Fast automatic beam-based alignment of the LHC collimator jaws

Abstract

The CERN Large Hadron Collider (LHC) in Geneva, Switzerland is the largest and most powerful particle accelerator ever built. With a circumference of 27 km, it is designed to collide particles in two counter-rotating beams, at a centre-of-mass energy of 14 TeV to explore the fundamental forces and constituents of matter. Due to its potentially destructive high energy particle beams, the LHC is equipped with several machine protection systems. The LHC collimation system is tasked with scattering and absorbing beam halo particles before they can quench the superconducting magnets. The 108 collimators also protect the machine from damage in the event of very fast beam losses, and shields sensitive devices in the tunnel from radiation over years of operation. Each collimator is made up of two blocks or 'jaws' of carbon, tungsten or copper material. The collimator jaws need be placed symmetrically on either side of the beam trajectory, to clean halo particles with maximum efficiency. The beam orbit and beam size need to be determined for each collimator, to be able to position the jaws Within a certain number of standard deviations (beam o-) from the beam centre. Beam-based alignment is used to determine these values at every collimator location. In the alignment procedure, each jawv is moved separately towards the beam trajectory, in 5 inn steps, until a spike appears in the signal of a Beam Loss Monitoring (BLM) detector positioned a couple of metres downstream of the collimator. A balance is required between scraping enough beam to obtain a signal, avoiding automatically triggered beam extractions (or dumps) in the event. of high beam losses, and completing the alignment in the shortest time possible to allow the LHC to produce maximum luminosity. In the 2010 LHC run, almost 30 hours were required for an alignment of all collimators, and 8 beam dumps were caused due to operator mistakes. A phased development, commissioning and usage of various algorithms in the 2011-2012 LHC runs allowed the alignment time to decrease to limit over 4 hours, with no more beam dumps. The algorithms range from automatic selection of BLM thresholds during the alignment, to BLM-based feedback loops and pattern recognition of the BLM signal spikes. The BLM-based feedback loop was also successfully used by the ALFA and TOTEM particle physics experiments in Roman Pot alignment campaigns. A Roman Pot is a detector that intercepts slightly deflected particles from head-on collisions to measure the total collision rate (cross-section). An alignment simulator was developed in MATLAB based on an empirical model of the BLM detector signal steady-state and crosstalk, as well as a beam diffusion model which allows the prediction of the characteristic BLM detector signal spike and decay. The simulator is targeted at validating possible future alignment algorithms which would otherwise require dedicated beam tests. A new collimator design for future LHC operation envisages Beam Position Monitor (BPM) pick-up buttons embedded inside the jaws. The BPM's will provide an accurate and continuous measurement of the beam centres without requiring BLM-based alignment. One quarter of the LHC collimators (tertiary collimators and IR6 secondary collimators) will be replaced with the new design, as foreseen since several years. Hence, an algorithm to automatically position the jaws around the beam centre at a large jaw gap was developed and tested with a prototype mock-up collimator installed in the Super Proton Synchrotron (SPS). Alignment times of approximately 20 s were reached. The work described in this dissertation was adopted by CERN for the first LHC running period (2008 - 2013). It will continue to be used in future operation post2015 after a two-year shutdown, in which the machine will be upgraded to be able to operate at the design parameters.