Quench Protection of the LHC Quadrupole Magnets

CERNs Large Hadron Collider (LHC) is a new high energy proton accelerator and storage ring. Its design allows to reach unprecedented beam energies and beam intensities, resulting in a largely increased particle physics discovery potential. The combination of its high beam energy and intensity may lead to beam losses which can have a severe impact on the LHC equipment and damage sensitive elements. To protect those and to measure operational losses, a Beam Loss Monitoring system has been installed all along the ring. The protection is achieved by extracting the beam from the ring in case thresholds imposed on measured radiation levels are exceeded. The thresholds are estimated through particle shower simulations. The simulated geometry and physic processes need to be precise in order to determine an optimum value, which therefore assures a high availability of the LHC for operation. This study is focused on the interconnection region between the main dipole and the main quadrupole magnet of the LHC. Six monitors are placed around the interconnection, three for each beam line. As proton impact location two loss patterns are assumed: one derived from halo particle tracking simulations and the other through analytic calculations relying on optical beam parameters. Particle shower simulations make the link between the amount of energy deposited in the superconducting coil and the signal measured in the ionisation chambers. The energ y deposition in the coil results in its temperature increase. In case a critical temperature is exceeded, a transition from the superconducting state to the normal conducting one will occur. This transition is called a quench and is analysed for steadystate and for fast transient losses. The fundamental parameters for the analysis are the critical power density and the enthalpy margin respectively. The combination of the detector signals allows the reconstruction of the loss pattern. Also the quench-protecting thresholds for two protection schemes have been evaluated and an optimisation of the detector positions in order to extend the protected area to the upstream main dipole magnet is proposed. First LHC results are used to verify the simulations and considerations with measurements, so that conclusions about the simulation accuracy and observed loss patterns could be drawn. For transient losses the quench protecting threshold used as initial setting in the Beam Loss Monitoring system for the 2010 run is for the first ionisation chamber of 1520 μGy at injection energy and 178 μGy at 7 TeV beam energy. For the second detector the respective values are 575 μGy and 29.7 μGy. For steady state losses the used threshold for the first ionisation chamber is 4960 μGy/s at injection energy and 1876 μGy/s at collision energy. For the second detector the respective values are 3789 μGy/s and 805 μGy/s.