LHC Report: Ion Age
The LHC starts the New Year facing a new challenge: proton-lead collisions in the last month before the shutdown in mid-February.
Commissioning this new and almost unprecedented mode of collider operation is a major challenge both for the LHC and its injector chain. Moreover, it has to be done very quickly to achieve a whole series of physics goals, requiring modifications of the LHC configuration, in a very short time. These include a switch of the beam directions halfway through the run, polarity reversals of the ALICE spectrometer magnet and Van der Meer scans.
The Linac3 team kept the lead source running throughout the end-of-year technical stop, and recovery of the accelerator complex was very quick. New proton and lead beams were soon ready, with a bunch filling pattern that ensures they will eventually match up in the LHC. The LEIR machine has even attained a new ion beam intensity record.
On Friday 11 January the first single bunches of protons and lead nuclei were injected into the LHC and successfully ramped to full energy, a very encouraging sign. Over the following night the LHC operations and beam physics teams sprang into action to commission and measure the optics through the completely new squeeze sequence which now includes lower β* values in the ALICE and LHCb experiments. The data were later used successfully to correct the squeeze optics to the quality required for physics operation. A new feature of proton-lead operation, arising from the slightly different speeds of the particles, is the need to run the two beams “off-momentum” with opposite orbit offsets. A new correction scheme to correct the resulting orbit perturbations has worked well.
Unfortunately, starting at the weekend, substantial amounts of beam time have been lost due to various power and cryogenics failures, slowing down what would otherwise have been rapid progress through the commissioning plan. The frequency of re-schedulings and unforeseen interventions, driven by the urgency of the programme, has made heavy demands on many CERN teams.
The first collisions of low intensity proton and lead are nevertheless expected imminently and the LHC will then push ahead into new beam physics territory, exploring higher intensity with asymmetric beams.
by John Jowett for the LHC team