LHC Report: Timeout is over!

Over the last two weeks the LHC has been collecting luminosity at a steady pace, but not delivering the canonical 1 fb-1 per week. This is because machine timeouts were necessary to solve some beam-stability problems. Also, the beam development programme was moved forward, taking advantage of a timeout caused by an emergency ramp-down of the CMS solenoid magnet. With all these problems solved and with good injector performance, the past week has seen the LHC back to new record luminosities.

Previous LHC reports have mentioned that the peak luminosity at the beginning of the “stable beams” period had gone down by about 10% with respect to previous records. This is explained by the reduction of bunch intensities, as higher bunch intensities were leading to beam instabilities and important beam losses. When beams become unstable, octupole magnets can be used to correct them. These magnets can be powered at two different polarities and several days were needed to find new optimum machine settings with the octupole polarity reversed relative to the polarity used over recent years. This paid off, as with the reversed polarity the beams became more stable when brought into collision and the bunch currents could be increased from 1.5 x 1011 to 1.6 x 1011 protons per bunch. Together with an important effort from the injector team to deliver beams with a smaller cross-section, a peak luminosity of 7.4 x 1033 cm-2s-1 in ATLAS and CMS was reached, which is about 10% above previous records.

While the machine was ready for weeks of stable operation with the new optimised settings, the CMS solenoid magnet had an emergency ramp-down, leading to the magnet being warmed up to 70 K and four days required for recovery. To minimize the time subtracted from data taking for CMS, the LHC teams quickly put together a machine development programme. The programme included a check of the octupole polarities; LHCb polarity switching; successful commissioning of injection and RF-capture using new SPS optics, called the Q20 optics, which allows for even higher bunch intensities; development of tune measurement using the transverse damper and the polarity switch of the ALICE experimental magnets, which was followed by a 48-hour commissioning programme with the new ALICE magnet polarity.

With the beams more stable than before and no special gymnastics announced for the coming weeks, the stage is set for a new period of record luminosity production.