LHC Report: squeezing, low-energy collisions and an unidentified object

Commissioning of the nominal cycle – beam injection, ramp, squeezing – with low-intensity (probe) beams is progressing well. In parallel, the operators have started commissioning the machine with higher-intensity beams: a nominal bunch in each beam was taken to 6.5 TeV on Saturday, 2 May and, four days later, collisions were delivered to the experiments at the injection energy (450 GeV).

The other main commissioning thread involves preparations for higher beam intensities. To safely handle the higher number of protons per bunch and the higher number of bunches, a number of key systems have to be fully operational and set up with beam. These include the beam dump system, the beam interlock system and the collimation system. The latter involves around 100 individual pairs of jaws, each of which has to be positioned with respect to the beam during all phases of the machine cycle. Confirmation that everything is as it should be is made by deliberately provoking beam losses and checking that the collimators catch the losses as they are supposed to (the so-called loss maps). The set-up so far has allowed a nominal bunch in each beam to be taken to 6.5 TeV and collisions to be delivered to the experiments at the injection energy. One of the key phases of the LHC cycle is the squeeze. During this phase, the strengths of the magnetic fields either side of a given experiment are adjusted to reduce the beam size at the corresponding interaction point. In regular operation mode the whole process takes around 15 minutes. During commissioning we perform careful beam-based measurements to verify the correctness of the changes in magnet focusing. With low-intensity bunches (up to 10¹⁰ protons in each bunch) this has been successfully performed down to the smallest interaction-point beam-sizes planned for 2015.

An unexpected obstacle is sitting at the bottom of the beam 2 beam-pipe in a dipole in sector 8-1. Although it is not causing too many problems for the moment, regular scans are being performed to make sure that the situation remains stable and that it doesn’t result in a more serious aperture restriction.

In recent days operations have been dogged by a series of technical problems ranging from cooling problems to a router issue. Despite the rocky machine availability, about half of the planned beam commissioning has been completed and the LHC remains on track for its first high-energy collisions in a few weeks from now. 

by Mike Lamont for the LHC team