On November 14, 2008, the Compact Muon Solenoid (CMS) successfully generated a nominal magnetic field of 4 tesla. This success rewards IRFU efforts for the design and construction of what constitutes the largest superconducting solenoid magnet in the world. Over a period of approximately one month, CMS teams conducted a continuous data acquisition campaign with the detector operating under nominal conditions. Approximately 300 million cosmic events were recorded. This also provided an excellent opportunity to showcase the specific expertise of IRFU teams, particularly in areas such as detection systems, electronics, trace data reconstruction techniques and laser control systems.
After an initial series of ground-level tests completed during the second semester of 2006, the magnet was disassembled into 15 subassemblies that were lowered separately into an underground chamber and reassembled. After installation of the detector assembly, in October 2008 the magnet produced an operational magnetic field of 3.8 tesla without any problems. The magnet continued to operate at this value for approximately one month for detector tests with cosmic rays. On November 14th, the magnetic field was increased to the nominal value of 4 tesla. The magnet and the detector assembly are now shut down and will undergo maintenance and optimization prior to the restart of the Large Hadron Collider (LHC), scheduled for the third quarter of 2009. The magnet will be used again for cosmic event data acquisition as of early next year.
Further to the incident concerning the LHC on September 19, the test program has been revised. CMS teams have prepared a new data acquisition plan so as to build on the significant efforts to date and establish the best possible conditions for the future restart of the LHC. This plan consists of conducting data acquisition experiments using cosmic rays under operating conditions as close as possible to LHC operating conditions.
With this objective in mind, the data acquisition system has been tested under conditions similar to those expected during collisions, including the calibration of all electromagnetic calorimeter (ECAL) crystals during physical data acquisition, and the use of the selective readout processor (SRP) to reduce the data flow received from the detector.
The high-level data selection system has also been tested in a configuration close to that considered for the restart of the LHC, including, for the first time, the use of a processor farm of 4500 processors for online event processing.
The ECAL selective readout processor (SRP) was tested in June 2008 and has been used continuously during cosmic data acquisition campaigns.
The SRP is an innovative system that reduces the size of acquisition data by a factor of 20, i.e. to an acceptable level for the data acquisition system, while preserving full accuracy of the ECAL. The method is based on a real-time definition of the detector regions necessary for the reconstruction of high-energy electromagnetic objects (electron or gamma-ray showers in the calorimeter). All the channels in these regions are then read, and the rest of the detector is read after application of a threshold value. SPP and SEDI teams have designed and developed the system for real-time selection of these regions. This selection is performed in less than 5 µs, 100 000 times per second. Using the latest technological breakthroughs in optical communication and programmable logic chips (FGPAs with high integrability), SEDI researchers have developed a compact solution that meets time constraint requirements. This project has allowed SEDI researchers to acquire a genuine expertise in parallel optical multi-gigabyte data links and on-chip systems (microprocessors integrated into FGPAs).
The selective readout processor has demonstrated extremely high reliability during the various data acquisition campaigns completed since its implementation.
The laser-based control system designed and developed by IRFU researchers can measure and correct the transparency of CMS electromagnetic calorimeter crystals in nearly real-time by injecting a calibrated light beam into each crystal via a complex fiber optic distribution system. In order to operate in real time, the laser-based control system must take measurements during normal detector operation without interfering with physical data acquisition processes. Associated systems such as the trip signal synchronization system, the physical data flow calibration event extraction system and the online event processing system have also been successfully tested.
This culmination of 10 years of work allows the restart of the LHC to be planned with full confidence.