LHC Report: A spring clean for the beam pipe
After a successful calibration run at 1.38TeV, the LHC went into a four-day technical stop on Monday 28 March. Work conducted during the technical stop included wrapping solenoids around vacuum pipes to help counteract electron-cloud effects. X-rays of the cryogenic piping line in Sector 4-5 were also taken, and a cryogeniccompressor was replaced at Point 4.
The recently installed solenoids suppress the electron cloud effect by creating a longitudinal magnetic field that bends back the emitted electrons avoiding their escape from the beam pipe surface and thus preventing their participation in the avalanche process in the beam pipe.
After coming out of the technical stop on 1 April, a series of rigorous tests with low-intensity beams was performed to make sure that everything was working as it should. This is standard procedure, as a number of hardware (and software) changes are made during a technical stop and it is imperative that we make sure that none of these have impacted machine protection. These tests continued into the weekend of 2-3 April and a number of technical issues were resolved.
The LHC then moved into a week-long scrubbing run, beginning on Monday 4 April. Until now, the LHC has been running with a relatively low number of widely spaced bunches (2010 saw 368 bunches with 150 nanosecond spacing; 2011 has seen 200 bunches with 75 nanosecond spacing). Achieving smaller bunch spacing and more bunches is the target for the next two years of LHC operation as it results in higher luminosity.
| What is a scrubbing run? Combining small bunch spacing with a high number of bunches leads to an effect known as "electron cloud". This accumulation of electrons in the beam pipes results from a synchronous effect between the bunches and the multiplication of impinging electrons on the beam pipe inner surface. Indeed, electrons generated inside the beam pipe are accelerated by the bunches and hit the opposite side with enough energy to generate more electrons. During a scrubbing run, a high beam current at low energy is injected into the beam pipe to induce electron clouds under controlled conditions. This is meant to release as many gas molecules trapped inside the metal as possible in order to later pump them out and to decrease the yield of generation of electrons at the surface. This improves the surface characteristics of the beam pipe. |
