The circulating beams will store an unprecedented amount of energy when the LHC is in operation. If even a small fraction of this beam deviates from the correct orbit, it may induce a quench in the superconducting magnets or even cause physical damage to system components. The LHC beam-loss monitoring (BLM) system is the key to protecting the machine against dangerous beam “losses” of this kind.
The BLM system generates a beam abort trigger when the measured rate of lost beam exceeds pre-determined safety thresholds. The lost beam particles initiate hadronic showers through the magnets, which are measured by monitors installed outside of the cryostat around each quadrupole magnet. About 4000 BLMs – mainly ionization chambers – will be installed around the LHC ring. They are the result of a successful collaboration between CERN and the Institute for High Energy Physics (IHEP) in Protvino, Russia. CERN developed the monitors and IHEP manufactured them during the past year, using industry-produced components.
Signal speed and robustness against aging were the main design criteria. The monitors are about 60 cm long with a diameter of 9 cm and a sensitive volume of 1.5 l. Each one contains 61 parallel aluminium electrode plates separated by 0.5 cm and is filled with nitrogen at 100 mbar overpressure and permanently sealed inside a stainless-steel cylinder. They operate at 1.5 kV and are equipped with a low-pass filter at the high-voltage input. The collection time of the electrons and ions is 300 ns and 80 μs, respectively.
The radiation dose on the detectors over 20 years of LHC operation is estimated at 2 × 108 Gy in the collimation sections and 2 × 104 Gy at the other locations. To avoid radiation aging, production of the chamber components included a strict ultra-high vacuum (UHV) cleaning procedure. As a result, impurity levels from thermal and radiation-induced desorption should remain in the range of parts per million. Standardized test samples analysed at CERN periodically helped to check the cleaning performance.
The team at IHEP designed and built a special UHV stand to ensure suitable conditions for building the monitors. They performed checks throughout the production phase and documented the results. The quality of the welding is a critical aspect, so the team tested all of the welds for leak tightness at several stages. They also monitored constantly the vacuum and the purity of the filling gas. It was necessary to test the components before welding, and the assembled monitors during and after production, to ensure that the leakage current of the monitors stayed below 1 pA. Overall, IHEP achieved a consistently high quality for the monitors during the whole production period and kept to the tight production schedule. Tests at CERN’s Gamma Irradiation Facility of all 4250 monitors found fewer than 1% to be outside of the strict tolerance levels.
