LHC Dipole Research Articles

IRIS - The Italian research infrastructure on Applied Superconductivity for Particle Accelerators and Societal Applications

The Italian Minister for University and Research has recently funded a large program for an Innovative Research Infrastructure on applied Superconductivity (IRIS) in Italy. Based on the LASA laboratory in Milan, it is a partnership, in the form of a strongly coordinated work, of existing laboratories of various institutes: INFN (leader, participating with 4 labs: Frascati, Genoa, Milan, Salerno); CNR (SPIN institute in Genoa, Naples and Salerno); five Universities: Genoa, Milan, Naples, Salento and Salerno. IRIS will be an upgrade of existing infrastructures, with new state-of-the-art instruments, reinforcing the capability of Italy in the domain of superconductivity aimed to accelerators. IRIS foresees a strong coordination of the activity of the participating laboratories until 2035, at least, thus enhancing the participation of Italian laboratories to future projects requiring advanced superconducting technology, like FCC or the Muon-Collider, and also for developing societal applications of technologies, pursued for high-energy accelerators, especially for the energy domain and the medical sector. In this paper, we present the two novel demonstrators, part of the initial IRIS program: 1) a green superconducting line, 130 m long and designed for 40 kA current capability at 25 kV; 2) a 1 m long HTS dipole magnet with some characteristics similar to LHC dipoles: 10 T, 50 mm × 80 mm bore, but operating at 20 K rather than 1.9 K.

Cryogenic performances of a heat exchanger prototype suitable for the superconducting HL-LHC recombination dipole D2

In the framework of the future High Luminosity upgrade of the Large Hadron Collider (HL-LHC) at CERN, most superconducting magnets in the Long Straight Sections will be replaced. Among them, the new D2 recombination dipole will be a He II conduction-cooled magnet with a larger aperture than the LHC dipoles. To provide the required cooling (up to 70 W) to the D2 and to comply with its cryostat integration constraints, a compact heat exchanger was designed by the CEA Département des Systèmes Basses Températures (DSBT) based on a CERN preliminary analysis and a CEA review of the possible cooling schemes. This heat exchanger provides the required heat transfer between the He II pressurized bath and the He II saturated bath to cool the D2 magnet in different operating conditions at 1.8 K and 2 K. The detailed design of the heat exchanger was defined and one prototype was manufactured by industry under the CEA supervision. The heat exchanger prototype is composed of roughly one hundred oxygen-free high purity copper tubes, electron beam welded to the stainless steel He II bath enclosure. The present paper describes the successful cryogenic performance tests of the prototype of the D2 heat exchanger measured in the CEA 400W@1.8K test facility in Grenoble.

Optical Polarimetry for Fundamental Physics

Sensitive magneto-optical polarimetry was proposed by E. Iacopini and E. Zavattini in 1979 to detect vacuum electrodynamic non-linearity, in particular Vacuum Magnetic Birefringence (VMB). This process is predicted in QED via the fluctuation of electron–positron virtual pairs but can also be due to hypothetical Axion-Like Particles (ALPs) and/or MilliCharged Particles (MCP). Today ALPs are considered a strong candidate for Dark Matter. Starting in 1992 the PVLAS collaboration, financed by INFN, Italy, attempted to measure VMB conceptually following the original 1979 scheme based on an optical cavity permeated by a time-dependent magnetic field and heterodyne detection. Two setups followed differing basically in the magnet: the first using a rotating superconducting 5.5 T dipole magnet at the Laboratori Nazionali di Legnaro, Legnaro, Italy and the second using two rotating permanent 2.5 T dipole magnets at the INFN section of Ferrara. At present PVLAS is the experiment which has set the best limit in VMB reaching a noise floor within a factor 7 of the predicted QED signal: Δn(QED)=2.5×10−23 @ 2.5 T. It was also shown that the noise floor was due to the optical cavity and a larger magnet is the only solution to increase the signal to noise ratio. The PVLAS experiment ended at the end of 2018. A new effort, VMB@CERN, which plans to use a spare LHC dipole magnet at CERN with a new modified optical scheme, is now being proposed. In this review, a detailed description of the PVLAS effort and the comprehension of its limits leading to a new proposal will be given.

Analysis of Geometry and Field Quality Along the Series Production of the 11 T Dipole for the High Luminosity LHC

The first series production of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Accelerator Magnets is approaching its end. The LHC dipole upgrade within the scope of the High Luminosity project consists of collared assemblies of 5.6 m-long coils targeted for a nominal field of 11 T at 11.85 kA and 60 mm aperture. As part of the quality control of the production, geometrical and magnetic measurements are performed at different steps. The experience gathered during manufacturing of 28 coils and 11 collared coils is summarized here. Using the available data, we analyzed the distributions of coil geometry, collared coil size, main field, and low order field errors in warm conditions. In some cases, the trends were traced back to displacements of the conductor with respect to the nominal layout. Three magneto-static conductor deformation models aim to explain the variability of field harmonics, confronted with measured data.

Scalable haloscopes for axion dark matter detection in the 30 \u03bceV range with RADES

RADES (Relic Axion Detector Exploratory Setup) is a project with the goal of directly searching for axion dark matter above the 30μeV scale employing custom-made microwave filters in magnetic dipole fields. Currently RADES is taking data at the LHC dipole of the CAST experiment. In the long term, the RADES cavities are envisioned to take data in the BabyIAXO magnet. In this article we report on the modelling, building and characterisation of an optimised microwave-filter design with alternating irises that exploits maximal coupling to axions while being scalable in length without suffering from mode-mixing. We develop the mathematical formalism and theoretical study which justifies the performance of the chosen design. We also point towards the applicability of this formalism to optimise the MADMAX dielectric haloscopes.

Design of the consolidated LHC dipole diode busbar insulation system

During the second long shutdown of the LHC in 2019 and 2020, the electrical insulation of the 1232 dipole diodes and diode busbars is presently consolidated. The newly designed insulation system, mainly consisting of polyimide laminated fiber reinforced epoxy sheets and injection molded pure resin inserts, can be installed in the very constricted space of the LHC tunnel, without removing the presently installed insulation. The consolidated insulation will prevent shorts to ground caused by metal debris in the LHC cold masses and allow preparing the LHC for operation at 7 TeV.

Thermal engineering of optical mirrors for use at cryogenic temperature inside a LHC magnet cryostat

In the frame of the HL-LHC project, innovative technical solutions are sought to measure accurately the position of the magnet helium vessel inside the cryostat. To this end, a system based on laser-interferometry is being designed to monitor the displacement of the vessel through dedicated openings in the new HL-LHC magnet cryostats. In order to test such a system on a full-scale setup in representative operating conditions, a LHC dipole cryostat was modified to integrate the system optical lines of sight and the reflective mirrors were mounted onto the magnet helium vessel.Upon the first cool down of the magnet to 80 K, severe ice-like condensation started forming on the reflective surface of the mirrors hence making the system unusable at cold. This was attributed to the condensation of the residual gas remaining in the cryostat insulation vacuum on the mirror surface. In this configuration the mirrors acted as local “cold spots” since they were purposefully sticking out of the multi-layer insulation (MLI) that is otherwise covering the magnet helium vessel.In order to cope with this condensation issue, a dedicated study was carried out to design and manufacture a passive temperature regulation system based on a thermal insulating support and a thermal radiation intercept in order to keep the mirrors just above the expected freezing temperature in operational conditions.This paper details the thermal engineering study leading to the design of the insulated mirrors and presents the technical solution retained as well as the latest test results.

Superfluid Helium cooling and compact Heat Exchanger for HL-LHC D2 Recombination Dipoles

New D2 recombination dipoles with a larger aperture than in the LHC dipoles are required for the future High Luminosity LHC at CERN. These 13.5 m-long D2 magnets are proposed to be conduction cooled in a static bath of pressurized He II. Their cooling is provided via pressurized He II channels located in the D2 iron yoke and thermally connected to a saturated bath installed at one end of each D2 dipole. The heat transfer between the pressurized He II static bath and the bath pumped down to 16.4 mbar (1.8 K) is performed in a heat exchanger under study at CEA. Various design solutions were studied and evaluated to define the more suitable solution fulfilling on the one hand D2 cooling requirements (up to 70 W) and on the other hand D2 cryostat integration constraints. The paper will report on the D2 cooling needs and constraints, present the studied options and detail the main design features of the selected solution for a compact heat exchanger for D2 dipoles.

OSQAR chameleon afterglow search experiment

Abstract OSQAR experiment has been extended to the quest of chameleon particles with environment-dependent mass from the search of a magnetic afterglow effect. OSQAR-CHASE (chameleon afterglow search) has been run in 2017 using one spare LHC dipole providing a 9 T transverse magnetic field with an 18.5 W laser and state-of-the art CCD detector. Experimental results with and without magnetic field were preliminary analyzed and no clear evidence for a photon-coupled chameleons was confirmed. The very weak false signals was observed with decay time 4300 s. It can be explained by decrease of dark current from initial higher values during CCD cooling in detection phase. Increasing of the sensitivity of the OSQAR-CHASE experiment over the previous GammeV reference experiment assumes that the present coupling constant limit can be reduced around factor of 4.

NbTi/Nb/Cu Multilayer Shield for the Superconducting Shield (SuShi) Septum

A passive superconducting shield was proposed earlier to realize a high-field (3-4 T) septum magnet for the Future Circular Collider. This paper presents the experimental results of a potential shield material, a NbTi/Nb/Cu multilayer sheet. A cylindrical shield was constructed from two halves, each consisting of 4 layers with a total thickness of 3.2~mm, and inserted into the bore of a spare LHC dipole corrector magnet (MCBY). At 4.2~K, up to about 3.1~T at the shield's surface only a leakage field of 12.5~mT was measured inside the shield. This can be attributed to the mis-alignment of the two half cylinders, as confirmed by finite element simulations. With a better configuration we estimate the shield's attenuation to be better than $\mathbf{4\times 10^{-5}}$, acceptable for the intended application. Above 3.1~T the field penetrated smoothly. Below that limit no flux jumps were observed even at the highest achievable ramp rate of more than 50~mT/s at the shield's surface. A 'degaussing' cycle was used to eliminate the effects of the field trapped in the thick wall of the shield, which could otherwise distort the homogeneous field pattern at the extracted beam's position. At 1.9~K the shield's performance was superior to that at 4.2~K, but it suffered from flux jumps.

Progress on HL-LHC Nb3Sn Magnets

The high-luminosity Large Hadron Collider (HL-LHC) project aims at allowing to increase the collisions in the LHC by a factor of ten in the decade 2025-2035. One essential element is the superconducting magnet around the interaction region points, where the large aperture magnets will be installed to allow to further reduce the beam size in the interaction point. The core of this upgrade is the Nb3Sn triplet, made up of 150-mm aperture quadrupoles in the range of 7-8 m. The project is being shared between the European Organization for Nuclear Research and the US Accelerator Upgrade Program, based on the same design, and on the two strand technologies. The project is ending the short model phase, and entering the prototype construction. We will report on the main results of the short model program, including the quench performance and field quality. A second important element is the 11 T dipole that replaces a standard dipole making space for additional collimators. The magnet is also ending the model development and entering the prototype phase. A critical point in the design of this magnet is the large current density, allowing increase of the field from 8 to 11 T with the same coil cross section as in the LHC dipoles. This is also the first two-in-one Nb3Sn magnet developed so far. We will report the main results on the test and the critical aspects.

CP -violation in the two Higgs doublet model: From the LHC to EDMs

We study the prospective sensitivity to CP-violating Two Higgs Doublet Models from the 14 TeV LHC and future electric dipole moment (EDM) experiments. We concentrate on the search for a resonant heavy Higgs that decays to a $Z$ boson and a SM-like Higgs h, leading to the $Z(\ell\ell)h(b\bar{b})$ final state. The prospective LHC reach is analyzed using the Boosted Decision Tree method. We illustrate the complementarity between the LHC and low energy EDM measurements and study the dependence of the physics reach on the degree of deviation from the alignment limit. In all cases, we find that there exists a large part of parameter space that is sensitive to both EDMs and LHC searches.

Suitability of Different Quench Protection Methods for a 16 T Block-Type Nb3Sn Accelerator Dipole Magnet

Within the future circular collider study, a 100-km long circular hadron collider is being designed for 100 TeV center-of-mass collision energies. The design of the 16 T Nb 3 Sn bending dipole magnets is carried out within the EuroCirCol collaboration. Three different type of dipole designs have been developed, each aiming to be as compact as possible, accounting for the design criteria. Quench protection a critical aspect of the magnet design and potentially limits the magnet compactness. The EuroCirCol magnets were designed assuming a protection system with significantly improved efficiency compared to the present LHC dipole protection. In this paper, we consider present state-of-the-art quench protection technologies, such as quench heaters and CLIQ, and apply them into the designed 16 T Block-type dipole. Two different simulation models are used to estimate the magnet hotspot temperature and voltages after a quench and consequently estimate the suitability of the different methods.

Dipoles for High-Energy LHC

For the high-energy LHC, a study of a 33 TeV center of mass collider in the LHC tunnel, main dipoles of 20 T operational field are needed. In this paper, we first review the conceptual design based on block coil proposed in the Malta workshop, addressing the issues related to coil fabrication and assembly. We then propose successive simplifications of this design, associating a cost estimate of the conductor. We then analyze a block layout for a 15 T magnet. Finally, we consider two layouts based on the D20 and HD2 short models built by LBL. A first analysis of the aspects related to protection of these challenging magnets is given.

Vacuum birefringence by Compton backscattering through a strong field

We propose a novel scheme to measure nonlinear effects in electrodynamics arising from QED corrections. Our theoretical starting point is the Heisenberg-Euler-Schwinger effective Lagrangian which predicts that a vacuum with a strong static electromagnetic field turns birefringent. We propose to employ a pulsed laser to create Compton backscattered photons off a high energy electron beam. These photons will pass through a strong static magnetic field, which according to the QED prediction changes the state of polarization of the radiation---an effect proportional to the photon energy. This change will be measured by using an aligned single crystal, since a large difference in the pair production cross sections at high energies can be achieved with proper orientation of the crystal. As an example we will consider the machine, LHeC, under consideration at CERN as the source of these electrons, and an LHC dipole magnet as the source of the strong static magnetic field. In the proposed experimental setup the birefringence effect will be manifested in a difference in the number of pairs created in the polarizer crystal as the initial laser light has a varying state of polarization, achieved with a rotating quarter wave plate. This will be seen as a clear peak in the Fourier transform spectrum of the pair-production rate signal, which can be obtained with 3 hours of measurement. We also comment on the sensitivity of the experiment, to the existence of an axion, a hypothetical spin-0 particle that couples to two photons.

Fast Method to Quantify the Collective Magnetization in Superconducting Magnets

The magnetization of the superconductor is one of the most important parameters determining the field quality of accelerator magnets. A fast method to quantify the magnetization effect in an entire magnet was developed at CERN based on a voltage-current measurement during a powering cycle. The collective magnetization includes the effect due to hysteresis losses in the magnet superconducting filaments, coupling losses in the magnet conductor, and magnetization of the iron yoke. It is calculated by means of an energy balance between the work done by the power converter and the change of magnetic energy in the system. Also, the energy dissipated at any time is calculated. In the magnet test facility at CERN, LHC dipole magnets have been cycled between ±600 A with a ramp-rate of 10 A/s. The magnetization curves deduced from these measurements show a good precision and high reproducibility, mainly due to the high precision of the power converter and the current measurement system. The results have been compared with numerical simulations performed with the computer code ROXIE. The proposed test method can be applied to any type of magnet, is rather easy and fast, and is therefore interesting for checking the reproducibility of the magnetization among a series production of magnets.

Axion search by laser-based experiment OSQAR

Laser-based experiment OSQAR in CERN is aimed to the search of the axions by two methods. The photon regeneration experiment is using two LHC dipole magnets of the length 14.3m and magnetic field 9.5T equipped with an optical barrier at the end of the first magnet. It looks as light shining through the wall. No excess of events above the background was detected at this arrangement. Nevertheless, this result extends the exclusion region for the axion mass. The second method wants to measure the ultra-fine vacuum magnetic birefringence for the first time. An optical scheme with electro-optical modulator has been proposed, validated and subsequently improved. Cotton–Mouton constant for air was determined in this experiment setup.

Predicting the Quench Behavior of the LHC Dipoles During Commissioning

The LHC hardware commissioning has shown a considerable difference of performance between the three dipole manufacturers (Firm1, Firm2 and Firm3). More than 90% of the quenches occurred in the dipoles made by Firm3, less than 10% in Firm2, and no one in Firm1. In this paper we propose a Monte-Carlo method based on the quench performance data of individual magnets that accounts for this behavior. The model relies on the data of the virgin training and on correlations with quench after a thermal cycle as measured on a sample of magnets. This model is used to derive estimates of the training behavior in the machine. A comparison with data gathered during the hardware commissioning shows that the model works well for low fields, and that, starting from fields corresponding to an energy of about 6.3 TeV, a slower training in the magnets of one manufacturer is observed.

Searching for Solar Axions with CAST

The CAST (CERN Axion Solar Telescope) experiment is the worldwide most sensitive helioscope searching for solar axions by their conversion into photons inside the magnet pipes of a decommissioned LHC dipole magnet. The expected converted photons can be measured by four independent X-ray detectors installed at each end of the two magnet bores. These detectors are an X-ray telescope and a Micromegas detector on the western end of the CAST magnet and on the eastern end, a Time Projection Chamber (TPC) was installed until 2006, covering the two ends of the magnet pipes. In 2007 the TPC has been replaced by two upgraded Micromegas detectors, based on newest technology. The first phase of the CAST experiment (2003-2004) with vacuum in the conversion region resulted in an upper limit on the axion-photon coupling that supersedes the best limits derived from stellar energy-loss arguments. Since 2005, CAST entered in its second phase where it operates with a low Z buffer gas in the conversion region in order to extend the sensitivity of the experiment to higher axion rest masses. During 2005 and 2006, 4 He was used to search for an axion signal with axion rest masses in the range from 0.02 eV up to 0.4 eV. In order to search for even higher axion rest masses up to 1.2 eV the experiment was adapted for 3 He data taking with an upgraded gas system in 2007. The data taking with 3 He inside the magnet pipes has started successfully in March 2008.

Difference in Stability Between Edge and Center in a Rutherford Cable

Keystoned superconducting Rutherford cables are widely used in accelerator magnets like in the LHC at CERN. An essential requirement in the cable design is its stability against local heat releases in the magnet windings originating from for example, strand movement or beam loss. Beam loss is the highest at the coil inner radius of the magnet, where also the magnetic field peaks. Also the local compaction of the cable is maximum here and hence the helium content minimum. When performing stability measurements on several superconducting Nb-Ti cables used in LHC dipole and quadrupole magnets, we observed that the stability against point-like heat disturbances is much worse very close to the cable edges as compared to the central part of the cable. The main reason is related to the geometry of the cable causing variation of many parameters across the cable width, like inter-strand electrical resistance, inter-strand heat conductivity, cooled strand surfaces and RRR. In this paper we show results of new stability experiments and thoroughly compare the data with results obtained with the numerical network model CUDI, which is updated for stability simulations.