On 12 January, after 23 months of hard work involving around 1000 people each day, the key to the LHC was symbolically handed back to the operations team. The team will now perform tests on the machine in preparation for the restart this spring.
Tests include training the LHC’s superconducting dipole magnets to the current level needed for 6.5 TeV beam energy. The main dipole circuit of a given sector is ramped up until a quench of a single dipole occurs. The quench-protection system then swings into action, energy is extracted from the circuit, and the current is ramped down. After careful analysis, the exercise is repeated. On the next ramp, the magnet that quenched should hold the current (i.e. is trained), while at a higher current another of the 154 dipoles in the circuit quenches. For 2015, the target current is 11,080 A for operation at 6.5 TeV (with some margin). Sector 6-7 was brought to this level successfully at the end of 2014, having taken 20 training quenches to get there. Getting all eight sectors to this level will be an important milestone.
The next big step is the first sector test, in which beam would enter the LHC for the first time since February 2013.
The next big step is the first sector test, in which beam would enter the LHC for the first time since February 2013. The aim is to send single bunches from the Super Proton Synchrotron into the LHC through the injection regions at points 2 and 8 for a single pass through the available downstream sectors. This will allow testing of synchronization, the injection system, beam instrumentation, magnet settings, machine aperture and the beam dump.
A full circuit of the machine with beam and the start of beam commissioning are foreseen for March. It should then take about two months to re-commission the operational cycle, commission the beam-based systems (transverse feedback, RF, injection, beam dump system, beam instrumentation, power converters, orbit and tune feedbacks, etc) and commission and test the machine-protection system to re-establish the high level of protection required. This will open the way for the first collisions of stable beams at 6.5 TeV – foreseen currently for May – initially with a low number of bunches.
On 26 January, the CMS collaboration installed their new Pixel Luminosity Telescope (PLT). Designed with LHC Run 2 in mind, the PLT uses radiation-hard CMS pixel sensors to provide near-instantaneous readings of the per-bunch luminosity – thereby helping LHC operators to provide the maximum useful luminosity to CMS. The PLT is comprised of two arrays of eight small-angle telescopes situated on either side of the CMS interaction point. Each telescope hovers only 1 cm away from the CMS beam pipe, where it uses three planes of pixel sensors to take separate, unique measurements of luminosity.