Large Hadron Collider Magnets Research Articles

Stability modeling of the LHC Nb-Ti Rutherford cables subjected to beam losses

The Large Hadron Collider (LHC) at CERN is being prepared for its full energy exploitation during run III, i.e., an increase of the beam energy beyond the present 6.5 TeV, targeting the maximum discovery potential attainable. This requires an increase of the operating field of the superconducting dipole and quadrupole magnets, which in turn will result in more demanding working conditions due to a reduction of the operating margin while the energy deposited by particle loss will increase. Beam-induced magnet quenches, i.e., the transition to normal conducting state, will become an increasing concern, because they could affect the availability of the LHC. It is hence very important to understand and be able to predict the quench levels of the main LHC magnets for the required values of current and generated magnetic fields. This information will be used to set accurate operating limits of beam loss, with sufficient but not excessive margin, so to achieve maximal beam delivery to the experiments. In this study we used a one-dimensional, multistrand thermal-electric model to analyze the maximum beam losses that can be sustained by the LHC magnets, still remaining superconducting. The heat deposition distribution due to the beam losses is given as an input for the stability analysis. Critical elements of the model are the ability to capture heat and current distribution among strands, and heat transfer to the superfluid helium bath. The computational model has been benchmarked against energy densities reconstructed from beam-induced main dipole quenches during LHC operation at 6.5 TeV. The model was then used to evaluate the stability margin of both main dipole and main quadrupole magnets at different beam energies, up to the expected ultimate operating energy of the LHC, 7.5 TeV. The comparison between the quench levels underlines how the increase of beam energy implies a substantial reduction of magnets stability and will require much stricter setting on the allowable beam losses to avoid resistive transitions during operation.

Using LSTM recurrent neural networks for monitoring the LHC superconducting magnets

The superconducting LHC magnets are coupled with an electronic monitoring system which records and analyzes voltage time series reflecting their performance. A currently used system is based on a range of preprogrammed triggers which launches protection procedures when a misbehavior of the magnets is detected. All the procedures used in the protection equipment were designed and implemented according to known working scenarios of the system and are updated and monitored by human operators.This paper proposes a novel approach to monitoring and fault protection of the Large Hadron Collider (LHC) superconducting magnets which employs state-of-the-art Deep Learning algorithms. Consequently, the authors of the paper decided to examine the performance of LSTM recurrent neural networks for modeling of voltage time series of the magnets. In order to address this challenging task different network architectures and hyper-parameters were used to achieve the best possible performance of the solution. The regression results were measured in terms of RMSE for different number of future steps and history length taken into account for the prediction. The best result of RMSE=0.00104 was obtained for a network of 128 LSTM cells within the internal layer and 16 steps history buffer.

Electromechanical Analysis of the Busbar Support System in the LHC Electrical Feed Boxes and Their Consolidation With Remotely Installed Clamps

The Large Hadron Collider (LHC) main superconducting circuits are powered by cryogenic electrical feed boxes located at each end of the eight arc segments of the LHC. Preliminary electromechanical analysis identified poorly supported busbars in the cryostat interfacing with the LHC magnets and supporting the beam pipes. To account for the nonlinear electromechanical behavior of the busbars, a refined iterative method of analysis was developed to accurately predict their displacement and improve the support system design. Inspection of the busbars at the start of the first long shutdown of the LHC confirmed the calculated displacements, and the decision was taken to consolidate the support system by installing clamps on the busbars. To avoid deforming the cryostat and misaligning the beam pipes, clamps were developed for remote installation through the existing busbar ducts. They were validated with a specific test program to ensure their structural, cryogenic, and electrical performance. Sixteen clamps were successfully installed in the eight feed boxes of the LHC using an endoscope and specifically developed tooling.

Integration of the 11-T Nb3Sn Dipoles and Collimators in the LHC

The Large Hadron Collider (LHC) collimation system upgrade plan comprises new collimators in the dispersion suppressors. The length required for each collimator along the LHC lattice is obtained by replacing an LHC main dipole and its cryostat with two shorter but stronger 11-T Nb 3 Sn magnets keeping the equivalent integrated field of the dipole removed. This requires a modification of the continuous cryostat, in order to create room-temperature beam vacuum sectors for the integration of the new collimators. In this paper, we present a new cryostat designed to allow the installation of a collimator between the 11-T magnets, while ensuring the continuity of the cryogenics, vacuum, and magnet powering systems of the LHC continuous cryostat. Challenging constraints, in terms of fabrication, alignment, and space, led to the development of a cryostat composed of three independent modules. Two of the modules house the 11-T dipole cold masses, which are cooled in the same 1.9-K pressurized superfluid helium bath of the main dipoles. These make use of the same design features of the LHC magnet cryostats, in order to contain construction and assembly costs and benefit from well-established procedures. A third module, which is placed between the two magnets, is equipped with cold to warm transitions on the beam lines and creates the space for the collimator between the vacuum vessel of the two 11-T magnet cryostats. The main functionalities, requirements, and implemented design solutions for this new cryostat are presented and discussed, in the context of the challenging integration in the LHC continuous cryostat and its tunnel.

The magnetic behaviour of the LHC at 6.5 TeV

After two years of shutdown, the Large Hadron Collider (LHC) operated in 2015 at an energy of 6.5 TeV. In this paper, we give the first outlook of the behavior of the LHC magnets operating at this field level, corresponding to 8-T peak field in the main dipoles. The main magnetic features are reconstructed through the beam measurements, mainly the tune (quadrupolar components) and the chromaticity (sextupolar components). The decay and snapback at injection and the start of the ramp are expected to increase by 50% due to the higher operational current. The behavior at high field will be affected by the saturation components, which were barely visible in one magnet family in the 4-TeV runs and totally invisible in the rest. Although the initial plan aims at a rather conservative value of β*, during the squeeze, the triplet and matching section quadrupole will become important. Beam measurements will provide interesting information about the magnetic behavior of these quadrupoles; in particular, through beta beating, we will estimate the absolute precision of the knowledge of the quadrupole strength. The statistics of the beam measurements will be rather limited in the first stages of commissioning, but should already reveal the main features of the 6.5-TeV operation.

Composition and connectivity variability of the A15 phase in PIT Nb3Sn wires

Powder-in-tube (PIT) Nb3Sn wires are competing with Restacked-Rod-Process (RRP®) for the realization of the high luminosity upgrade of the Large Hadron Collider (LHC) at CERN. These two conductors have different properties and microstructures that are in both cases averages of an inhomogeneous A15 microstructure. PIT has in general a smaller fraction of A15 in the non-Cu cross-section than RRP® and a lower non-Cu Jc (12 T, 4.2 K) (2500–2700 A mm−2 versus 2900–3000 A mm−2) but it can be made in smaller filament diameters, which is an important property for LHC magnets. Another characteristic of PIT A15 is that ∼25% is made up of ∼1–2 μm sized grains (typically ∼10 times the small grain (SG) diameter) and their contribution to transport is uncertain. Here we studied a 192 filament Ta-doped, 1 mm diameter PIT wire and combined multiple characterization techniques in order to distinguish the different wire components, to determine their individual properties and to identify which components are current-carriers. We found multiple evidence that the large A15 grains, which are also the highest-Tc grains, do not contribute to transport at high field and that the only current-carrying A15 is the SG with Tc <17.7 K. However, because of the high density of grain boundaries in the SG A15 layer, PIT has an exceptionally high SG-layer Jc and high specific grain boundary pinning force, QGB. These findings clearly show that it is essential to increase the ratio of small to large and disconnected grains in order to improve PIT performance.

Deduction of Steady-State Cable Quench Limits for Various Electrical Insulation Schemes With Application to LHC and HL-LHC Magnets

Undesired quenches of superconducting magnets can be a limiting factor for the operation of the Large Hadron Collider (LHC) accelerator, both for its forthcoming exploitation at full energy as well as for its future upgrades. An accurate knowledge of the quench limit, the maximum amount of heat deposit the magnets can withstand, is required to be able to prevent beam induced quenches. In this paper we provide an overview of the heat extraction through the multitude of cable insulation schemes used in particle accelerators in the past 20 years and foreseen for the coming years. Based on the relevant heat transfer measurements, we deduce steady-state cable quench limits both for the LHC Nb-Ti magnets and for the future HL-LHC Nb3Sn ones. We provide them for different operating conditions and different locations within the coil.

Development of superconducting links for the Large Hadron Collider machine

In the framework of the upgrade of the Large Hadron Collider (LHC) machine, new superconducting lines are being developed for the feeding of the LHC magnets. The proposed electrical layout envisages the location of the power converters in surface buildings, and the transfer of the current from the surface to the LHC tunnel, where the magnets are located, via superconducting links containing tens of cables feeding different circuits and transferring altogether more than 150 kA. Depending on the location, the links will have a length ranging from 300 m to 500 m, and they will span a vertical distance of about 80 m. An overview of the R&D program that has been launched by CERN is presented, with special attention to the development of novel types of cables made from MgB2 and high temperature superconductors (Bi-2223 and REBCO) and to the results of the tests performed on prototype links. Plans for future activities are presented, together with a timeline for potential future integration in the LHC machine.

Low uncertainty position transducers for the LHC collimators: tutorial 49

The Large Hadron Collider (LHC) is the largest particle accelerator ever built. The transverse energy density of the nominal beam is 1000 times higher than what was previously achieved in proton storage rings. Tiny fractions of the stored beam suffice to quench a super-conducting LHC magnet (i.e., a situation where part of the magnet exits the superconducting state) or even to destroy parts of the accelerator. The energy in the two LHC beams is sufficient to melt almost one ton of copper.

Spallation-driven Ultracold Neutron Sources: Concepts for a Next Generation Source

Abstract We present concepts for a next-generation, spallation-driven ultracold neutron source. Our source is based on a 40 liter volume of liquid He held at 1.6 K, with heat removed by “sub-cooled” He technology developed for the Large Hadron Collider's magnet systems. We report on neutronics modeling for two geometries which utilize well-vetted scattering and absorption data developed for the Lujan Center Mark-3 target, as well as promising moderator materials for the cold neutron “pre-moderator” for this source.

Slip–Stick Mechanism in Training the Superconducting Magnets in the Large Hadron Collider

Superconducting magnets can exhibit training quenches during successive powering to reaching nominal performance. The slip-stick motion of the conductors is considered to be one of the mechanisms of training. In this paper, we present a simple quantitative model where the training is described as a discrete dynamical system matching the equilibrium between the energy margin of the superconducting cable and the frictional energy released during the conductor motion. The model can be explicitly solved in the linearized case, showing that the short sample limit is reached via a power law. Training phenomena have a large random component. A large set of data of the large hadron collider magnet tests is postprocessed according to previously defined methods to extract an average training curve for dipoles and quadrupoles. These curves show the asymptotic power law predicted by the model. The curves are then fit through the model, which has two free parameters. The model shows good agreement over a large range, but it fails to describe the very initial part of the training.

Mathematical formulation to predict the harmonics of the superconducting Large Hadron Collider magnets: III. Precycle ramp rate effects and magnet characterization

The Large Hadron Collider (LHC) at CERN is equipped with a feed-forward control system known as the field description for the LHC (FiDeL) which is designed to predict the magnetic field and its multipoles, hence reducing the burden on beam based feedback. FiDeL consists of a physical and empirical parametric field model based on magnetic measurements at warm and in cryogenic conditions. It is particularly critical during beam injection when the field decays and at the beginning of acceleration when the field snaps back. It is known that the decay amplitude is largely affected by the powering history of the magnet, particularly by the precycle flattop current and duration and the preinjection preparation duration. Recently, we have collected data that quantify the dependence of the decay amplitude on the precycle ramp rate. This paper presents the results of the measurements performed to investigate this effect, and the method included in FiDeL to model the precycle dependence. With this complete picture of dynamic changes, we finally discuss the effect on the data taken at nominally constant field, along the magnet loadline. We show that a correction for dynamic changes is required for adequate magnet characterization.

Modeling of Quench Limit for Steady State Heat Deposits in LHC Magnets

A quench, the transition of a conductor from the superconducting to the normal conducting state, occurs irreversibly in the accelerator magnets if one of the three parameters: temperature, magnetic field or current density exceeds a critical value. Energy deposited in the superconductor by the particle beams provokes quenches detrimental for the accelerator operation. In particular if particles impacting on the vacuum chamber and their secondary showers depose energy in the magnet coils. The large hadron collider (LHC) nominal beam intensity is 3.2 ldr 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> protons. A quench occurs if a fraction of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> protons per second is lost locally. A network model is used to simulate the thermodynamic behavior of the magnets. The heat flow in the network model was validated with measurements performed in the CERN magnet test facility. A steady state heat flow was introduced in the coil by using the quench heaters implemented in the LHC magnets. The value of the heat source current is determined by the network model and the magnet coil current which is required to quench the coil is predicted accordantly. The measured and predicted value comparison is regarded as a sensitive test of the method.

Mathematical formulation to predict the harmonics of the superconducting Large Hadron Collider magnets. II. Dynamic field changes and scaling laws

A superconducting particle accelerator like the LHC (Large Hadron Collider) at CERN, can only be controlled well if the effects of the magnetic field multipoles on the beam are compensated. The demands on a control system solely based on beam feedback may be too high for the requirements to be reached at the specified bandwidth and accuracy. Therefore, we designed a suitable field description for the LHC (FIDEL) as part of the machine control baseline to act as a feed-forward magnetic field prediction system. FIDEL consists of a physical and empirical parametric field model based on magnetic measurements at warm and in cryogenic conditions. The performance of FIDEL is particularly critical at injection when the field decays, and in the initial part of the acceleration when the field snaps back. These dynamic components are both current and time dependent and are not reproducible from cycle to cycle since they also depend on the magnet powering history. In this paper a qualitative and quantitative description of the dynamic field behavior substantiated by a set of scaling laws is presented.

Review of Quench Performance of LHC Main Superconducting Magnets

The regular lattice of the large Hadron collider (LHC) will make use of more than 1600 main magnets and about 7600 corrector magnets, all superconducting and working in pressurized superfluid helium bath. This complex magnet system will fill more than 20 km of the LHC underground tunnel. In this paper an overview of the cold test program and quality assurance plan to qualify all LHC superconducting magnets will be presented. The quench training performance of more than 1100 LHC main dipoles and about 300 main quadrupoles, cold tested to date, will be reviewed. From these results an estimate of the number of quenches that will be required to start operation of the whole machine at nominal energy will be discussed. The energy level at which the machine could be operated at the early phase of the commissioning without being disturbed by training quenches will be addressed. The LHC magnet program required the development of many new tools and techniques for the testing of superconducting magnet coils, magnet protection systems, cryogenics, and instrumentation. This paper will also present a summary of this development work and the results achieved.

Mathematical formulation to predict the harmonics of the superconducting Large Hadron Collider magnets

CERN is currently assembling the LHC (Large Hadron Collider) that will accelerate and bring in collision 7 TeV protons for high energy physics. Such a superconducting magnet-based accelerator can be controlled only when the field errors of production and installation of all magnetic elements are known to the required accuracy. The ideal way to compensate the field errors obviously is to have direct diagnostics on the beam. For the LHC, however, a system solely based on beam feedback may be too demanding. The present baseline for the LHC control system hence requires an accurate forecast of the magnetic field and the multipole field errors to reduce the burden on the beam-based feedback. The field model is the core of this magnetic prediction system, that we call the field description for the LHC (FIDEL). The model will provide the forecast of the magnetic field at a given time, magnet operating current, magnet ramp rate, magnet temperature, and magnet powering history. The model is based on the identification and physical decomposition of the effects that contribute to the total field in the magnet aperture of the LHC dipoles. Each effect is quantified using data obtained from series measurements, and modeled theoretically or empirically depending on the complexity of the physical phenomena involved. This paper presents the developments of the new finely tuned magnetic field model and, using the data accumulated through series tests to date, evaluates its accuracy and predictive capabilities over a sector of the machine.

Industrialisation of the insulation vacuum barrier for the large hadron collider (LHC) magnet cryostats

The insulation vacuum (<10 −4 Pa) of the large hadron collider magnet cryostats, thermally protecting the superconducting magnets which operate at 1.9 K in superfluid helium, is divided in to 214 m long segments separated by means of insulation vacuum barriers. The insulation vacuum barrier is a leak-tight stainless steel welded structure, composed of two concentric corrugated cylinders and one internal bellows linked together by a 6 mm thick central plate. As the vacuum barrier mechanically links the cryostat vacuum vessel operating at ambient temperature and the 1.9 K superconducting magnets, it is designed to have minimum heat conductivity. Conduction heat in-leak is intercepted at 65 K by a high-purity copper ring brazed onto the stainless steel central plate and thermally linked to a cryogenic line by a copper–aluminium soldering. The thermal performance has been experimentally validated by cryogenic testing. This paper presents the results obtained after industrialisation, manufacture and testing of prototypes and series units. Qualification of leak-tight welds in thin-sheet stainless steel (thickness 0.15–1.3 mm) has been carried out. Ultrasonic testing is performed on all brazing and soldering. Helium leak testing is performed, using dedicated tooling, to ensure a leak-tightness to a rate better than 10 −9 Pa m 3 s −1.

Correlation between voltage–current relation and current distribution in superconducting cables

For many years it has been known that the properties and operating margins of superconducting magnets are influenced by the distribution of transport and induced currents among the strands in the cables with which the magnet is wound. In FRESCA, CERN’s test station for measuring electrical cable properties at 1.9 and 4.3 K, the voltage–current curve of Rutherford type cables and the homogeneity of the current distribution can be measured simultaneously. The longitudinal variation of the cable self-field is measured along one twist pitch, and taken as a measure for the variation of the strand currents. A change in the background field, which is applied non-uniformly along the length of the cable, will induce boundary induced coupling currents. These coupling currents decay very slowly and cause a variation in the strand currents, resulting in a different electro-magnetic behaviour of the cable, even long after the field sweep has finished. In this paper the experimental results on about 40 cables for large hadron collider magnets are presented in terms of quench current and apparent critical current and n-value. The measurements will be sustained by calculations.

Heat transfer in electrical insulation of LHC cables cooled with superfluid helium

The electrical insulation of the Large Hadron Collider (LHC) cables constitutes a thermal barrier between the conductor and the superfluid helium bath. This can prevent removal of the heat dissipated in the cable by the current rise in the dipoles or by the beam losses. The main experimental results, obtained with stacks of insulated conductors representing a piece of the actual coil, are given. The mock-ups vary only by the material composition and the structure of the electrical insulation. Analysis of the temperature distribution measured in the conductors as a function of the dissipated heat power makes it possible to determine the dominant heat transfer mode in each type of tested insulation and to classify these according to their permeability to superfluid helium. Thermal numerical modelling of the experimental mock-ups clarifies the heat transfer path in the complex structure of the insulation and enables calculating values of the thermal quantities characteristic of each insulation. The results of these studies have led to the choice of the cable insulation of the LHC magnets.

Status of the Large Hadron Collider and magnet program

The Large Hadron Collider (LHC), approved by the CERN Council in December 1994, is a 7+7 TeV proton accelerator-collider, to be installed in the existing 27 km long LEP tunnel. It will represent a unique research facility for particle physics, allowing proton-proton collisions with a luminosity of 10/sup 34/ cm/sup -2/ s/sup -1/ capable of providing also heavy ion (Pb-Pb) collisions with a luminosity of 10/sup 27/ cm/sup -2/ s/sup -1/, using the existing CERN heavy ion source. The main technological challenges of the machine are the superconducting magnet system, in total over 8000 magnet units immersed in superfluid helium, with the lattice dipoles operating at 8.4 T, and the very large cryogenic system, which maintains the entire string of cryomagnets at its working temperature below 2 K. The paper discusses briefly the main issues which have led to the present layout of the LHC, gives an overview of the different machine components and characteristics and describes in more detail the recent development work and results of the LHC magnet program.