LHC Report: superconducting circuit powering tests

After the long maintenance and consolidation campaign carried out during LS1, the machine is getting ready to start operation with beam at 6.5 TeV… the physics community can’t wait! Prior to this, all hardware and software systems have to be tested to assess their correct and safe operation.


Most of the cold circuits (those with high current/stored energy) possess a sophisticated magnet protection system that is crucial to detect a transition of the coil from the superconducting to the normal state (a quench) and safely extract the energy stored in the circuits (about 1 GJ per dipole circuit at nominal current). LHC operation relies on 1232 superconducting dipoles with a field of up to 8.33 T operating in superfluid helium at 1.9 K, along with more than 500 superconducting quadrupoles operating at 4.2 or 1.9 K. Besides, many other superconducting and normal resistive magnets are used to guarantee the possibility of correcting all beam parameters, for a total of more than 10,000 magnets. About 1700 power converters are necessary to feed these superconducting circuits.

The commissioning of the superconducting circuits is a lengthy process.  All interlock and protection systems have to be tested, before and while ramping up the current in steps.  The typical time needed to commission a dipole circuit is in the order of 3 to 5 weeks. A total of more than 10,000 test steps have to be performed on the LHC’s circuits and analysed by the protection experts.

An additional challenge for this year’s commissioning of the superconducting circuits is the increase in energy (and therefore the current feeding each circuit) up to near the design values. In fact, when a superconducting magnet approaches its maximum performance, it experiences repetitive quenches (a phenomenon also known as training) before reaching the target magnetic field. The quenches are caused by the sudden release of the electromechanical stresses and the local increase in temperature above the transition level. The entire coil is then warmed up and needs to be cooled down again - for the LHC dipoles, this might take several hours. The LHC now has two sectors where the dipole magnets have been already trained. 20 and 7 quenches per sector respectively were necessary to reach the equivalent operational energy of 6.5 TeV, with a net time of 10 and 4 days spent in training the magnets.

As concerns the general preparation of the machine, the powering tests have now started in five of the eight LHC sectors: about 30% of the total number of test steps have been executed and the main dipoles and quadrupoles have been prepared for tests in half of the machine; all preparation activities should be completed in about three weeks’ time and the commissioning of all circuits is expected to finish sometime in March.

by Mirko Pojer, Matteo Solfaroli