Text How the Damage to the Lhc Was Repaired

1. INTRODUCTION

The Large Hadron Collider (LHC) is a two-ring superconducting-hadron accelerator and collider installed in an existing 26.7-km tunnel, which was constructed between 1984 and 1989 for the CERN Large Electron-Positron Collider (LEP). The LEP tunnel has eight straight sections and eight arcs and lies between 45 m and 170 m below the surface on a plane inclined at 1.41%, sloping toward Lake Léman. Approximately 90% of the tunnel's length is in molasse rock, which has excellent characteristics for this application, and 10% is in limestone under the Jura Mountains. There are two transfer tunnels, each approximately 2.5 km in length, linking the LHC to the CERN accelerator complex, which acts as injector. Full use has been made of the existing civil engineering structures, but modifications and additions were also needed. Broadly speaking, the underground and surface structures at Points 1 and 5 for the ATLAS and CMS experiments, respectively, are new, whereas those for ALICE and LHCb, at Points 2 and 8, respectively, were originally built for the LEP.

Approval for the LHC project was granted by the CERN Council in December 1994. At that time, the plan was to build a machine in two stages, starting with a center-of-mass energy of 10 TeV, to be upgraded later to 14 TeV. However, during 1995 and 1996, intense negotiations secured substantial contributions to the project from nonmember states, and in December 1996, the CERN Council approved construction of the 14-TeV machine in a single stage.

The LHC design depends on some basic principles linked with the latest technology. As a particle-particle collider, the LHC contains two rings with counterrotating beams, unlike particle-antiparticle colliders, in which both beams can share the same ring. The tunnel in the arcs has a finished internal diameter of 3.7 m, which makes it extremely difficult to install two completely separate proton rings. This hard limit on space led to the adoption of the twin-bore magnet design, which was proposed by John Blewett (1) at Brookhaven National Laboratory (BNL) in 1971. At that time, it was known as the two-in-one superconducting-magnet design and was put forward as a cost-saving measure (2, 3), but in the case of the LHC, the overriding reason for adopting this solution was the lack of space in the existing tunnel.

In the second half of the twentieth century, it became clear that higher energies could be reached only through better technologies, principally superconductivity. The first use of superconducting magnets in an operational collider was in the Intersecting Storage Rings at CERN, but always at 4 K to 4.5 K (4). However, research was moving toward operation at and below 2 K to take advantage of the increased temperature margins and the enhanced heat transfer at the solid-liquid interface and in the bulk liquid (5). The French Tokamak Tore II Supra demonstrated this new technology (6, 7), which was then proposed for the LHC (8) and advanced from the preliminary study to the final concept design and validation in six years (9).

In a short review, it is impossible to describe in detail all the different systems needed to operate the LHC. Instead, we concentrate on the principal new technologies developed for the machine. A detailed description of the machine as built can be found in the three-volume LHC Design Report (10). This review ends with a brief description of the commissioning of the LHC and its first year of operation.

2. MAIN MACHINE LAYOUT AND PERFORMANCE

2.1. Performance Goals

The aim of the LHC is to reveal the physics beyond the Standard Model with center-of-mass collision energies of up to 14 TeV. The number of events per second generated in the LHC collisions is given by

1.

where σ _event is the cross section for the event under study and L is the machine luminosity. The machine luminosity depends only on the beam parameters and can be written for a Gaussian beam distribution as

2.

where N_b is the number of particles per bunch, n_b is the number of bunches per beam, f _rev is the revolution frequency, γ_r is the relativistic γ factor, ε_n is the normalized transverse beam emittance, β* is the β function at the collision point, and F is the geometric luminosity reduction factor due to the crossing angle at the interaction point (IP):

3.

Here, θ_c is the full crossing angle at the IP, σ_z is the root-mean-square (rms) bunch length, and σ* is the transverse rms beam size at the IP. The above expression assumes round beams, with σ_z ≪ β and with equal beam parameters for both beams. The exploration of rare events in the LHC collisions therefore requires both high beam energies and high beam intensities.

The LHC has two high-luminosity experiments, ATLAS (11) and CMS (12); both aim for a peak luminosity of L = 10³⁴ cm⁻² s⁻¹ for proton operation. There are also two low-luminosity experiments, LHCb (13) for B physics, which aims for a peak luminosity of L = 10³²cm⁻² s⁻¹, and TOTEM (14) for the detection of protons from elastic scattering at small angles, which aims for a peak luminosity of L = 2 × 10²⁹ cm⁻² s⁻¹ with 156 bunches. In addition to the proton beams, ion beams also operate at the LHC. The LHC has one dedicated ion experiment, ALICE (15), which aims for a peak luminosity of L = 10²⁷ cm⁻² s⁻¹ for nominal lead-lead ion operation.

The high beam intensity required for a luminosity of L = 10³⁴ cm⁻² s⁻¹ excludes the use of antiproton beams and, hence, excludes the particle-antiparticle collider configuration of a common vacuum and magnet system for both circulating beams (this setup is used, for example, in the Tevatron). Colliding two counterrotating proton beams requires opposite magnetic dipole fields in both rings. The LHC is therefore designed as a proton-proton collider with separate magnet fields and vacuum chambers in the main arcs and with common sections only at the insertion regions where the experimental detectors are located. The two beams share an ∼130-m-long common beam pipe along the interaction regions (IRs).

There is not enough room for two separate rings of magnets in the LEP/LHC tunnel, and for this reason, as well as to reduce costs, the LHC uses twin-bore magnets that consist of two sets of coils and beam channels within the same mechanical structure and cryostat. The peak beam energy depends on the integrated dipole field around the storage ring, which implies a peak dipole field of 8.33 T for an energy of 7 TeV. This field can be achieved with an affordable niobium-titanium (NbTi) superconductor only by lowering the temperature to 1.9 K, which is below the phase transition of helium from a normal to a superfluid state.

2.2. Performance Limitations

The performance of the LHC is limited by many factors, including beam-beam and single-beam effects, and by the fundamental limitations of superconducting magnets.

2.2.1. Beam-beam limit.

When the beams collide, a proton in one beam is affected by the electromagnetic field of the other beam. The maximum particle density per bunch is limited by the nonlinearity of this beam-beam interaction, the strength of which is measured by the linear tune shift,

4.

where r_p is the classical proton radius: r_p = e ²/(4πε ₀ m_pc ²). Experience with existing hadron colliders indicates that the total linear tune shift summed over all IPs should not exceed 0.015. With three proton experiments requiring head-on collisions, the linear beam-beam tune shift for each IP should satisfy ξ < 0.005.

2.2.2. Maximum dipole field and magnet quench limits.

The maximum beam energy that can be reached in the LHC is limited by the peak dipole field in the storage ring. The nominal field is 8.33 T, corresponding to an energy of 7 TeV. Operating at this very high field level requires the magnets to be cooled in a bath of superfluid helium at 1.9 K.

2.2.3. Energy stored in the circulating beams and in the magnetic fields.

A total beam current of 0.584 A corresponds to a stored energy of approximately 362 MJ. In addition to the energy stored in the circulating beams, the LHC magnet system has a stored electromagnetic energy of approximately 600 MJ, yielding a total stored energy of more than 1 GJ. This stored energy must be absorbed safely at the end of each run or in case of a malfunction or an emergency. The beam-dumping system and the magnet system therefore provide additional limits on the maximum attainable beam energies and intensities.

2.2.4. Heat load.

Although the synchrotron radiation in hadron storage rings is small compared with that generated in electron rings, it can still impose practical limits on the maximum attainable beam intensities if the radiation has to be absorbed by the cryogenic system. In addition to the synchrotron-radiation heat load, the LHC cryogenic system must absorb heat deposition from luminosity-induced losses, impedance-induced losses (i.e., the resistive wall effect), and electron-cloud bombardment.

2.2.5. Field quality and dynamic aperture.

Field-quality errors compromise the particle stability in the storage ring, and hence loss-free operation requires high field quality. A characteristic feature of superconducting magnets is the decay of persistent currents and their "snap back" (a rapid change in persistent current at the beginning of the ramp). Achieving small beam losses therefore requires tight control of the magnetic field errors during magnet production and during machine operation. Assuming fixed limits for the beam losses (set by the quench levels of the superconducting magnets), the accuracy of the field-quality correction during operation and its limitation on machine performance can be estimated.

2.2.6. Collective beam instabilities.

The interaction among the charged particles in each beam via electromagnetic fields, and between the particles and the conducting boundaries of the vacuum system, can result in collective beam instabilities. Generally speaking, the collective effects are a function of the vacuum system geometry and its surface properties. They are usually proportional to the beam currents and can therefore limit the maximum attainable beam intensities.

2.3. Lattice Layout

The basic layout of the LHC follows the LEP tunnel geometry (Figure 1). The LHC has eight arcs and eight straight sections. Each straight section is approximately 528 m long and can serve as an experimental or utility insertion. The two high-luminosity experimental insertions are located at diametrically opposite straight sections: The ATLAS experiment is located at Point 1 and the CMS experiment at Point 5. Two more experimental insertions are located at Point 2 and Point 8, which also include the injection systems for Beam 1 and Beam 2, respectively. The injection kick occurs in the vertical plane, with the two beams arriving at the LHC from below the LHC reference plane. The beams cross from one magnet bore to the other at four locations. The remaining four straight sections do not have beam crossings. Each insertion at Points 3 and 7 contains two collimation systems. The insertion at Point 4 contains two radio-frequency (rf) acceleration systems: one independent system for each LHC beam. The straight section at Point 6 contains the beam-dump insertion, where the two beams can be safely extracted from the machine if needed. Each beam features an independent abort system.

The arcs of the LHC lattice are made of 23 regular arc cells. The arc cells are 106.9 m long and consist of two 53.45-m-long half-cells. Each half-cell contains one 5.355-m-long cold mass (6.63-m-long cryostat), a short straight section (SSS) assembly, and three 14.3-m-long dipole magnets. The LHC arc cell is optimized for a maximum integrated dipole field along the arc with a minimum number of magnet interconnections and with the smallest possible beam envelopes.

The two apertures of Ring 1 and Ring 2 are separated by 194 mm. The two coils in the dipole magnets are powered in series, and all the dipole magnets of one arc form one electrical circuit. The quadrupoles of each arc form two electrical circuits: All focusing quadrupoles in Beam 1 and Beam 2 are powered in series, and all defocusing quadrupoles in Beam 1 and Beam 2 are powered in series. The optics of Beam 1 and Beam 2 in the arc cells are therefore strictly coupled via the powering of the main magnetic elements.

A dispersion suppressor (DS) is located at each transition between an LHC arc and a straight section, yielding a total of 16 DS sections. The aim of the DSs is threefold: They (a) adapt the LHC reference orbit to the geometry of the LEP tunnel, (b) cancel the horizontal dispersion that arises in the arc and is generated by the separation/recombination dipole magnets and the crossing-angle bumps, and (c) facilitate matching of the insertion optics to the periodic optics of the arc.

2.4. High-Luminosity Insertions

IRs 1 and 5 house the high-luminosity experiments of the LHC and are identical in terms of hardware and optics, except that the crossing angle is in the vertical plane in Point 1 and in the horizontal plane in Point 5. The small β-function values at the IPs are generated between quadrupole triplets that leave ±23 m of free space about the IP. In this region, the two rings share the same vacuum chamber, the same low-β triplet magnets, and the D1 separation dipole magnets. The remaining matching section (MS) and the DS consist of twin-bore magnets with separate beam pipes for each ring. From the IP up to the DS insertion, the layout has the following components.

1.	A 31-m-long superconducting low-β triplet assembly that operates at a temperature of 1.9 K and provides a nominal gradient of 205 T m−1.
2.	A pair of separation/recombination dipoles separated by approximately 88 m.
3.	The D1 dipole, located next to the triplet magnets, that has a single bore and consists of six 3.4-m-long, conventional warm magnet modules that yield a nominal field of 1.38 T.
4.	The following D2 dipole, which is a 9.45-m-long, twin-bore, superconducting dipole magnet that operates at a cryogenic temperature of 4.5 K and has a nominal field of 3.8 T. The bore separation in the D2 magnet is 188 mm and thus is slightly smaller than the arc bore separation.
5.	Four matching quadrupole magnets. The first quadrupole following the separation dipole magnets, Q4, is a wide-aperture magnet that operates at a cryogenic temperature of 4.5 K and yields a nominal gradient of 160 T m−1. The remaining three quadrupole magnets are normal-aperture quadrupole magnets that operate at a cryogenic temperature of 1.9 K and have a nominal gradient of 200 T m−1.

Figure 2 shows a schematic layout of the right side of IR1.

2.5. Medium-Luminosity Insertion

The straight section of IR2 (Figure 3) houses the injection elements for Ring 1, as well as for the ion-beam experiment ALICE. During injection, the optics must obey the special constraints imposed by the beam injection for Ring 1, and the geometrical acceptance in the IR must be large enough to accommodate both beams in the common part of the ring with a beam separation of at least 10 σ.

2.6. Beam-Cleaning Insertions

The IR3 insertion houses the momentum cleaning systems (which capture off-momentum particles) of both beams, and IR7 houses the betatron cleaning systems (which control the beam halo) of both beams. Particles with a large momentum offset are scattered by the primary collimator in IR3, and particles with large betatron amplitudes are scattered by the primary collimator in IR7. In both cases, the scattered particles are absorbed by secondary collimators. Figures 4 and 5 show the right-hand sides of IRs 3 and 7, respectively.

In IR7, the layout of the long straight section between Q7L and Q7R (L and R refer to left and right, respectively) is mirror symmetric with respect to the IP, which allows a symmetrical installation for the collimators of the two beams and minimizes the space conflicts in the insertion. Starting from Q7L, the superconducting quadrupole Q6 is followed by a dogleg structure made of two sets of MBWs, namely warm, single-bore, wide-aperture dipole magnets (two warm modules each). The dogleg dipole magnets are labeled D3 and D4 in the LHC sequence; D3 is closer to the IP. The primary collimators are located between the D4 and D3 magnets, which allows for neutral particles produced in the jaws to point out of the beam line and for most charged particles to be swept away. The interbeam distance between the dogleg assemblies to the left and right of the IP is 224 mm, that is, 30 mm larger than in the arc. This increased beam separation allows a substantially higher gradient in the Q4 and Q5 quadrupoles, which are not superconducting because of the heavy irradiation from the collimators. The space between Q5L and Q5R from the IP is used to house the secondary collimators at appropriate phase advances, with respect to the primary collimators.

2.7. Radio-Frequency Insertion

IR4 (Figure 6) houses the rf and feedback systems, as well as some of the LHC beam instrumentation. The rf equipment is installed in the old ALEPH (LEP) cavern, which provides a large space for the power supplies and klystrons. To provide the transverse space for two independent rf systems for Beams 1 and 2, the separation must be increased to 420 mm. This increase is achieved through two pairs of dogleg dipole magnets, D3 and D4. In contrast to those in IR3 and IR7, the dogleg magnets in IR4 are superconducting because the radiation levels are low.

2.8. Beam-Abort Insertion

IR6 (Figure 7) houses the beam-abort systems for Beams 1 and 2. Beam abort from the LHC is done by kicking the circulating beam horizontally into an iron septum magnet, which deflects the beam in the vertical direction away from the machine components to absorbers in a separate tunnel. Each ring has its own system. To minimize the length of the kicker and of the septum, large drift spaces are provided. Matching the β functions between the ends of the left and right DSs requires only four independently powered quadrupoles. In each DS, up to six quadrupoles can be used for matching. The total of 16 quadrupoles is more than sufficient to match the β functions and the dispersion and to adjust the phases. However, other constraints concerning apertures inside the insertion should be taken into account.

Special detection devices protect the extraction septum and the LHC machine against losses during the abort process. The target collimator dump septum absorber is located in front of the extraction septum, and the target collimator dump quadrupole is in front of the Q4 quadrupole magnet, downstream of the septum magnet.

2.9. Medium-Luminosity Insertion

IR8 houses the LHCb experiment and the injection elements for Beam 2. The small β-function values at the IP are generated with the help of a triplet quadrupole assembly that leaves ±23 m of free space around the IP. In this region, the two rings share the same vacuum chamber, the same low-β triplet magnets, and the D1 separation dipole magnet. The remaining MS and the DS consist of twin-bore magnets with separate beam pipes for each ring. From the IP up to the DS insertion, the layout comprises the following.

1.	Three warm dipole magnets to compensate for the deflection generated by the LHCb spectrometer magnet.
2.	A 31-m-long, superconducting low-β triplet assembly operated at 1.9 K that provides a nominal gradient of 205 T m−1.
3.	A pair of separation/recombination dipole magnets separated by approximately 54 m. The D1 dipole located next to the triplet magnets is a 9.45-m-long, single-bore superconducting magnet. The following D2 dipole is a 9.45-m-long, double-bore superconducting dipole magnet. Both magnets are operated at 4.5 K. The bore separation in the D2 magnet is 188 mm and thus is slightly smaller than the arc bore separation.
4.	Four matching quadrupole magnets. The first quadrupole following the separation dipole magnets, Q4, is a wide-aperture magnet that operates at 4.5 K and yields a nominal gradient of 160 T m−1. The remaining three MS quadrupole magnets are normal-aperture quadrupole magnets that operate at 1.9 K and have a nominal gradient of 200 T m−1.
5.	The injection elements for Beam 2 on the right-hand side of IP8. To provide sufficient space for the spectrometer magnet of the LHCb experiment, the beam-collision point is shifted by 15 half-rf wavelengths (3.5 times the nominal bunch spacing ≈11.25 m) toward IP7. This shift of the collision point must be compensated for before the beam returns to the DS sections, and it requires a nonsymmetric magnet layout in the MS. Figure 8 shows a schematic layout of the right side of IR8. Figure 8

3. MAGNETS

3.1. Overview

The LHC relies on superconducting magnets that are at the frontier of present technology. All the other large superconducting accelerators [e.g., the Tevatron at Fermi National Accelerator Laboratory (FNAL), HERA at DESY, and RHIC at BNL] use classical NbTi superconductors, cooled by supercritical helium at temperatures slightly above 4.2 K, with fields below or around 5 T. The LHC magnet system, while still making use of the well-proven technology based on NbTi Rutherford cables, cools the magnets to below 2 K by using superfluid helium; it operates at fields above 8 T. Because electromagnetic forces increase with the square of the field, the structures retaining the conductor motion must be mechanically much stronger than in earlier designs. In addition, space limitations in the tunnel and the need to keep costs down have led to the adoption of the so-called two-in-one or twin-bore design for almost all of the LHC superconducting magnets. The two-in-one design accommodates the windings for the two beam channels in a common cold mass and cryostat, with magnetic flux circulating in the opposite sense through the two channels.

3.2. Superconducting Cable

The transverse cross section of the coils in the LHC's 56-mm-aperture dipole magnet (Figure 9) shows two layers of different cables distributed in six blocks. The cable used in the inner layer has 28 strands, each of which has a diameter of 1.065 mm, whereas the cable used in the outer layer is formed from 36 strands, each of which has a diameter of 0.825 mm.

The filament size is 7 μm for the strand of the inner-layer cable and 6 μm for the strand of the outer-layer cable. These strands are optimized to reduce the effects of the persistent currents on the sextupole field component at injection. The residual errors are corrected by small sextupole and decapole magnets located at the end of each dipole.

3.3. Main Dipole Cold Mass

The LHC ring accommodates 1,232 main dipoles: 1,104 in the arc and 128 in the DS regions. All have the same basic design. The geometric and interconnection characteristics have been targeted to be suitable for the DS region, which is more demanding than the arc. The cryodipoles are a critical part of the machine, both from the point of view of machine performance and in terms of cost. Figure 10 shows a cross section of the cryodipole.

The successful operation of the LHC requires the main dipole magnets to have practically identical characteristics. The relative variations of the integrated field and the field shape imperfections must not exceed ∼10⁻⁴, and their reproducibility must be better than 10⁻⁴ after magnet testing and during magnet operation. The reproducibility of the integrated field strength requires (a) tight control over coil diameter and length and the stacking factor of the laminated magnetic yokes and, possibly, (b) fine-tuning of the length ratio between the magnetic and nonmagnetic parts of the yoke. The structural stability of the cold mass assembly is achieved by using very rigid collars and by opposing the electromagnetic forces acting at the interfaces between the collared coils and the magnetic yoke with the forces set up by the shrinking cylinder. A prestress between coils and the retaining structure (i.e., the collars, iron lamination, and the shrinking cylinder) is also built in. Because of the larger thermal contraction coefficient of the shrinking cylinder and austenitic steel collars, compared with that of the yoke steel, the force distribution inside the cold mass changes during cooldown from room temperature to 1.9 K.

3.4. Dipole Cryostat

The vacuum vessel consists of a long, cylindrical standard tube with an outer diameter of 914 mm (36 inches) and a wall thickness of 12 mm. It is made from alloyed low-carbon steel. The vessel has stainless-steel end flanges for vacuum-tight connection, via elastomer seals, to adjacent units. Three support regions feature circumferential reinforcement rings. Upper reinforcing angles support alignment fixtures. An ISO (International Organization for Standardization)-standard flanged port is located azimuthally on the wall of the vessel at one end. In normal operation, the vessel would be under vacuum. In case of a cryogenic leak, the pressure could rise to 0.14 MPa absolute, and a sudden local cooling of the vessel wall to approximately 230 K could occur. The steel selected for the vacuum-vessel wall has been tested to demonstrate adequate energy absorption during a standard Charpy test (a test to determine the temperature-dependent ductile-to-brittle transition) at −50°C. A front view of the cryodipole is shown in Figure 11.

In the main dipoles, the magnetic field is up in one aperture and down in the other. In straight sections 2 and 8, the beams are separated into two apertures with special superconducting dipoles: D1, with a single aperture, and D2, with two apertures where the field direction is identical. Such special dipoles are also used in the rf insertion at Point 4, where the beams are separated further to make way for the cavities. All these special dipoles were designed and built at BNL.

3.5. Short Straight Sections of the Arcs

Figure 12 shows a perspective view and Figure 13 the cross section of an SSS. The cold masses of the arc SSSs contain the main quadrupole magnets (MQs) and various corrector magnets. On the upstream end, these can be octupoles, tuning quadrupoles, or skew quadrupole correctors. On the downstream end are the combined sextupole-dipole correctors.

Because of the lower electromagnetic forces in the MQs than in the dipoles, the two apertures do not need to be combined. Instead, they are assembled in separate annular collaring systems.

3.6. Insertion Magnets

The insertion magnets are superconducting or normal conducting and are used in the eight insertion regions of the LHC. Four of these insertions are dedicated to experiments, whereas the others are used for major collider systems (one for the rf, two for beam cleaning, and one for beam dumping). The various functions of the insertions are fulfilled by various magnets, most of which are based on the technology of NbTi superconductors cooled by superfluid helium at 1.9 K. Numerous stand-alone magnets in the MSs and beam-separation sections are cooled to 4.5 K, and in the radiation areas, specialized normal-conducting magnets have been installed.

3.7. Matching Section Quadrupoles

Tuning of the LHC insertions is provided by the individually powered quadrupoles in the MS and the DS sections. The MSs consist of stand-alone quadrupoles arranged in four half-cells, but the number and parameters of the magnets are specific to each insertion. Apart from the cleaning insertions, where specialized, normal-conducting quadrupoles are used in the high-radiation areas, all matching quadrupoles are superconducting magnets. Most of them are cooled to 4.5 K, except the Q7 quadrupoles, which are the first magnets in the continuous-arc cryostat and are cooled to 1.9 K.

CERN has developed two superconducting quadrupoles for the MSs: (a) the MQM quadrupole, featuring a 56-mm-aperture coil, that is also used in the DSs, and (b) the MQY quadrupole, with an enlarged, 70-mm-coil aperture. Both quadrupoles use narrow cables so that the nominal current is less than 6 kA, which substantially simplifies the warm and cold powering circuits. Each aperture is powered separately, but a common return is used, so a three-wire bus-bar system is sufficient for full control of the apertures.

In the cleaning insertions IR3 and IR7, each of the matching quadrupoles Q4 and Q5 consists of a group of six normal-conducting MQW magnets. This choice is dictated by the high radiation levels arising from scattered particles from the collimation system, and therefore the use of superconducting magnets is not possible. The MQW features two apertures in a common yoke (the two-in-one design), which is atypical for normal-conducting quadrupole magnets but is required because of transverse space constraints in the tunnel. The two apertures may be powered in series in a standard focusing/defocusing configuration (MQWA) or, alternatively, in a focusing/focusing configuration (MQWB) to correct asymmetries of the magnet. In a functional group of six magnets, five are configured as MQWA and are corrected by one configured as MQWB.

3.8. Low-Beta Triplets

The low-β triplet is composed of four single-aperture quadrupoles with a coil aperture of 70 mm. These magnets are cooled with superfluid helium at 1.9 K through the use of an external heat-exchanger system capable of extracting up to 10 W m⁻¹ of power deposited in the coils by the secondary particles emanating from the proton collisions. Two types of quadrupoles are used in the triplet: 6.6-m-long MQXA magnets designed and developed by KEK, Japan, and 5.7-m-long MQXB magnets designed and built by FNAL. The magnets are powered in series with 7 kA, with an additional inner loop of 5 kA for the MQXB magnets. Together with the orbit correctors (MCBX), skew quadrupoles (MQSX), and multipole spool pieces supplied by CERN, the low-β quadrupoles are completed in their cold masses and cryostated by FNAL. The cryogenic feed-boxes, which provide a link to the cryogenic distribution line and power converters, were designed and built by Lawrence Berkeley National Laboratory. Alongside the LHC main dipoles, the high-gradient, wide-aperture, low-β quadrupoles are the most demanding magnets in the collider. They must operate reliably at 215 T m⁻¹, sustain extremely high heat loads in the coils and high-radiation doses during their lifetime, and have a very good field quality within the 63-mm aperture of the cold bore.

4. THE RADIO-FREQUENCY SYSTEMS

4.1. Introduction

The injected beam is captured, accelerated, and stored using a 400-MHz superconducting-cavity system, and the longitudinal injection errors are damped using the same system. This choice defines the maximum allowable machine impedance, particularly for higher-order modes in cavities (16). Transverse injection errors are damped by a separate system of electrostatic deflectors that also ensures subsequent transverse stability against resistive wall instability (17). All rf and beam-feedback systems are concentrated at Point 4 and extend from the UX45 cavern area into the tunnel on either side.

The beam and machine parameters that are directly relevant to the design of the rf and beam-feedback systems are given in Table 1. At nominal intensity in the Super Proton Synchrotron, an emittance of 0.6 eV has been achieved, yielding a bunch length of 1.6 ns at 450 GeV. The phase variation along the batch due to beam loading is within 125 ps. This emittance is lower than originally assumed, and as a result, an rf system of 400.8 MHz can be used to capture the beam with minimal losses, accelerate the beam, and finally store it at top energy. Higher frequencies, although better for producing the short bunches required in storage, cannot accommodate the injected bunch length.

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Table 1

The main beam and radio-frequency (rf) parameters

There is some emittance increase at injection, but it is within the capabilities of the rf system for acceleration and, in particular, is below the final emittance needed in storage. This final emittance is defined by the intrabeam scattering lifetime in the presence of damping due to synchrotron radiation, rf lifetime, and instability-threshold considerations. Controlled emittance increase is provided during acceleration by excitation with band-limited noise; the emittance increases with the square root of the energy to optimize both narrow- and broadband instability thresholds (16).

The final emittance at 7 TeV (2.5 eV) and the maximum bunch length given by luminosity considerations in the experiments lead to a required maximum voltage of 16 MV per beam. There are many advantages of having a separate rf system for each beam. However, the standard distance between beams in the machine, 194 mm, is insufficient. Consequently, the beam separation is increased in the rf region to 420 mm by means of special superconducting dipoles. With the increased separation, and by staggering the cavities longitudinally, the second beam can pass outside the cavity. However, it must still pass through the cryostat.

4.2. Main 400-MHz Radio-Frequency Accelerating System

Each of the two independent rf systems must provide at least 16 MV in coast, whereas at injection approximately 8 MV are needed. The frequency of 400 MHz is close to that of LEP (352 MHz) and allows the same proven technology of niobium-sputtered superconducting cavities to be applied. The present design—which uses single-cell cavities, each of which has 2 MV of accelerating voltage, corresponding to a conservative field strength of 5.5 MV m⁻¹—minimizes the power carried by the rf window. A large tuning range is required to compensate for the average reactive beam component. Each rf system has eight cavities with R/Q = 45 Ω and a length of λ/2, grouped in two sets of four with a spacing of 3λ/2 in one common cryostat (18). Each cavity is driven by an individual rf system with klystron, circulator, and load. Complex feedback loops around the cavities allow precise control of the field in each cavity, which is important for the unforgiving high-intensity LHC proton beam.

The use of niobium sputtering on copper for construction of the cavities has an important advantage over the use of solid niobium: Susceptibility to quenching is greatly reduced. Local heat generated by small surface defects or impurities is quickly conducted away by the copper. During the low-power tests, all 21 cavities reached an acceleration field of twice the nominal field without quenching. The niobium-sputtered cavities are insensitive to the Earth's magnetic field, and special magnetic shielding, required for solid-niobium cavities, is not required. Four cavities, each equipped with a helium tank, tuner, higher-order-mode couplers, and a power coupler, are grouped together in a single cryomodule. The conception of the cryomodule is itself modular; all cavities are identical and can be installed in any position. If a problem arises with a cavity, it can be replaced. The cavities are tuned by elastic deformation, which requires pulling on a harness via stainless-steel cables that are wound around a shaft. A stepping motor, fixed to the outside of the cryostat, drives the shaft. The motor therefore works in normal ambient conditions and can be easily accessed for maintenance or repair. Figure 14 shows a four-cavity module during assembly.

5. VACUUM SYSTEM

5.1. Overview

The LHC has three vacuum systems: the insulation vacuum for cryomagnets, the insulation vacuum for helium distribution (the QRL), and the beam vacuum. The insulation vacua before cooldown do not have to be better than 10⁻¹ mbar, but at cryogenic temperatures, in the absence of any significant leak, the pressure stabilizes around 10⁻⁶ mbar. The requirements for the beam vacuum are much more stringent and are driven by the required beam lifetime and background at the experiments. Rather than being expressed as equivalent pressures at room temperature, the requirements at cryogenic temperature are quoted as gas densities normalized to hydrogen and take into account the ionization cross sections for each gas species. Equivalent hydrogen gas densities should remain below 10¹⁵ H₂ m⁻³ to ensure the required 100-h beam lifetime. In the IRs around the experiments, the densities should be below 10¹³ H₂ m⁻³ to minimize the background to the experiments. In the room-temperature parts of the beam-vacuum system, the pressure should be in the range of 10⁻¹⁰ to 10⁻¹¹ mbar.

Numerous dynamic phenomena have to be taken into account in the design of the beam-vacuum system. Synchrotron radiation strikes the vacuum chambers, in particular the arcs, and electron clouds (multipacting) could affect almost the entire ring. Extra care has to be taken during design and installation to minimize these effects, but conditioning with the beam will be required to reach nominal performance.

5.2. Beam-Vacuum Requirements

The design of the beam-vacuum system takes into account the requirements of operation at 1.9 K and the need to shield the cryogenic system from heat sources, as well as the more typical constraints set by vacuum-chamber impedances. Four main heat sources have been identified and quantified at nominal intensity and energy: (a) synchrotron light radiated by the circulating proton beams (0.2 W m⁻¹ per beam, with a critical energy of approximately 44 eV); (b) energy loss by nuclear scattering (30 mW m⁻¹ per beam); (c) image currents (0.2 W m⁻¹ per beam); and (d) energy dissipated during the development of electron clouds, which form when the surfaces seen by the beams have a secondary electron yield that is too high.

Intercepting these heat sources at a temperature above 1.9 K necessitated the introduction of a beam screen. The more classical constraints on the vacuum-system design are set by the stability of the beams, which sets the acceptable longitudinal and transverse impedance (19, 20), and by the background conditions in the IRs.

The vacuum lifetime is dominated by the nuclear scattering of protons on the residual gas. The cross sections for such an interaction at 7 TeV vary with the gas species (21, 22); they are given in Table 2, together with the gas density and pressure (at 5 K) that are compatible with the requested 100-h lifetime. This value ensures that the contribution of beam-gas collisions to the decay of the beam intensity is small compared with other loss mechanisms; it also reduces the energy lost by scattered protons in the cryomagnets to below the nominal value of 30 mW m⁻¹ per beam.

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Table 2

The nuclear scattering cross sections at 7 TeV for different gases and the corresponding densities and equivalent pressures for a 100-h lifetime

5.3. Beam Vacuum in the Arcs and Dispersion Suppressors

The two beams are confined in independent vacuum chambers from one end of the continuous-arc cryostat to the other, extending from Q7 in one octant to Q7 in the next octant. Cold bores (part of the cryomagnets) with an inner diameter of 50 mm are connected together by so-called cold interconnects, which compensate for length variations and alignment errors. A beam-position monitor (BPM), with an actively cooled body, is mounted on each beam in each SSS (i.e., at each quadrupole). An actively cooled beam screen (Figure 15) is inserted into the cold bore of all magnets. The racetrack shape of the beam screen optimizes the available aperture while leaving space for the cooling tubes. The nominal horizontal and vertical apertures are 44.04 mm and 34.28 mm, respectively. Slots, covering a total of 4% of the surface area, are perforated in the flat parts of the beam screen to allow condensation of the gas on surfaces protected from the direct impact of energetic particles (ions, electrons, and photons). The pattern of the slots was chosen to minimize longitudinal and transverse impedance, and their size was chosen to keep the rf losses through the holes below 1 mW m⁻¹. A thin (75-μm) copper layer on the inner surface of the beam screen provides a low-resistance path for the image current of the beam. A sawtooth pattern on the inner surface in the plane of bending helps the absorption of synchrotron radiation.

The beam screen is cooled by two stainless-steel tubes with an inner diameter of 3.7 mm and a wall thickness of 0.53 mm; the tubes allow the extraction of up to 1.13 W m⁻¹ in nominal cryogenic conditions. The helium temperature is regulated to 20 K at the output of the cooling circuit at every half-cell, resulting in a cooling-tube temperature between 5 K and 20 K for nominal cryogenic conditions. The cooling tubes are laser-welded onto the beam-screen tube and are fitted at each end with adaptor pieces that allow their routing out of the cold bore without any fully penetrating weld between the helium circuit and the beam vacuum. Sliding rings with a bronze layer directed toward the cold bore are welded onto the beam screen every 750 mm to ease the insertion of the screen into the cold bore tube, to improve the centering, and to provide good thermal insulation. Finally, because the electron clouds can deposit significant power into the cold bore through the pumping slots, the latter are shielded with copper beryllium shields clipped onto the cooling tubes. The net pumping speed for hydrogen is reduced by a factor of two, which remains acceptable.

5.3.1. Cold interconnects.

Beam-vacuum interconnects ensure continuity of the vacuum envelope and of the helium flow, as well as a smooth geometrical transition between beam screens along the 1,642 twin-aperture superconducting cryomagnets installed in the continuous-arc cryostat. The physical beam envelope must have a low electrical resistance for image currents and must minimize coupled-bunch instabilities. It must also have a low inductance for the longitudinal single-bunch instability. The maximum dc resistance allowed at room temperature for a complete interconnect is 0.1 mΩ. To meet these requirements, a complex interconnect module integrates a shielded bellows to allow thermal expansion, as well as to permit compensation of mechanical and alignment tolerances between two adjacent beam screens. The shielding of the bellows is achieved with a set of sliding contact fingers made out of gold-plated copper-beryllium that slide on a rhodium-coated copper tube.

5.3.2. Beam vacuum in the insertions.

Room-temperature chambers alternate with stand-alone cryostats in the IRs. The room-temperature part includes beam instrumentation, acceleration cavities, experiments, collimation equipment, and the injection and ejection kickers and septa, as well as some of the dipole and quadrupole magnets in which superconducting elements are not used. In these regions, the vacuum systems for the two beams sometimes merge, notably in the four experimental insertions but also in some special equipment such as the injection kickers and some beam stoppers.

5.3.3. Beam screen.

The beam screen is required only in the cold bores of the stand-alone cryostats. It is derived from the arc type but uses a smaller (0.6-mm) steel thickness and comes in various sizes to match different cold bore diameters. The orientation of the beam screen in the cold bore is adapted to the aperture requirements, which means that the flat part with the cooling tubes can be either vertical or horizontal. The sawtooth structure was abandoned for these beam screens because (a) the synchrotron radiation hitting the screen in these regions is at least 10 times less intense than in the arc and (b) fitting the sawteeth at the appropriate location of the beam screen would be too expensive.

5.3.4. Room-temperature beam vacuum in the field-free regions.

The baseline for the room-temperature beam-vacuum system is to use 7-m-long oxygen-free copper chambers that have an inner diameter of 80 mm and are fitted with standard DN100 Conflat™ flanges. The thickness of the copper is 2 mm, and the chambers are coated with TiZrV nonevaporable getter (23). Following activation at low temperature (200°C), the getter provides distributed pumping and low outgassing to maintain a low residual gas pressure, as well as a low secondary electron-emission yield to avoid electron multipacting. The chambers are connected by means of shielded bellows modules, some of which include pumping and diagnostic ports.

6. CRYOGENIC SYSTEM

6.1. Overview

The superconducting-magnet windings in the arcs, the DSs, and the inner triplets are immersed in a pressurized bath of superfluid helium at approximately 0.13 MPa (1.3 bar) and a maximum temperature of 1.9 K (24), allowing a sufficient temperature margin for heat transfer across the electrical insulation. As the specific heat of the superconducting alloy and its copper matrix falls rapidly with decreasing temperature, the full benefit in terms of stability margin of operation at 1.9 K (instead of at the conventional 4.5 K) may be gained only by effectively using the transport properties of superfluid helium, for which the temperature of 1.9 K also corresponds to a maximum in the effective thermal conductivity. The low bulk viscosity enables the coolant to permeate the heart of the magnet windings. The large specific heat (typically 10⁵ times that of the conductor per unit mass, 2 × 10³ times per unit volume), combined with the enormous heat conductivity at moderate flux (3,000 times that of cryogenics-grade oxygen-free high-conductivity copper, peaking at 1.9 K), can have a powerful stabilizing effect on thermal disturbances. To achieve this effect, the electrical insulation of the conductor must preserve sufficient porosity and provide a thermal percolation path while fulfilling its demanding dielectric and mechanical duties.

The cryogenic system must be able to cope with the load variations and a large dynamic range induced by the operation of the accelerator; it must also be able to cool down and fill the huge cold mass (37 × 10⁶ kg) of the LHC within a maximum delay of 15 days while avoiding thermal differences higher than 75 K in the cryomagnet structure. Further, the cryogenic system must be able to cope with resistive transitions of the superconducting magnets, which occasionally occur in the machine, while minimizing loss of cryogen and system perturbations. It must handle the resulting heat release and its consequences, which include fast pressure rises and flow surges. The system must limit the propagation to neighboring magnets and recover in a time period that does not seriously affect the operational availability of the LHC. A resistive transition extending over one lattice cell should not result in a downtime of more than a few hours. The cryogenic system must also be able to rapidly warm up and cool down limited lengths of the lattice for magnet exchange and repair. Finally, it must be able to handle, without endangering the safety of personnel or equipment, the largest credible incident of the resistive transition of a full sector. The system is designed to have some redundancy in its subsystems.

6.2. General Architecture

The main constraints on the cryogenic system arise from the need to install the system in the existing LEP tunnel and to reuse LEP facilities, including four refrigerators. The limited number of access points to the underground area is reflected in the architecture of the system. The cooling power required at each temperature level is produced by eight refrigeration plants and is distributed to the adjacent sectors over distances up to 3.3 km. To simplify the magnet string design, the cryogenic headers that distribute the cooling power along a machine sector, as well as all remaining active cryogenic components in the tunnel, are contained in a QRL. The QRL runs alongside the cryomagnet strings in the tunnel and feeds each 106.9-m-long lattice cell in parallel via a jumper connection (Figure 16).

The LHC tunnel is inclined at 1.41% with respect to the horizontal, thereby yielding height differences of up to 120 m across the tunnel diameter. This slope generates hydrostatic heads in the cryogenic headers and could generate flow instabilities in two-phase, liquid-vapor flow. To avoid these instabilities, all fluids should be transported over large distances in a monophase state, that is, in the superheated-vapor or supercritical region of the phase diagram. Local two-phase circulation of saturated liquid can be tolerated over limited lengths in a controlled direction of circulation. Equipment is installed aboveground whenever possible to avoid the need for further excavation, but certain components have to be installed underground near the cryostats. For reasons of safety, the use of nitrogen in the tunnel is forbidden, and the discharge of helium is restricted to small quantities.

Figure 17 shows the general layout of the cryogenic system. There are five so-called cryogenic islands at Points 1, 2, 4, 6, and 8, at which all refrigeration equipment and ancillary equipment are concentrated. Equipment at ground level includes electrical substations, warm compressors, cryogen storage (helium and liquid nitrogen), cooling towers, and cold boxes. Underground equipment includes lower cold boxes, refrigeration-unit boxes at 1.8 K, interconnecting lines, and interconnection boxes. Each cryogenic island houses one or two refrigeration plants that feed one or two adjacent tunnel sectors, which requires distribution and recovery of the cooling fluids underground over distances of 3.3 km.

6.3. Temperature Levels

Given the high thermodynamic cost of refrigeration at 1.8 K, the LHC cryogenic components have been designed to intercept the main heat influx at higher temperatures, hence the multistage temperature levels in the system. The temperature levels are (a) 50 K to 75 K for the thermal shield protecting the cold masses; (b) 4.6 K to 20 K for lower-temperature interception and for cooling the beam screens that protect the magnet bores from beam-induced loads; (c) 1.9-K quasi-isothermal superfluid helium for cooling the magnet cold masses; (d) 4 K, at very low pressure, for transporting the superheated helium flow coming from the distributed 1.8-K heat-exchanger tubes across the sector length to the 1.8-K refrigeration units; (e) 4.5-K normal saturated helium for cooling some insertion-region magnets, rf cavities, and the lower sections of the high-temperature superconducting current leads; and (f) 20 K to 300 K for cooling the resistive upper sections of the high-temperature superconducting current leads (4). These temperature levels are attained through the use of helium in several thermodynamic states. The cryostats and QRL combine several techniques for limiting heat influx, such as low-conduction support posts, insulation vacuum, multilayer reflective insulation wrapping, and low-impedance thermal contacts, all of which have been successfully applied on an industrial scale.

7. BEAM INSTRUMENTATION

An accurate and complete set of beam instrumentation is essential for efficient commissioning and operation of the LHC. This instrumentation includes beam-position measurement all around the ring, beam-loss monitors, current and profile measurements, and specialized instrumentation to measure beam properties such as chromaticity and tune.

7.1. Beam-Position Measurement

The majority (860 of 1,032) of the LHC BPMs are of the arc type (Figure 18), consisting of four 24-mm-diameter button-electrode feed-throughs mounted orthogonally in the beam pipe with an inner diameter of 48 mm. The electrodes are curved to follow the beam-pipe aperture and are retracted by 0.5 mm to protect the buttons from direct synchrotron radiation from the main bending magnets. Each electrode has a capacitance of 7.6 ± 0.6 pF and is connected to a 50-Ω coaxial, glass-ceramic, ultrahigh-vacuum feed-through.

The inner triplet BPMs in all IRs are equipped with 120-mm, 50-Ω directional stripline couplers that can distinguish between counterrotating beams in the same beam pipe. The locations of these BPMs (in front of Q1, in the Q2 cryostat, and after Q3) were chosen to be as far as possible from parasitic crossings to optimize the directivity. The 120-mm stripline length was chosen to provide a signal similar to that obtained through the button electrode, thereby allowing use of the same acquisition electronics as for the arcs. The cold directional couplers use an Ultem® dielectric intended for a cryogenic environment, whereas the warm couplers use a Macor® dielectric to allow bake-out to above 200°C.

The cleaning insertions in Points 3 and 7 are equipped with warm, 34-mm-diameter, button-electrode BPMs fitted to either side of the MQWA magnets. The electrodes are an enlarged version of the arc BPM button. The same button electrodes are also used for the cold BPMs in the MSs on either side of the four IRs, as well as for the warm BPMs located near the D2 magnets and on either side of the transverse damper. The BPMs installed in Point 4 are combined monitors consisting of (a) one BPM using standard 24-mm button electrodes for use by the orbit system and (b) one BPM using 150-mm shorted stripline electrodes for use by the transverse damper system.

7.2. Beam-Current Measurement

Two types of beam-current transformers provide intensity measurements for the beams circulating in the LHC rings, as well as for the transfer lines from the Super Proton Synchrotron to the LHC and from the LHC to the dumps. All the transformers are installed in sections where the vacuum chamber is at room temperature and where the beams are separated.

The fast beam-current transformers can integrate the charge of each LHC bunch. This process provides good accuracy for both bunch-to-bunch measurements and average measurements intended mainly for low-intensity beams, for which the accuracy of the dc current transformers (DCCTs) will be limited. For redundancy, two transformers with totally separate acquisition chains will be placed in each ring and will be located at Point 4. The measurement precision for the pilot beam of 5 × 10⁹ protons in a single bunch is approximately 5% (10% in the worst-case scenario), and for the nominal beam the measurement precision is below 1%.

The DCCTs are based on the principle of magnetic amplifiers; they measure the mean intensity or current of the circulating beam and can be used to measure the beam lifetime. Because of their operational importance, two of these devices will be installed in each ring. Currently, a resolution of 2 μA can be reached, but a resolution of 1 μA, corresponding to 5 × 10⁸ circulating particles, is the target.

7.3. Beam-Loss System

The loss of a very small fraction of the circulating beam may induce a quench of the superconducting magnets or even physical damage to machine components. The detection of the lost beam protons allows for the protection of the equipment against quenches and damage through generation of a beam-dump trigger when the losses exceed thresholds. In addition to providing the advantages of quench prevention and damage protection, loss detection allows for the observation of local aperture restrictions, orbit distortion, beam oscillations, and particle diffusion.

Loss measurement is based on the detection of secondary shower particles by use of ionization chambers located outside of the magnet cryostats. The secondary particle energy flux is linear with the initiating protons' parameters. To observe a representative fraction of the secondary particle flux, detectors are placed at likely loss locations. Calibration of the damage and quench level thresholds, with respect to the measured secondary particle energy deposition, is simulation based.

7.4. Transverse Profile Measurement

User requirements led to the definition of four functional modes to be mapped onto the different types of hardware monitors. These modes are as follows.

1.	A single-pass monitor of high sensitivity (pilot beam), with a modest demand on accuracy and few restrictions on the beam blowup due to the traversal.
2.	A so-called few-pass monitor (typically 20 turns) dedicated to the intermediate-to-nominal-intensity range of the injected beam for calibration or matching studies. The blowup per turn should be small compared with the effect to be measured.
3.	A circulating beam monitor that works over the entire intensity range. No blowup is expected from such a monitor.
4.	A circulating beam-tail monitor optimized to scan low beam densities. In this mode, one may not be able to measure the core of the beam. The measurement should not significantly disturb the tail density.

The monitor types include wire scanners, residual gas ionization monitors, and synchrotron light monitors using light from D2-type superconducting dipoles. Synchrotron light monitors for using light from superconducting undulators in each ring are also under development. Point 4 is the default location for all such instrumentation.

7.5. Tune, Chromaticity, and Betatron Coupling

Reliable measurement of betatron tune and the related quantities tune spread, chromaticity, and betatron coupling is essential for all phases of LHC running, from commissioning to full-performance luminosity runs. For injection and ramping, the fractional part of the betatron tune must be controlled to ±0.003, whereas in collision, the required tolerance shrinks to ±0.001. With the exception of Schottky scans and the ac-dipole excitation outside the tune peak, all tune-measurement techniques involve some disturbance to the beam. The resulting emittance increase, although acceptable for some modes of running, has to be strongly limited for full-intensity physics runs. Therefore, different tune-measurement systems have been installed.

7.5.1. General tune-measurement system.

The general tune-measurement system makes use of standard excitation sources (single-kick, chirp, slow swept frequency, and noise). It operates with all filling patterns and bunch intensities. Even with oscillation amplitudes down to 50 μm, a certain increase in the emittance results, which limits the frequency at which measurements can be made. It is therefore unsuitable for generating measurements for an online tune-feedback system.

7.5.2. High-sensitivity tune-measurement system.

The beam is excited by applying a signal of low amplitude and high frequency to a stripline kicker. This frequency is close to half the bunch-spacing frequency (40 MHz for the nominal 25-ns bunch spacing). The equivalent oscillation amplitude is a few micrometers or less for a β function of approximately 200 m. A notch filter in the transverse feedback loop suppresses the loop gain at this frequency, at which instabilities are not expected to be a problem. If the excitation frequency divided by the revolution frequency corresponds to an integer plus the fractional part of the tune, then coherent betatron oscillations of each bunch build up turn by turn (this effect is termed resonant excitation). A batch structure with a bunch every 25 ns "carries" the excitation frequency as sidebands of the bunch-spacing harmonics. A beam-position pickup is tuned to resonate at one of these frequencies.

8. COMMISSIONING AND OPERATION

By September 10, 2008, seven of the eight sectors of the LHC had been successfully commissioned to 5.5 TeV in preparation for a run at 5 TeV. Due to lack of time, the eighth sector had been taken only to 4 TeV. Beam commissioning started by threading Beam 2, the beam traveling counterclockwise around the ring, stopping it at each long straight section sequentially to correct the trajectory. In less than an hour, the beam had completed a full turn, which was witnessed by a second spot on a fluorescent screen intercepting both injected and circulating beams (Figure 19).

A beam circulating for a few hundred turns was quickly established. Figure 20 shows the capture process when the rf cavities are switched on. Each horizontal line on the mountain-range display records the bunch intensity every 10 turns. Without the rf, the beam debunches as it should in ∼250 turns, or 25 ms. An initial attempt to capture the beam was made, but as shown in the figure, the injection phase was completely wrong. Adjusting the phase allowed a partial capture, but at the wrong frequency. Adjusting the frequency finally resulted in a perfect capture. The closed orbit was then corrected. Figure 21 shows the first orbit correction, where—remarkably, at this early stage—the rms orbit was less than 2 mm. In the horizontal plane, the mean orbit is displaced radially by ∼1 mm, indicating an energy mismatch of ∼10⁻³.

On March 30, 2010, the first collisions were obtained at a center-of-mass energy of 7 TeV. Since then, operating time has been split between machine studies and physics data taking.

Given the very large stored energy in the beams, particular attention must be paid to the machine-protection and collimation systems. More than 120 collimators are arranged in a hierarchy of primary, secondary, and tertiary collimators. Tight control of the orbits in the region of the collimators is achieved with a feedback system.

The collimation system also works very efficiently. Figure 22 shows a loss map around the ring, obtained by provoking beam loss. The losses are located precisely where they should be, with a factor-of-10,000 difference between the losses on the collimators and those in the cold regions of the machine.

The machine's performance at this early stage has been very impressive. A single-beam lifetime of more than 1,000 h—an order of magnitude better than expected—has been observed, which proves that the vacuum is considerably better than expected and that the noise level in the rf system is very low. The nominal bunch intensity of 1.1×10¹¹ has been exceeded, and the β* at the experimental collision points has been squeezed to 2 m. The closed orbit can be kept to better than 1 mm rms, with very good reproducibility. During the 200 days of running in 2010, the luminosity increased by five orders of magnitude to 2.1 × 10³² cm⁻² s⁻¹, with a bunch separation of 150 ns. More than 6 pb⁻¹ of integrated luminosity have been produced in each of the two large detectors.

disclosure statement

The author is not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.

acknowledgments

The LHC is the most complex scientific instrument ever constructed. It has taken 15 years to build, and many problems have been encountered on the way. All these problems have been overcome thanks to the resourcefulness and resilience of the people who built it, both inside CERN and in our collaborating laboratories around the world. Now the machine is moving into its operational phase, and I am confident that an equally competent team will exploit it to its full potential over the coming years.

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Text How the Damage to the Lhc Was Repaired

Source: https://www.annualreviews.org/doi/10.1146/annurev-nucl-102010-130438

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