The collimation system: defence against beam loss

19 August 2013

The LHC employs the largest and most advanced cleaning system ever built for a particle accelerator.

Multi-stage cleaning

Ideally, a storage ring like the LHC would never lose particles: the beam lifetime would be infinite. However, a number of processes will always lead to losses from the beam. The manipulations needed to prepare the beams for collision – such as injection, the energy ramp and “squeeze” – all entail unavoidable beam losses, as do the all-important collisions for physics. These losses generally become greater as the beam current and the luminosity are increased. In addition, the LHC’s superconducting environment demands an efficient beam-loss cleaning to avoid quenches from uncontrolled losses – the nominal stored beam energy of 362 MJ is more than a billion times larger than the typical quench limits.

The tight control of beam losses is the main purpose of the collimation system. Movable collimators define aperture restrictions for the circulating beam and should intercept particles on large-amplitude trajectories that could otherwise be lost in the magnets. Therefore, the collimators represent the LHC’s defence against unavoidable beam losses. Their primary role is to clean away the beam halo while maintaining losses at sensitive locations below safe limits. The current system is designed to ensure that peak losses below a few 0.01% of the energy lost from the beam is deposited in the cold magnets. As the closest elements to the circulating beams, the collimators provide passive machine protection against irregular fast losses and failures. They also control the distribution of losses around the ring by ensuring that the largest activation occurs at optimized locations. Collimators are also used to minimize background in the experiments.

The LHC collimation system provides multi-stage cleaning where primary, secondary and tertiary collimators and absorbers are used to reduce the population of halo particles to tolerable levels (figure 1). Robust carbon-based and non-robust but high-absorption metallic materials are used for different purposes. Collimators are installed around the LHC in seven out of the eight insertion regions (between the arcs), at optimal longitudinal positions and for various transverse rotation angles. The collimator jaws are set at different distances from the circulating beams, respecting the optimum setting hierarchy required to ensure that the system provides the required cleaning and protection functionalities.

The design was optimized using state-of-the-art numerical-simulation programs

The detailed system design was the outcome of a multi-parameter optimization that took into account nuclear-physics processes in the jaws, robustness against the worst anticipated beam accidents, collimation-cleaning efficiency, radiation impact and machine impedance. The result is the largest and most advanced cleaning system ever built for a particle accelerator. It consists of 84 two-sided movable collimators of various designs and materials. Including injection protection collimators, there are a total of 396 degrees-of-freedom, because each collimator jaw has two stepping motors. By contrast, the collimation system of the Tevatron at Fermilab had less than 30 degrees-of-freedom for collimator positions.

The design was optimized using state-of-the-art numerical-simulation programs. These were based on a detailed model of all of the magnetic elements for particle tracking and the vacuum pipe apertures, with a longitudinal resolution of 0.1 m along the 27-km-long rings. They also involved routines for proton-halo generation and transport, as well as aperture checks and proton–matter interactions. These simulations require high statistics to achieve accurate estimates of collimation cleaning. A typical simulation run involves tracking some 20–60 million primary halo protons for 200 LHC turns – equivalent to monitoring a single proton travelling a distance of 0.03 light-years. Several runs are needed to study the system in different conditions. Additional complex energy- deposition and thermo-mechanical finite-element computations are then used to establish heat loads in magnets, radiation doses and collimator structural behaviour for various loss scenarios. Such a highly demanding simulation process was possible only as a result of computing power developed over recent years.

The backbone of the collimation system is located at two warm insertion regions (IRs): the momentum cleaning at IR3 and betatron cleaning at IR7, which comprise 9 and 19 movable collimators per beam, respectively. Robust primary and secondary collimators made of a carbon-fibre composite define the momentum and betatron cuts for the beam halo. In 2012, in IR7 they were at ±4.3–6.3σ (with σ being the nominal standard deviation of the beam profile in the transverse plane) from the circulating 140 MJ beams, which passed through collimator apertures as small as 2.1 mm at a rate of around 11,000 times per second.

Additional tungsten absorbers protect the superconducting magnets downstream of the warm insertions. While these are more efficient in catching hadronic and electromagnetic showers, they are also more fragile against beam losses, so they are retracted further from the beam orbit. Further local protection is provided for the experiments in IR1, IR2, IR5 and IR8: tungsten collimators shield the inner triplet magnets that otherwise would be exposed to beam losses because they are the magnets with the tightest aperture restrictions in the LHC in collision conditions. Injection and dump protection elements are installed in IR2, IR8 and IR6. The collimation system must provide continuous cleaning and protection during all stages of beam operation: injection, ramp, squeeze and physics.

An LHC collimator

An LHC collimator consists of two jaws that define a slit for the beam, effectively constraining the beam halo from both sides (figure 2). These jaws are enclosed in a vacuum tank that can be rotated in the transverse plane to intercept the halo, whether it is horizontal, vertical or skew. Precise sensors monitor the jaw positions and collimator gaps. Temperature sensors are also mounted on the jaws. All of these critical parameters are connected to the beam-interlock system and trigger a beam dump if potentially dangerous conditions are detected.

At the LHC’s top energy, a beam size of less than 200 μm requires that the collimators act as high-precision devices. The correct system functionality relies on establishing the collimator hierarchy with position accuracies to within a fraction of the beam size. Collimation movements around the ring must also be synchronized to within better than 20 ms to achieve good relative positioning of devices during transient phases of the operational cycle. A unique feature of the control system is that the stepping motors can be driven according to arbitrary functions of time, synchronously with other accelerator systems such as power converters and radio-frequency cavities during ramp and squeeze.

These requirements place unprecedented constraints on the mechanical design, which is optimized to ensure good flatness along the 1-m-long jaw, even under extreme conditions. Extensive measurements were performed during prototyping and production, both for quality assurance and to obtain all of the required position calibrations. The collimator design has the critical feature that it is possible to measure a gap outside the beam vacuum that is directly related to the collimation gap seen by the beam. Some non-conformities in jaw flatness could not be avoided and were addressed by installing the affected jaws at locations of larger β functions (therefore larger beam size), in a way that is not critical for the overall performance.

Set-up and performance

The first step in collimation set-up is to adjust the collimators to the stored beam position. There are unavoidable uncertainties in the beam orbit and collimator alignment in the tunnel, so a beam-based alignment procedure has been established to set the jaws precisely around the beam orbit. The primary collimators are used to create reference cuts in phase space. Then all other jaws are moved symmetrically round the beam until they touch the reference beam halo. The results of this halo-based set-up provide information on the beam positions and sizes at each collimator. The theoretical target settings for the various collimators are determined from simulations to protect the available machine aperture. The beam-based alignment results are then used to generate appropriate setting functions for the collimator positions throughout the operational cycle. For each LHC fill, the system requires some 450 setting functions versus time, 1200 discrete set points and about 10,000 critical threshold settings versus time. Another 600 functions are used as redundant gap thresholds for different beam energies and optics configurations.

This complex system worked well during the first LHC operation with a minimum number of false errors and failures, showing that the choice of hardware and controls are fully appropriate for the challenging accelerator environment at the LHC. Collimator alignment and the handling of complex settings have always been major concerns for the operation of the large and distributed LHC collimation system. The experience accumulated in the first run indicates that these critical aspects have been addressed successfully.

Beam losses

The result of the cleaning mechanism from the LHC collimation process is always visible in the control room. Unavoidable beam losses occur continuously at the primary collimators and can be observed online by the operations team as the largest loss spikes on the fixed display showing the beam losses around the ring. The local leakage to cold magnets is in most cases below 0.00001 of the peak losses, with a few isolated loss locations around IR7 where the cleaning reaches levels up to a few 0.0001 (figure 3). So far, this excellent performance has ensured a quench-free operation, even in cases of extreme beam losses from circulating beams. Moreover, this was achieved throughout the year with only one collimator alignment in IR3 and IR7, thanks to the remarkable stability of the machine and of the collimator settings.

However, collimators in the interaction regions required regular setting up for each new machine configuration that was requested for the experiments. Eighteen of these collimators are being upgraded in the current long shutdown to reduce the time spent on alignment: the new tertiary collimator design has integrated beam-position monitors to enable a fast alignment without dedicated beam-based alignment fills. This upgrade will also eventually contribute to improving the peak luminosity performance by reducing further the colliding beam sizes, thanks to better control of the beam orbit next to the inner triplet.

The LHC collimation system performance is validated after set-up with provoked beam losses, which are artificially induced by deliberately driving transverse beam instabilities. Beam-loss monitors then record data at 3600 locations around the ring. As these losses occur under controlled conditions they can be compared in detail with simulations. As predicted, performance is limited by a few isolated loss locations, namely the IR7 dispersion-suppressor magnets, which catch particles that have lost energy in single diffractive scattering at the primary collimator. This limitation of the system will be addressed in future upgrades, in particular in the High Luminosity LHC era.

The first three-year operational run has shown that the LHC’s precise and complex collimation system works at the expected high performance, reaching unprecedented levels of cleaning efficiency. The system has shown excellent stability: the machine was regularly operated with stored beam energies of more than 140 MJ, with no loss-induced quenches of superconducting magnets. This excellent performance was among the major contributors to the rapid commissioning of high-intensity beams at the LHC as well as to the squeezing of 4 TeV beams to 60 cm at collision points – a crucial aspect of the successful operation in 2012 that led to the discovery of a Higgs boson.

•The success of the collimation system during the first years of LHC operation was the result of the efforts of the many motivated people involved in this project from different CERN departments and from external collaborators. All of these people, and Ralph Assmann who led the project until 2012, are gratefully acknowledged.

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