LHC report: Intensity ramp-up - Familiar demons

The increase in total beam currents has flushed out two major but familiar issues


The first 2015 scrubbing run ended on Friday, 3 July and successfully delivered a well-scrubbed machine ready for operation with a 50 ns beam. This opened the way for the first phase of the so-called beam intensity ramp-up. The last couple of weeks have seen the number of bunches increase from 3 to 476 per beam via periods of 50, 144 and 300 bunches per beam.

The graph plots the rate of LHC beam dumps due to single-event effects (SEE) versus beam luminosity. It is an indication of the importance of tackling this issue (Image: LHC Operations team/CERN)

To verify the full and proper functioning of all systems, operators need at least 3 fills and 20 hours of stable beams without significant problems. After 20 hours, an extensive checklist is signed off by the system experts before the next step up in the number of bunches. The systems involved include magnet protection, radio-frequency, beam instrumentation, collimation, operations, feedback, beam dump and injection.

Increasing the total beam intensity poses a number of operational challenges, and the higher beam currents demand attention to a number of important details, ranging from well-optimised injection to sufficiently good control of the key beam parameters such as tuning, chromaticity and the closed orbit.

The increase in total beam currents has flushed out two major but familiar issues. The first are the unidentified falling objects (UFOs) – micrometre-sized dust particles falling through the beam and generating localised beam loss. These have been observed as expected and, with the increase in intensity, have caused beam dumps and even magnet quenches at high energy. A particular worry was the unidentified lying object (ULO) near Point 8 which has been associated with particularly big UFOs. Things have been quiet in this region recently but earlier this week two ramps were lost due to UFOs in the vicinity of the ULO. In Run 1, the number of UFOs observed decreased with time. It is hoped that we will experience the same sort of conditioning in Run 2.

A lot of experience was gained in Run 1 on the effect of radiation on tunnel electronics. This issue was addressed through a well-coordinated campaign during LS1, which deployed a number of wide-ranging measures to alleviate the effects, including the relocation and improved shielding of electronics. However, a specific weakness in components of the quench protection system has emerged following the increase in beam intensity. Although it does not compromise machine safety, the weakness has led to some premature beam dumps. Test mitigation measures have been deployed in the tunnel and hopefully the problem will not compromise progress too much.

Next week the LHC moves into a five-day machine development period to be followed by a two-week scrubbing period aimed at preparing the machine for 25 ns operation.