D Wollmann

Simulations and measurements of beam loss patterns at the CERN Large Hadron Collider

The CERN Large Hadron Collider (LHC) is designed to collide proton beams of unprecedented energy, in order to extend the frontiers of high-energy particle physics. During the first very successful running period in 2010--2013, the LHC was routinely storing protons at 3.5--4 TeV with a total beam energy of up to 146 MJ, and even higher stored energies are foreseen in the future. This puts extraordinary demands on the control of beam losses. An un-controlled loss of even a tiny fraction of the beam could cause a superconducting magnet to undergo a transition into a normal-conducting state, or in the worst case cause material damage. Hence a multi-stage collimation system has been installed in order to safely intercept high-amplitude beam protons before they are lost elsewhere. To guarantee adequate protection from the collimators, a detailed theoretical understanding is needed. This article presents results of numerical simulations of the distribution of beam losses around the LHC that have leaked out of the collimation system. The studies include tracking of protons through the fields of more than 5000 magnets in the 27 km LHC ring over hundreds of revolutions, and Monte-Carlo simulations of particle-matter interactions both in collimators and machine elements being hit by escaping particles. The simulation results agree typically within a factor 2 with measurements of beam loss distributions from the previous LHC run. Considering the complex simulation, which must account for a very large number of unknown imperfections, and in view of the total losses around the ring spanning over 7 orders of magnitude, we consider this an excellent agreement. Our results give confidence in the simulation tools, which are used also for the design of future accelerators.

Testing beam-induced quench levels of LHC superconducting magnets

In the years 2009-2013 the Large Hadron Collider (LHC) has been operated with the top beam energies of 3.5 TeV and 4 TeV per proton (from 2012) instead of the nominal 7 TeV. The currents in the superconducting magnets were reduced accordingly. To date only seventeen beam-induced quenches have occurred; eight of them during specially designed quench tests, the others during injection. There has not been a single beam- induced quench during normal collider operation with stored beam. The conditions, however, are expected to become much more challenging after the long LHC shutdown. The magnets will be operating at near nominal currents, and in the presence of high energy and high intensity beams with a stored energy of up to 362 MJ per beam. In this paper we summarize our efforts to understand the quench levels of LHC superconducting magnets. We describe beam-loss events and dedicated experiments with beam, as well as the simulation methods used to reproduce the observable signals. The simulated energy deposition in the coils is compared to the quench levels predicted by electro-thermal models, thus allowing to validate and improve the models which are used to set beam-dump thresholds on beam-loss monitors for Run 2.

Heavy Ions in 2011 and beyond

The LHCs first heavy ion run set and tested the operational pattern for 2011 and later years: a rapid commissioning strategy intended to ensure delivery of integrated luminosity despite the risks associated with the short time-frame. It also gave us hard data to test our understanding of the beam physics that will limit performance. The 2010 experience is fed into the commissioning plan, parameter choices and projected performance for 2011. The prospects for future stages of the LHC ion program, Pb-Pb collisions at higher energy and luminosity, hybrid collisions and other species, depend critically on the scheduling of certain hardware upgrades.

Experimental Studies and Theoretical Interpretation of Hydrodynamic Tunneling of SPS Protons in Solid Targets

The phenomenon of hydrodynamic tunneling has been experimentally studied at the CERN HiRadMat facility using the SPS beam [1]. The beam parameters include, proton energy = 440 GeV, bunch intensity = 1.5x10 , bunch length = 0.5 ns, bunch separation = 50 ns and σ of the transverse intensity distribution = 0.2 mm. In two experiments, 108 and 144 proton bunches, respectively, were used and the protons were delivered in sets of 36 bunches each while a separation of 250 ns was considered between the bunch packets.

Numerical Simulations of Hydrodynamic Tunneling Experiments Performed at HiRadMat Facility Using SPS Proton Beam

Extensive simulations carried out over the past 10 years to study the full impact of the LHC beam on solid targets has revealed substantial hydrodynamic tunneling of protons and their shower [1-3]. This effect has very important implications on the LHC machine protection design. In order to confirm the validity of these simulations, experiments have been carried out at the HiRadMat facility using 440 GeV protons impacting on solid targets. Detailed numerical simulations have been done to interpret the experiments. The experimental results together with a comparison with the simulations has been reported elsewhere [4,5]. In the present contribution we present the details about the simulations.