Particles are back in the LHC!
Last weekend (23-25 October) particles once again entered the LHC after the one-year interruption following the incident of September 2008. Particles travelled through one sector clockwise and one anticlockwise. ALICE and LHCb, the two experiments sitting along the portion of the beam lines in question, were able to observe the first effects of real beams in the machine. The first ion beam entering point 2 of the LHC, just before the ALICE detector (23 October 2009).
On Friday 23 October, a first beam of ions entered the clockwise beam pipe of the LHC. Previous tests, on 25-26 September, had involved injecting lead ion beams through the whole injection chain right up to the threshold of the LHC. This time, the lead ions entered the LHC just before point 2, where the ALICE experiment is installed, and were dumped before point 3. These tests enabled machine experts to test the operation of the whole injection chain and an entire sector (sector 1-2) of the LHC.
Several sub-detectors of the ALICE experiment were switched on and saw their first beam. This helped them synchronise with the LHC clock, which scans the entrance of the particle bunches into the machine. The capability of the sub-detectors to measure high particle multiplicities – up to 400 000 hits in the Silicon Pixel Detector – as well as multiplicity variations in response to the changing beam steering parameters was also tested.
On Saturday afternoon, the first proton beam also made its way through the TI8 transfer line up to the anticlockwise beam pipe of the LHC. Protons passed through the LHCb experiment and were dumped just before point 7.
Most of the LHCb sub-detectors were switched off to keep the experiment safe during these delicate operations. Only the beam and background monitors remained switched on and operators were able to study the performance of these systems, learn about the types and levels of background to be expected during normal operation and how to correlate them. In particular, this type of exercise provides an opportunity to commission the beam monitoring software (and not just the instrumentation itself), which will be used to monitor the data-taking conditions and allow the operators to take safe and appropriate action. A highlight of the weekend was the switching-on of the LHCb magnet, with operators able to measure its effect on the LHC beam and adjust the magnetic compensators around LHCb to bring the beam back into orbit. This operation was also very successful.
At the same time as these injection tests, magnet powering tests are being performed in seven of the eight sectors of the LHC. Five sectors are already in phase 2, where operators progressively increase the current up to 2 kAmps, allowing the passage and guidance of beams at about 1.2 TeV. Magnet powering tests in the last sector, 3-4, will begin in the first week of November. Furthermore, the qualification of the new quench protection system is progressing well and all splices in three sectors have been checked. All the measured values comply with the stringent standards.
After a very busy weekend, the machine operators and the ALICE and LHCb teams were very happy with the excellent way their equipment operated.
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