a high current into the accelerator and ‘force’ it to use that secondary network," explains Hugues Thiesen, the CSCM tests coordinator. “We run the tests at a temperature of 20 K - a temperature at which the LHC cables can’t act as superconductors. They therefore impede the current, forcing it to travel via the secondary network." 

Sensors already installed in the LHC’s electrical circuit allow the team running the CSCM tests to check that everything is going as planned. “If we detect a problem of too much resistance, we immediately cut the power, which dissipates straight away since almost no energy is accumulated in the machine during the CSCM tests," Thiesen stresses. During the LHC’s normal operation, on the other hand, the amount of energy that accumulates is enormous and it takes no less than 300 seconds for it to be evacuated via the secondary circuit, risking damage to the equipment. During the CSCM tests, this time is reduced to a few hundred microseconds, which in turn reduces the risk to the accelerator. 

The tests are currently being carried out in Sector 6-7, where a current of 7 kA has just been injected – it is being progressively increased in seven stages, from 400 A to 11 kA. The CSCM tests in this sector should be completed in the next few days and, depending on the results, a decision will then be made as to whether to run the tests in the LHC's other seven sectors too.

Meanwhile, elsewhere...

At the LHC, cool-down of Sector 8-1 has begun. Sector 1-2 is next on the list, where cool-down should begin at the end of next week. Vacuum teams are preparing for beam, conducting leak tests and bake-outs of the beam pipe in different sectors.

Meanwhile, final checks - including warm magnet tests, warm busbar measurements and ELQA tests - continue to take place throughout the accelerator. Preparations for one of these tests revealed damaged surface cables; during the shutdown, these cables had become tasty treats for martens! The damaged lines have been replaced and no further issues were found.

Over at the SPS, hardware tests have revealed a damaged TIDVG. Its replacement is scheduled for August and will not affect the works currently on-going.


by Anaïs Schaeffer & Katarina Anthony