As the cool-down phase of commissioning the LHC came towards a successful conclusion at the beginning of August, CERN announced that the first attempt to circulate a beam in the LHC will be made on 10 September. The announcement was soon followed by the first sight of protons – albeit a small number – in the LHC during tests to synchronize the LHC’s clockwise beam transfer system.
The LHC is unlike any other particle collider, being the first to have two beams of particles travelling in opposite directions in separate channels within the same magnetic structure, and it is the first to operate with superfluid helium at 1.9 K. It truly is its own prototype.
Starting up a machine like this is not as simple as flipping a switch. Commissioning is a long process that starts with the cooling down of each of the machine’s eight sectors. This is followed by electrical testing of the 1600 superconducting magnet systems, and their individual powering to nominal operating current. Once these steps are completed, the powering together of all the circuits of each sector can begin. Only then can the eight independent sectors be powered up in unison to operate as a single machine.
There are around 1400 tests of varying complexity to be performed on each sector after it reaches 1.9 K. These include: electrical quality assurance to check that all the wiring is in place after the magnets have contracted during cool down; individual testing of protection systems; and power testing. These tests are done by the Operations Group together with teams of equipment experts from the Accelerators and Beams, Accelerator Technology and Technical Support Departments. A dedicated hardware commissioning team coordinates this effort.
After all these tests have been completed, the sectors are then handed over to the Operations Group to commence “dry runs”, where the machine is run as it would be with the beam. There are also safety tests that must be done before the beam can circulate, to prevent people from being in the tunnel at the same time as the beam.
By the end of July, this work was approaching completion, with the whole machine fully loaded with 130 tonnes of liquid helium for the first time, and the final commissioning of the hardware progressing apace. All eight sectors were at or close to the operating temperature of 1.9 K required to reach the high magnetic-field strengths necessary to bend the beams at 7 TeV.
The next phase in the process is the synchronization of the LHC with the SPS accelerator, which forms the last link in the LHC’s injector chain. Timing between the two machines has to be accurate to within a fraction of a nanosecond. The synchronization of the LHC’s clockwise beam-transfer system was successfully achieved on the weekend beginning 8 August, when a single bunch of protons was taken down the transfer line from the SPS accelerator to the LHC. After a period of optimization, one bunch was kicked up from the transfer line into the LHC beam pipe and steered about 3 km around the LHC itself on the first attempt. The following day, the test was repeated several times to optimize the transfer before the Operations Group handed the machine back for hardware commissioning to resume. The anti-clockwise synchronization systems will be tested over the weekend of 22 August.
These tests will prepare the LHC for the first circulating beam on 10 September at the injection energy of 450 GeV. Once stable circulating beams have been established they will be brought into collision, and the final step will be to commission the LHC’s acceleration system to boost the energy to 5 TeV per beam – the target energy for 2008. The decision to run the LHC at 5 TeV rather than 7 TeV this year is related to the need to re-train the superconducting magnets in the tunnel to reach the nominal field after some “de-training” occurred during transport and installation.
