Etienne Carlier

Protection of the CERN Large Hadron Collider

TheLargeHadronCollider(LHC)atCERNwillcollidetwocounter- rotating proton beams, each with an energy of 7TeV. The energy stored in the superconducting magnet system will exceed 10GJ, and each beam has a stored energy of 362MJ which could cause major damage to accelerator equipment in the case of uncontrolled beam loss. Safe operation of the LHC will therefore rely on a complex system for equipment protection. The systems for protection of the superconducting magnets in case of quench must be fully operational before powering the magnets. For safe injection of the 450GeV beam into the LHC, beam absorbers must be in their correct positions and specific procedures must be applied. Requirements for safe operation throughout the cycle necessitate early detection of failures within the equipment, and active monitoring of the beam with fast and reliable beam instrumentation, mainly beam loss monitors (BLM). When operating with circulating beams, the time constant for beam loss after a failureextendsfrom ≈mstoafewminutes—failuresmustbedetectedsufficiently early and transmitted to the beam interlock system that triggers a beam dump. It is essential that the beams are properly extracted on to the dump blocks at the end of a fill and in case of emergency, since the beam dump blocks are the only elements of the LHC that can withstand the impact of the full beam.

BEAM LOSS SCENARIOS AND STRATEGIES FOR MACHINE PROTECTION AT THE LHC

At the Large Hadron Collider (LHC) with nominal parameters at 7 TeV, each proton beam has an energy of more than 330 MJ threatening to damage accelerator equipment in case of uncontrolled beam loss. To prevent such damage, kickers are fired in case of failure deflecting the beams into dump blocks. The dump blocks are the only elements that can safely absorb the beams without damage. The time constant for particle losses depends on the specific failure and ranges from microseconds to several seconds. Starting with some typical failure scenarios, the strategy for the protection during LHC beam operation is illustrated. The systems designed to ensure safe operation, such as beam dump, beam instruments, collimators / absorbers and interlocks are discussed.

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.

A High Power Pulse System for the Beam Extraction from CERN's Large Hadron Collider

CERN, the European Organization for Nuclear Research, is close to starting operation of the large hadron collider (LHC). A beam dumping system must protect the LHC machine from damage, by reliably and safely extracting and absorbing the circulating beams when requested. For this purpose a beam extraction system has been designed, built, installed and tested. It is composed of 15 fast kicker magnets per beam line to extract the particles in one turn of the collider. Each magnet is powered by a dedicated pulse generator through special low impedance coaxial cables. The generator charging voltage is proportional to the beam momentum, which is 450 GeV/c at injection and will be 7 TeV/c at top energy. The current pulse has a maximum amplitude of 19 kA with a rise time of 2.8degs and a fall time of 2 ms; the first 89degs of the fall time are used to dump the beam. Each kicker magnet consists of a window frame of Si-Fe tape wound cores and high voltage insulated single turn conductors. They are built around a ceramic vacuum chamber which is metallized on the inside. The measures taken to ensure a high reliability of the system, which was one of the main considerations during the design, construction and testing of the system, are discussed. Results of measurements on the series systems are presented.

Studies of beam losses from failures of sps beam dump kickers

The SPS beam dump extraction process was studied in detail to investigate the possibility of operation with reduced kicker voltage and to fully understand the trajectories and loss patterns of miss-kicked beam. This paper briefly describes the SPS beam dump process, and presents the tracking studies carried out for failure cases. The simulation results are compared to the results of measurements made with low intensity beam.

High intensity commissioning of SPS LSS4 extraction for CNGS

The SPS LSS4 fast extraction system will serve both the anti-clockwise ring of the LHC and the CERN Gran Sasso Neutrino project (CNGS). CNGS requires 2 fast extractions of 10.5 microsecond long batches, 50 milliseconds apart. Each batch will consist of 2.4 times 1013 protons at 400 GeV. These intensities are factor of 10 above the equipment damage limit in case of beam loss. Active (interlock system) and passive protection systems have to be in place to guarantee safe operation and to respect the radiation limits in zones close to the extraction region. In summer 2006 CNGS was commissioned including extraction with high intensity. A thorough setting-up of the CNGS extraction was carried out as part of the CNGS commissioning, including aperture and beam loss measurements, and defining and checking of interlock thresholds for extraction trajectory, beam loss monitors and radiation monitors. The relevant systems and risks are introduced in this paper, the commissioning results are summarised and comparisons with simulation predictions are presented.

Beam Simulation Studies for the Upgrade of the SPS Beam Dumping System

The SPS at CERN currently uses a beam dumping system that is installed in the long straight section 1 (LSS1) of the SPS. This system consists of two beam stopper blocks for low and high energy beams, as well as two vertical and three horizontal kicker magnets, which deflect and dilute the beam on the dumps. Within the frame of the LHC injector upgrade project (LIU) the beam dumping system will be relocated to long straight section 5 (LSS5) and upgraded with an additional vertical kicker, new main switches and a single new beam dump, which covers the full energy range. The impact of a possible increase of the vertical kicker rise time on the beam has been studied in simulations with MAD-X for the different optics in the SPS. Furthermore, the impact on the beam in failure scenarios such as the non-firing of one kicker has been investigated. The results of these studies will be presented and discussed in this paper. Operational mitigation methods to deal with an increased rise time will also be discussed. INTRODUCTION The SPS presently uses an internal beam dumping system, which consists of two separate beam dump blocks for low and high energy beams installed in the long straight section 1 (LSS1, Fig. 1) [1]:  TIDH, energy range 14 – 28.9 GeV  TIDVG, energy range 102.2 – 450 GeV There exists a forbidden zone for beam energies between 28.9 GeV and 102.2 GeV, where no programmed beam dump is possible. The deflection onto the dump blocks is performed with two vertical kicker magnets (MKDV). Three horizontal kicker magnets (MKDH) dilute the beam on the dump blocks to reduce the beam density on their front faces. Figure 1: Layout of the present SPS beam dumping system (Figure courtesy of F. M. Velotti). The present SPS beam dumping system (SBDS) has several limitations and issues [1, 2]:  Incompatibility of upgrading present TIDVG for High-Luminosity LHC (HL-LHC) beam parameters;  Production of high air activation;  High activation of the narrow area around the TIDVG without the possibility of full shielding;  Interference with SPS injection system, which is also located in LSS1;  The above mentioned forbidden energy zone;  MKDV magnet reliability issues at high energy. These limitations will be addressed in an upgrade of the SBDS [2, 3] in the frame of the LHC injector upgrade project (LIU) [4], which aims for upgrading the LHC injector chain to enable the production of high-brightness beams required for the HL-LHC era. For the LIU SBDS upgrade, the following changes are planned [2]:  The relocation of the SBDS from LSS1 to LSS5. This will solve the interferences with the injection system;  Replacing the two separate beam dump blocks by one newly designed for the entire energy range;  Installation of an additional MKDV magnet with the full system operated with reduced voltage to decrease the risk of high-voltage breakdowns;  Upgrade of MKDV main generators with new solid state switches;  A new dump external shielding to reduce the surrounding dose. The upgrade of the generators is currently under development and it is not clear if the present MKDV rise time of 1.1 μs can be preserved with the new system. A slight increase of the rise time to 1.3 μs might be necessary. To study the impact of the increased rise time on the beam, tracking simulations were performed using MAD-X [5]. MKDV RISE TIME The studies were performed using the present kicker waveforms, shown in Fig. 2, since the new waveforms are not yet available, scaled to the expected operational voltage. For the 3rd MKDV, which will be installed during the Figure 2: Waveforms of the vertical (MKDV) and horizontal kickers (MKDH) used for the simulation. ________________________________________ * Email: christoph.hessler@cern.ch 9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-TUPAF031 04 Hadron Accelerators T12 Beam Injection/Extraction and Transport TUPAF031 747 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 18 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I.

Upgrade of the power triggering system of the LHC beam dumping system

The beam dumping system of CERNs Large Hadron Collider (LHC) is equipped with fast solid state closing switches composed of a stack of ten series connected Fast High Current Thyristors (FHCT). The triggering circuit of these switches consists of a 10:1 trigger transformer, with stray inductance of 5 μH, powered by two redundant Power Trigger Modules (PTM) delivering 520 A peak gate current with rate of rise of 460 A/μs. Operational experience gained since the commissioning of the system in 1998 has identified performance limitation of the LHC Beam Dumping System (LBDS) that could be solved by increasing the triggering current. In view of the operation of the LHC with higher luminosity beams in the coming years, an upgrade of the LBDS triggering system is proposed. The objective is an increase of the FHCT gate current to 2 kA peak with a rate of rise of 4 kA/μs, which will increase the FHCT lifetime and reduce the switching time and losses. These new performances will be obtained by the design of a faster low inductance trigger transformer; a reduction of the trigger cable inductance and an optimization of the present PTM electrical circuit. This paper will present the different modifications proposed for the PTM. First encouraging results obtained with a slightly modified PTM and new prototype trigger transformer will also be discussed.

Prospects for an optical re-triggering system for the LHC beam dumping system at CERN

The LHC (Large Hadron Collider) beam extraction kicker system, composed of 15 fast kicker magnets per beam, is used to extract the particles in one turn from the collider and to dispose of them, after dilution, on an external absorber. Each of the 15 magnets is powered by a separate pulse generator, all of which are simultaneously triggered when a beam extraction from the machine is requested. Spontaneous firing of a single generator will create beam oscillations that are likely to exceed the accelerator aperture, resulting in beam losses and potential damage to the machine. In order to protect against occurrence of such events, a Re-Triggering System (RTS) has been implemented to redistribute, as fast as possible, a trigger request issued from the spontaneous-firing generator to all 15 generators. A prospect for a RTS based on passively generated and transmitted optical power to all others generators has been studied as an alternative to existing re-triggering line solution. This can be accomplished by coupling light from a number of diode laser arrays at re-trigger sources of one generator to bundles of optical fibres subsequently dispatched to all 15 generators. At each generator control stage we foresee a re-triggering switch which ensures the conversion of the light signal into an isolated electrical triggering pulse.

Options to upgrade the triggering system of the SPS beam dumping system at CERN

In order to prevent uncontrolled beam losses in the Super Proton Synchrotron (SPS) at CERN, which can cause thermal and radiation damages to machine components, an internal beam dumping system is used. Upgraded layout will consists of three fast pulsed magnets which deflect the circulating beam vertically and three that sweep it in horizontal axis onto an absorber block within one accelerator revolution. The excitation current for each magnet is generated by the discharge of a Pulse Forming Network (PFN) through the magnet into a matched terminating resistor. As triggering circuits are one of the most critical components that will determine the global performance of a pulsed power system, a matched triggering system with the stacks of FHCT (Fast High Current Thyristors) has been developed with the objectives to improve the switching performance. The baseline triggering solution will be discussed in this paper with few alternative solutions that have been evaluated during prototyping such as pulse compression or the use of a laser activated thyristor. Power switches identified for this application are briefly described and compared in terms of performance. The limiting factors of the different switching techniques are highlighted in this comparative review.