LHC Report: machine development

Machine development weeks are carefully planned in the LHC operation schedule to optimise and further study the performance of the machine. The first machine development session of Run 2 ended on Saturday, 25 July. Despite various hiccoughs, it allowed the operators to make great strides towards improving the long-term performance of the LHC.

The main goals of this first machine development (MD) week were to determine the minimum beam-spot size at the interaction points given existing optics and collimation constraints; to test new beam instrumentation; to evaluate the effectiveness of performing part of the beam-squeezing process during the energy ramp; and to explore the limits on the number of protons per bunch arising from the electromagnetic interactions with the accelerator environment and the other beam.

Unfortunately, a series of events reduced the machine availability for studies to about 50%. The most critical issue was the recurrent trip of a sextupolar corrector circuit – a circuit with 154 small sextupole magnets used to correct errors in the main dipoles – in arc 7-8 at high energy. This problem resulted in the cancellation of the last test runs at high energy and the MD session stopping some 8 hours earlier than planned. However, the time with beam was effective in terms of the results achieved. A large set of instruments were developed or tested, including high-resolution beam position monitors (DOROS), robust beam current monitors and two systems to examine the frequency content of the beam.

Thanks to the MD studies, the beam sizes at the two high-luminosity interaction points (where the ATLAS and CMS detectors are installed) were reduced by a factor of 1.4. The corresponding machine optics were finely tuned to be ready for high-intensity beams. However, before these optics can be used in operation, further studies are mandatory to understand and validate other important parameters, including the machine aperture, new collimator settings, a reduced crossing angle and, possibly, non-linear corrections in the quadrupole triplets next to the interaction points. These topics will be addressed in future MD weeks to pave the way towards higher luminosities in Run 2.

For the first time, operators were able to perform the beam-size squeeze during the energy ramp. This opens up the possibility of saving up to 10 minutes per fill in a slightly more ambitious configuration than that tested last week. Results on higher bunch populations require careful analysis of the collected beam data. These will soon be available in detailed reports to be published as LHC MD notes.

At the end of the MD period, the LHC went into its second scrubbing run, a two-week period that aims to prepare the machine fully for operation with 25-nanosecond bunch spacing, planned for the first weeks of August.

We would like to take this opportunity to thank all the MD teams, system experts, management, operators and physics experiments involved during the MDs for their high flexibility, dedication and endurance.

by Rogelio Tomás García for the LHC team