A Nordt

UFOs in the LHC after LS1

UFOs (“Unidentified Falling Objects”) are potentially a major luminosity limitation for nominal LHC operation. With large-scale increases of the BLM thresholds, their impact on LHC availability was mitigated in the second half of 2011. For higher beam energy and lower magnet quench limits, the problem is expected to be considerably worse, though. Therefore, in 2011, the diagnostics for UFO events were significantly improved, dedicated experiments and measurements in the LHC and in the laboratory were made and complemented by FLUKA simulations and theoretical studies. In this paper, the state of knowledge is summarized and extrapolations for LHC operation after LS1 are presented. Mitigation strategies are proposed and related tests and measures for 2012 are specified.

Handling of the LHC Beam Loss Monitoring system abort thresholds

The LHC Beam Loss Monitoring system (BLM) makes use of approximately 4000 detectors located around the 27 km ring. Its main purpose is to protect all critical elements of the LHC by requesting a beam abort when the measured losses exceed any of the predefined threshold levels. The BLM system integrates the acquired signals in 12 different continuously updated time intervals, spanning from 40 us to 83.8 s, enabling for a different set of abort thresholds depending on the duration of the beam loss. Furthermore, the system takes into account 32 energy steps, from 450 GeV to 7 TeV, as the energy density of a particle shower increases with the energy of the primary particle, i.e. the beam energy and the magnet coil quench level decreases with its increasing current. Due to the differences on the elements under protection and the position of the detector in the tunnel, the system is required to allow a unique set of thresholds per detector. Such thresholds are originally based on thermodynamical arguments and Monte Carlo simulations and tuned with data recorded during the LHC run. The evolution of the BLM thresholds is described in this document. Moreover, the necessity of one set of thresholds per detector requires approximately 1.5E6 to be handled and sent to the appropriate processing modules for the system to function. This thresholds are extremely critical for the safety of the machine. Thus,well established procedures to compute, store and check new or changed threshold values have been defined. In order to avoid human errors, discover non-conformities and voids in the protection during manipulations, sanity checks and constrains have been embedded in the tools. The procedures, as well as the tools developed to automate this process are described in detail in this document.