The LHC is back in action again after the technical stop that began on 6 December, with initial preparations for the 2011 run in full swing. On 19 February the previous few weeks of careful preparation paid off, with circulating beams being rapidly re-established. There then followed a programme of beam measurements and re-commissioning of the essential subsystems. The initial measurements show that the LHC is in good shape and magnetically little-changed from last year. The first collisions of 2011 were produced on 2 March, with stable beams and collisions for physics planned for later in the month.
In addition to the maintenance work, a number of modifications were made to the LHC during the technical stop. These included the installation of small solenoids to combat the build-up of electrons inside the vacuum chamber with increasing proton beam intensity; the replacement of a number of uninterruptible power-supply installations for essential systems such as the cryogenics; the installation of additional capacitors on the quench-protection system to prepare for a possible increase in beam energy in 2011; plus a host of other improvements to RF, beam instrumentation, power convertors and kickers, etc.
During the same period similar maintenance took place on the injector chain, namely LINAC2, the Booster, the Proton Synchrotron (PS) and the Super Proton Synchrotron (SPS). An example of this work is the programme to exchange eight magnets in the SPS machine. This is part of regular preventive maintenance in which the SPS magnets are exhaustively tested at the end of each year and those presenting any initial signs of weakness are changed during the accelerator stop.
At the PS, the technical stop was used to begin the commissioning of the new PS main power supply (POPS), which will replace the old rotating machine that has powered the PS magnets since 1968. The PS power supply must be capable of delivering extremely high-power (60 MW) electrical pulses to the magnets and then reabsorbing the energy at each accelerator cycle, less than 2s later. The rotating machine has been replaced by an enormous system of power converters and capacitors. The system is crucial because the PS is one of the lynchpins of CERN’s accelerator complex and any failure in the electrical system would practically paralyse all of the experiments.
POPS was inaugurated and tested on 10 SPS test magnets in 2010 and then hooked up to the 101 PS main magnets for testing on 31 January 2011. This system was tested with gradually increasing intensities, right up to 6000 A. It then took a few days to pass the operation of POPS from the specialists controlling it locally to the CERN Control Centre prior to the crucial beam test on 11 February.