The LHC status : commissioning plans and interface with the experiments

Abstract

The status of the ongoing Large Hadron Collider (LHC) installation is described with particular attention to the Long Straight Sections around the experiments. A summary of the present beam commissioning schedule is given with some details on the beam conditions during first collisions. The second part of this paper will address the experiment protection system from beam failures (including interlocks) and the exchange of data and control signals between the accelerator and the experiments. I. STATUS OF THE LHC INSTALLATION (31/8/07) The LHC [1] is a two-ring superconducting proton-proton collider made of eight 3.3 km long arcs separated by 528 m long straight sections (LSS). While the eight arcs are nearly identical, the straight sections are very different. Four straight sections house physics experiments, ATLAS, CMS, ALICE and LHCb; the latter two also include the beam injection systems. Two insertions are for the beam cleaning systems capturing off-momentum and halo particles, one insertion is for the superconducting RF cavities and beam instrumentation and finally, one insertion is for the beam dump systems to deposit the two beams onto external dump blocks. The LHC architecture allows the commissioning of each of the eight sectors independently from the others, before the installation of other sectors is complete. Therefore, both the LHC installation and the LHC commissioning is organized in sectors. The installation and commissioning of the cryogenic distribution line (QRL) is now completed. The installation started in July 2003 and finished in November 2006. It suffered a number of delays and problems and Sector 7-8 needed to be dismantled, repaired and re-installed by CERN. The last magnet was installed in April 2007. In particular, about 1700 cryo-assembles and about 200 warm magnets have been installed. To complete the magnet installation, the transport vehicles had to travel 30000 Km underground at a speed of about 2 Km/h. After installation and alignment, the cryomagnets have to be interconnected [2]. The interconections must ensure the continuity of several functions: vacuum enclosures, beam pipe image current (RF contacts), cryogenic circuits, electrical power supply, and thermal insulation. In the machine, about 1700 interconnections between cryomagnets are necessary. For each interconnection, various operations must be done in the tunnel: TIG welding of cryogenic channels (≈ 50000 welds), induction soldering of main superconducting cables (≈ 10000 joints), ultrasonic welding of auxiliary superconducting cables (≈ 20000 welds), mechanical assembly of various elements, and installation of the multi-layer insulation (≈ 200000 m). The interconnections of 3 Sectors have been completed while the remaing sectors are nearly finished. The LSS consist of a sequence of cold and room temperature (RT) elements. In general, the RT elements are the beam vacuum chambers, collimators, absorbers, experimental devices (like Roman Pots), beam diagnostic devices and RT magnets. Once installed and aligned, the RT elements need to be put under vacuum via permanent and mobile pumping stations. Then all components are baked at a nominal temperature of 250 °C (with a few exceptions at lower temperature). This allows simultaneoulsy the surface degassing and the activation of the Non-Evaporable-Getter (NEG) coatings which provides an additional distributed pumping needed to get the very low pressure required in the LSS. The installation and commissioning of the RT elements in LSS is very well advanced apart from LSS3 (problems in the installation of a few services) and the zones hosting collimators due to the delays in their delivery. Finally, the installation and commissioning of the experimental beam chambers is mainly driven by the installation schedule of the experiments: the installation in ALICE and ATLAS is nearly finished while for LHCb and CMS is still ongoing. II. STATUS OF THE LHC HARDWARE COMMISSIONING The Machine Hardware Commissioning [3] consists in the commissioning of a number of systems: magnets, vacuum, cryogenics, power converters, current leads, quench detection and energy extraction systems as well as the associated utility systems such as AC distribution, water cooling, ventilation, access control and safety systems. In the first phase each system and each utility is tested and qualified independently. This is followed by a second phase where most of the equipment in each sector is globally tested together. After the leak and pressure test, the preparation for the cool-down starts (flushing, filling with He, repairs etc) and the electrical quality assurance tests (ELQA) are performed at warm on all circuits. During cool-down, all the circuits go through different electrical quality assurance tests at several temperature levels. The powering tests of the LHC superconducting circuits start as soon as the cryogenic conditions for powering are met, 1.9 K and 4.5 K for the circuits in the arc and the long straight sections respectively. During the power test all power converters are connected to the magnets for the first time and tested up to nominal current.