LS1: reawakening the accelerators

CERN’s accelerator complex is gradually restarting, more than a year after the start of the first long shutdown (LS1). On 4 April, a team switched on the proton source and a week later re-commissioning began on Linac2. Beams are scheduled to be sent to the Proton Synchrotron Booster (PSB) at the end of May and then on to the Proton Synchrotron (PS) by mid-June, reaching the Super Proton Synchrotron (SPS) later this year, with the LHC restart on course for early 2015. One LHC sector has already begun the long cool down towards its operating temperature.

Since LS1 began, the Superconducting Magnets And Circuits Consolidation (SMACC) project has formed a “train” of workers in the LHC tunnel, with “wagons” of teams distributed across the eight sectors. The end of April marked the installation of the last of the 27,000 shunts – low-resistance connections to consolidate the 10,000 ”splices” that interconnect the superconducting magnet bus bars and can carry currents up to 13,000 A. The shunts provide an alternative path for a portion of this current in the event that a splice loses its superconducting state, to avoid repeating the incident in September 2008 that delayed the start-up of the LHC (CERN Courier September 2010 p27).

To install a shunt, the SMACC team had to first open the area around the interconnection, slide custom-built metallic bellows out of the way and remove the thermal shielding within. This revealed a series of metallic pipes linking the magnets to each other. One set of these pipes – the “M lines” – then had to be cut open to access the splices between the superconducting cables. The team opened up the last of the M lines in February this year (CERN Courier April 2014 p5), and since then has been continuing to install the shunts, finishing on 30 April.

Now, emphasis is shifting from installation to testing, including short-circuit tests to verify all of the LHC’s hardware components – cables, interlocks, energy extraction systems, power converters that provide current to the superconducting magnets, and the cooling system. The most complicated components are the superconducting circuits, which have a myriad of different failure modes with interlock and control systems. Testing these circuits now dramatically reduces the time spent in the tunnel during the powering tests at low temperatures, which are planned to start in August. Although the magnet circuits themselves cannot be tested at warm temperatures, the teams can verify the power converter and the circuits up to where the cables enter the magnets. These circuits are tested at the highest current the power converters can achieve – higher than the nominal current in the machine – for a few hours to a day. Teams then verify that the warming effect of the current stabilizes at a temperature that does not affect the behaviour of the cables. To do this, they use infrared cameras to check the temperature rise of the different parts of the circuit.

As well as short-circuit tests, electrical quality-assurance testing (ELQA) is ongoing. Sector 6-7 showed no non-conformities and, on 7 May, was the first of the LHC sectors to begin the cool down to 1.8 K. The other seven sectors will follow as the long journey of the SMACC train approaches its end and the teams complete their final tasks.