A year of challenges and successes

15 January 2016

2015 was a tough year for CERN’s accelerator sector. Besides assuring delivery of beam to the extensive non-LHC facilities such as the AD, ISOLDE, nTOF and the North Area, many teams also had to work hard to bring the LHC back into business after the far-reaching efforts of the long shutdown.

At the end of 2014 and start of 2015, the LHC was cooled down sector by sector and all magnet circuits were put through a campaign of powering tests to fully re-qualify everything. The six-month-long programme of rigorous tests involved the quench-protection system, power converters, energy extraction, UPS, interlocks, electrical quality assurance and magnet-quench behaviour. The powering-test phase eventually left all magnetic circuits fully qualified for 6.5 TeV.

Some understandable delay was incurred during this period and three things can be highlighted. First was the decision to perform in situ tests of the consolidated splices – the so called Copper Stabilizer Continuity Measurement (CSCM) campaign. These were a success and provided confirmation of the quality work done during the shutdown.

Second, dipole-quench re-training took some time – in particular, the dipoles of sector 45 proved a little recalcitrant and reached the target 11,080 A after some 51 training quenches.

Third, after an impressive team effort co-ordinated by the machine-protection team to conceive, prototype, test and deploy the system, a small piece of metallic debris that was causing an earth fault in a dipole in sector 34 was successfully burnt away on the afternoon of Tuesday 31 March.

First beam 2015 went around the LHC on Easter Sunday, 5 April. Initial commissioning delivered first beam at 6.5 TeV after five days and first “stable beams” after two months of careful set up and validation.

Ramp up

Two scrubbing runs delivered good beam conditions for around 1500 bunches per beam, after a concerted campaign to re-condition the beam vacuum. However, the electron cloud, anticipated to be more of a problem with the nominal 25 ns bunch-spacing beam, was still significant at the end of the scrubbing campaign.

The initial 50 ns and 25 ns intensity ramp-up phase was tough going and had to contend with a number of issues, including earth faults, unidentified falling objects (UFOs), an unidentified aperture restriction in a main dipole, and radiation affecting specific electronic components in the tunnel. Although operating the machine in these conditions was challenging, the teams succeeded in colliding beams with 460 bunches and delivered some luminosity to the experiments, albeit with poor efficiency.

The second phase of the ramp-up following the technical stop at the start of September was dominated by the electron cloud and the heat load that it generates in the beam screens of the magnets in the cold sectors. The challenge was then for cryogenics, which had to wrestle with transients and operation close to the cooling-power limits. The ramp-up in number of bunches was consequently slow but steady, culminating in a final figure for the year of 2244 bunches per beam.

Importantly, the electron cloud generated during physics runs at 6.5 TeV serves to slowly condition the surface of the beam screen and so reduce the heat load at a given intensity. As time passed, this effect opened up a margin for the use of more bunches. Cryogenics operations were therefore kept close to the acceptable maximum heat load, and at the same time in the most effective scrubbing regime.

The overall machine availability is a critical factor in integrated-luminosity delivery, and remained respectable with around 32% of the scheduled time spent in stable beams during the final period of proton–proton physics from September to November. By the end of the 2015 proton run, 2244 bunches per beam were giving peak luminosities of 5.2 × 1033 cm–2s–1 in ATLAS and CMS, with both being delivered an integrated luminosity of around 4 fb–1 for the year. Levelled luminosity of 3 × 1032 cm–2s–1 in LHCb and 5 × 1030 cm–2s–1 in ALICE was provided throughout the run.

Also of note were dedicated runs at high β* for TOTEM and ALFA. These provided important data on elastic and diffractive scattering at 6.5 TeV, and interestingly a first test of the CMS-TOTEM Precision Proton Spectrometer (CT-PPS), which aims to probe double-pomeron exchange.

As is now traditional, the final four weeks of operations in 2015 were devoted to the heavy-ion programme. To make things more challenging, it was decided to include a five-day proton–proton reference run in this period. The proton–proton run was performed at a centre-of-mass energy of 5.02 TeV, giving the same nucleon–nucleon collision energy as that of both the following lead–lead run and the proton–lead run that took place at the start of 2013.

Good intensities

Both the proton reference run and ion run demanded re-set-up and validation of the machine at new energies. Despite the time pressure, both runs went well and were counted a success. Performance with ions is strongly dependent on the beam from the injectors (source, Linac3, LEIR, PS and SPS), and extensive preparation allowed the delivery of good intensities, which open the way for delivery of a levelled design luminosity of 1 × 1027 cm–2s–1 to ALICE and more than 3 × 1027 cm–2s–1 to ATLAS and CMS. For the first time in an ion–ion run, LHCb also took data following participation in the proton–lead run. Dedicated ion machine development included crystal collimation and quench-level tests, the latter providing important input to future ion operation in the HL-LHC era.

The travails of 2015 have opened the way for a full production run in 2016. Following initial commissioning, a short scrubbing run should re-establish the electron cloud conditions of 2015, allowing operation with 2000 bunches and more. This figure can then be incrementally increased to the nominal 2700 as conditioning progresses. Following extensive machine development campaigns in 2015, the β* will be reduced to 50 cm for the 2016 run. Nominal bunch intensity and emittance will bring the design peak luminosity of 1 × 1034 cm–2s–1 within reach. Reasonable machine availability and around 150 days of 13 TeV proton–proton physics should allow the 23 fb–1 total delivered to ATLAS and CMS in 2012 to be exceeded.

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