Beam Screen Research Articles

Computation of the Magnetization of Type II Superconductors for Potential Beam Screen Coatings of the Future Circular Collider

High-temperature superconductor coatings have been proposed for the reduction of the beam coupling impedance for the future circular collider's hadron–hadron (FCC-hh) option. These coatings are, however, prone to hysteretic magnetization due to the ramp of the external magnet field. We present a mathematical formulation and implementation of Bean's model, which is a simplified, phenomenological model of persistent current loops and magnetization. After validating the model on simplified, analytically accessible examples, it is applied to the FCC setup under various conditions. The conclusion is that the magnetization might impair the magnetic field quality strongly for a complete coating, but a coating in the form of stripes can be a feasible tradeoff between field quality and impedance reduction.

Commissioning of a beam screen test bench experiment with a future circular hadron collider type synchrotron radiation beam

Coping with synchrotron radiation (SR) that originated at superconducting bending magnets is one of the major challenges in the design of the vacuum beam pipes of hadron colliders. In the case of the Future Circular hadron Collider (FCC-hh), similarly as for the LHC, a beam screen, operating at higher temperatures than the cold mass, has been designed in order to preserve the superconducting magnet cold bores from direct synchrotron irradiation. The quality of the beam screen vacuum can be severely compromised by the absorption of SR into its walls, enhancing the risk for numerous beam detrimental effects to arise. In order to experimentally study such effects and develop strategies for their minimization, a beam screen test bench experiment (BESTEX) has been conceived and installed in the Karlsruhe Research Accelerator storage ring at the Karlsruhe Institute for Technology. The BESTEX has been designed to explore photon stimulated desorption, photon reflectivity, photon heat loads, and photoelectron generation originated on beam screen prototypes under irradiation of the FCC-hh-like SR spectrum. A detailed description of the BESTEX, its commissioning, and its functionality is hereby presented.

The Cryogenic System and Planned Cryogenic Tests For the Future High Luminosity LHC IT Magnet String

The high luminosity upgrade of the Large Hadron Collider will require the replacement of the triplet of final focusing superconducting magnets at interaction points 1 and 5 with a new set of helium II cooled magnets. To validate the technologies and assembly procedures and to investigate the collective effects, a string of magnets representative of the final HL-LHC configuration, will be installed in the SM18 test facility at CERN in 2021. The local cryogenic distribution connected to the string of magnets will also be representative of the final configuration. This paper first describes the cryogenic configuration of the magnet string and the functional cryogenic requirements for the operation and test program. The cryogenic parameters for the different circuits and operation phases are presented. The paper details the cryogenic test program foreseen to investigate the specific challenges of cooling Nb3Sn magnets and their beam screens during high heat load (several hundred watts at 1.9 K), steady state, and transient operation. The paper presents the complete cryogenic system planned for the string, its integration into the SM18 facility, the required adaptation of the SM18 cryogenic infrastructure, and the conceptual design of the main components.

Advances in the study of HTS superconductors for the beam impedance mitigation in CERN-FCC: the thermal runaway problem

In CERN Future Circular Collider (FCC-hh), a possible next-generation high-energy hadron–hadron collider, the center-of-mass collision energy will be of 100 TeV, with opposite proton beams of 50 TeV steered in a 100-km circumference tunnel by 16 T superconducting magnets. The synchrotron radiation, emitted by the beams, is absorbed by a beam-facing screen held at 50 K. The surface impedance of this screen has a strong impact on the beam stability, and copper at 50 K allows only tight beam stability margin. This has motivated investigating the possibility of high-temperature superconductors (HTSs) coatings on the beam screen internal surface, as a possible solution. In this communication, we will briefly review the general theory of the surface resistance of HTS in high field, low frequency regimes and will present specific calculations for REBCO commercial tapes that represent one of the possible envisaged solutions. The possible “thermal runaway” problems arising using REBCO tapes are then discussed. In particular, the upper limit for the transverse thermal resistance that guarantees thermal stability is quantitatively determined as a function of the REBCO superconducting properties at FCC operating conditions.

Traveling wave method for simulating geometric beam coupling impedance of a beam screen with pumping holes

In particle accelerators, pumping holes in a vacuum chamber can be a source of unwanted broadband coupling impedance, leading to beam instabilities. Analytical methods have been previously developed to estimate the impedance of holes in circular-like chambers e.g. the beamscreen of the Large Hadron Collider (LHC). More sophisticated chamber designs like that of the High Energy LHC (HE-LHC) and the Future Circular Collider (FCC-hh) call for a different way to calculate the impedance. We propose using decomposition of the wakefield into synchronous traveling waves and employing a numerical solver to find the impedance of each wave. This method is compared to the direct time domain wakefield calculation method and its greater sensitivity to small impedances is shown.

Study on heat transfer between the High-Luminosity LHC beam screens and the He II-cooled beam tube

The new beam screens for the inner triplets in the HL-LHC are designed to intercept heat loads of up to 25 Wm−1 between 60 K and 80 K. The screens need to be mechanically supported by the beam tube at 1.9 K while limiting heat transfer to the same tube to less than 0.5 W m−1. Measurements for the thermal validation of the beam screens were carried out on a dedicated test stand at the Central Cryogenic Laboratory at CERN. This paper describes the first measurement campaign of a full-scale D1-type beam screen mock-up: several parameters such as base temperature and heating power were varied, and results include heat transfer mechanisms within the beam screen and more importantly to the 1.9 K beam tube that forms the physical boundary between beam screen and He II-cooled magnets.

Heat load profile estimates on LHC beam screens by thermal transient analysis

The LHC beams are producing significant dynamic heat loads on the LHC cryogenic system. These heat loads are deposited on beam screens, where they must be properly extracted with dedicated cooling loops between 4.6 K and 20 K. Since 2015, unexpected beam-induced heat loads are observed in specific locations of the machine and their origin is still not completely understood. In order to improve our understanding on the heat load origin and on the spread between them on different areas of the accelerator, the thermal transients occurring after the beam dumps have been analyzed to reconstruct the heat load profiles of the beam screens. Following a description of this specific issue, the paper presents the methodology used for the measurements and estimates of the heat load profiles and its validation against some experimental data and dynamic simulations.

Preliminary Conceptual design of FCC-hh cryoplants: Linde evaluation

In the framework of an international collaboration, a 100 TeV hadron collider in a 100-km long tunnel is under study as a future circular collider beyond LHC at CERN. Its design is based on 16-T superconducting magnets cooled at 1.9 K by 10 cryoplants with a unit equivalent capacity of 100 kW at 4.5 K, up to 4 times larger than the present state-of-the-art. Half of the entropic refrigeration load is due to the synchrotron radiation produced by the high-energy proton beams and deposited on beam screens cooled between 40 and 60 K. This non-conventional thermal load distribution is an additional challenge for the cryogenic system. An engineering study on FCC-hh cryoplants is in progress with world-leader industries to define the preliminary conceptual design of industrial solutions and to confirm innovative technologies. The paper recalls the FCC-hh cryogenic requirements, presents the main results of Linde Kryotechnik study and highlights some identified R&D efforts.

Magnetic frequency response of High-Luminosity Large Hadron Collider beam screens

Magnetic fields used to control particle beams in accelerators are usually controlled by regulating the electrical current of the power converters. In order to minimize lifetime degradation and ultimately luminosity loss in circular colliders, current-noise is a highly critical figure of merit of power converters, in particular for magnets located in areas with high beta-function, like the High-Luminosity Large Hadron Collider (HL-LHC) insertions. However, what is directly acting upon the beam is the magnetic field and not the current of the power converter, which undergoes several frequency-dependent transformations until the desired magnetic field, seen by the beam, is obtained. Beam screens are very rarely considered when assessing or specifying the noise figure of merit, but their magnetic frequency response is such that they realize relatively effective low pass filtering of the magnetic field produced by the system magnet-power converter. This work aims at filling this gap by quantifying the expected impact of different beam screen layouts for the most relevant HL-LHC insertion magnets. A well-defined postprocessing technique is used to derive the frequency response of the different multipoles from multiphysics finite element method (FEM) simulation results. In addition, a well-approximated analytical formula for the low-frequency range of multi-layered beam screens is presented.

First beam test of Laser Engineered Surface Structures (LESS) at cryogenic temperature in CERN SPS accelerator

Electron cloud mitigation is an essential requirement for accelerators of positive particles with high intensity beams to guarantee beam stability and limited heat load in cryogenic systems. Laser Engineered Surface Structures (LESS) are being considered, within the High Luminosity upgrade of the LHC collider at CERN (HL-LHC), as an option to reduce the Secondary Electron Yield (SEY) of the surfaces facing the beam, thus suppressing the electron cloud phenomenon. As part of this study, a 2.2 m long Beam Screen (BS) with LESS has been tested at cryogenic temperature in the COLD bore EXperiment (COLDEX) facility in the SPS accelerator at CERN. In this paper, we describe the manufacturing procedure of the beam screen, the employed laser treatment technique and discuss our first observations in COLDEX confirming electron cloud suppression.

Characterization of low-temperature solution-processed LiCoO2 thin-film cathode with molecular weight control of polyvinylpyrrolidone

Numerous LiCoO2 cathode thin-films have been prepared for all-solid-state thin-film battery applications. Preparation methods include sputtering, pulsed laser deposition, electron beam evaporation, screen printing, and spin coating. When compared to vacuum deposition methods, the solution-processed fabrication methods could result in significant cost savings and large-area production. Unfortunately, high-temperature annealing (Tanneal > 700 °C) of LiCoO2 cathode thin film is usually crucial for achieving a high performance layered phase. Here, we report on phase control of spin-coated LiCoO2 thin film by varying the molecular weight of polyvinylpyrrolidone (PVP) at low temperature. We could achieve thick and smooth film morphology by optimizing the PVP molecular weight. This method could be a promising cathode layer for low-cost thin-film batteries.

Design study of an YBCO-coated beam screen for the super proton-proton collider bending magnets.

In order to reduce the beam impedance and refrigeration power dramatically, we have designed a high temperature superconductor (HTS) coated beam screen to screen the cold chamber walls of the super proton-proton collider bending magnets from beam-induced heat loads. It employs an absorber, inspired by the future circular collider studies, to absorb the immense synchrotron radiation power of 12.8 W/m emitted from the 37.5 TeV proton beams. Such a structure has the advantage of decreasing the electron cloud effect and improving the beam vacuum. We have compared the critical magnetic field and current density and accessibility of two potential HTS materials for the beam screen, TlBa2Ca2Cu3O9-δ (Tl-1223) and Yttrium Barium Copper Oxide (YBCO) and finally chose YBCO for coating. The beam screen is tentatively designed to work at 55-70 K because of the limited development of the YBCO material. The thermal analysis with oxygen cooling fluid indicates that the YBCO conductor can maintain its superconductivity even if the synchrotron radiation hits the YBCO-coated surface and the mechanical analysis shows that the structure has the ability to resist the Lorenz force during magnet quenches.

High-temperature superconductor coating for coupling impedance reduction in the FCC-hh beam screen

The international Future Circular Collider study develops a conceptual design for a post Large Hadron Collider particle accelerator using 16 T superconducting dipoles for achieving p–p center-of-mass collision energies up to 100 TeV. One concern for this project is the beam coupling impedance especially at injection energy. A copper coated beam screen as in the LHC is planned, but preliminary studies indicate that copper at the high operating temperature of 50 K might not provide a sufficiently low impedance for a stable beam. In order to reduce the coupling impedance, we investigate high-temperature superconductors as a possible coating material in combination with copper as a hybrid system. The effect of different coating combinations are estimated through numerical calculations to identify the best hybrid beam screen coating system.

Examining mitigation schemes for synchrotron radiation in high-energy hadron colliders

At high proton-beam energies, beam-induced synchrotron radiation is an important source of heating, of beam-related vacuum pressure increase, and of primary photoelectrons, which can give rise to an electron cloud. We use the synrad3d code developed at Cornell to simulate the photon distributions in the arcs of several existing, planned, or proposed highest-energy hadron colliders to analyze the efficiency of several techniques developed, or proposed, to mitigate the negative effects of synchrotron radiation, such as a sawtooth surface and slots in the beam screen.

A coupled multiphysics FEM model to investigate electromagnetic, thermal and mechanical effects in complex assemblies: The design of the High-Luminosity Large Hadron Collider beam screen

• A multiphysics model is proposed to assess the mechanical response of a complex assembly to a magnetic field variation. • The one-way and two-way couplings between the magnetic, thermal and mechanical physics are fully investigated. • The two-way coupling between the magnetic and thermal physics turns out to lower the induced forces up to approximately 40%. • Dynamic effects such as self-inductance phenomena lower and delay in time the induced forces for highly conductive materials. In the framework of the High-Luminosity Large Hadron Collider (HL-LHC) project, new beam screens will be installed by 2024 within the cold bore of the superconducting magnets. The beam screen is an octagonal shaped pipe that shields the 1.9 K magnet cryogenic system from the heat loads and damage to the magnet coils that would be otherwise induced by the highly penetrating collision debris. It also ensures that proper vacuum conditions required for the stability of the beam are met. A failure scenario of the beam screen is represented by the magnet quench, a resistive transition of the superconducting magnets, that can compromise its mechanical integrity. During a quench the magnet gradient of the quadrupole, in which the beam screen is inserted, decays from 140 T/m to about 0 T/m in 0.4 s inducing high magnitude forces in the assembly. Understanding the magnetic, thermal and mechanical behaviours of the beam screen assembly during the quench is critical to enable its effective design and operation. A numerical model, that can accurately predict the behaviours of the beam screen during a magnet quench, has been developed. Compared to the analytical formulations used to design the beam screen currently installed in the LHC, the multiphysics FEM model developed in this research introduces multiple elements of novelty and improved performance. First, self-inductance effects are accounted for and found to reduce the induced forces up to approximately 2000% at high electrical conductivity values. Second, the one-way and two-way coupling of the magnetic with the mechanical and thermal interfaces are explored and the best trade-off is defined. Third, the mechanical response of the assembly is evaluated dynamically over the evolution of the magnetic field decay rather than just in a quasi-static manner. Fourth, three dimensional geometries can also be studied enabling the design of the components to be placed along the beam axis. The model has been verified by comparison to a closed form expression showing the advantages of considering self-inductance phenomena. The mechanical integrity of the new beam screen has been demonstrated and a less conservative design has been obtained, which has permitted to relax the tight constraints on interfacing systems. Amongst other applications, the model has already been applied at CERN to support the conceptual design of an ad-hoc beam screen for the Future Circular Collider (FCC).

Impact of large beam-induced heat loads on the transient operation of the beam screens and the cryogenic plants of the Future Circular Collider (FCC)

The Future Circular Collider (FCC) under study at CERN will produce 50-TeV high-energy proton beams. The high-energy particle beams are bent by 16-T superconducting dipole magnets operating at 1.9 K and distributed over a circumference of 80 km. The circulating beams induce 5 MW of dynamic heat loads by several processes such as synchrotron radiation, resistive dissipation of beam image currents and electron clouds. These beam-induced heat loads will be intercepted by beam screens operating between 40 and 60 K and induce transients during beam injection. Energy ramp-up and beam dumping on the distributed beam-screen cooling loops, the sector cryogenic plants and the dedicated circulators. Based on the current baseline parameters, numerical simulations of the fluid flow in the cryogenic distribution system during a beam operation cycle were performed. The effects of the thermal inertia of the headers on the helium flow temperature at the cryogenic plant inlet as well as the temperature gradient experienced by the beam screen has been assessed. Additionally, this work enabled a thorough exergetic analysis of different cryogenic plant configurations and laid the building-block for establishing design specification of cold and warm circulators.

Parametric study on the thermal performance of beam screen samples of the High-Luminosity LHC upgrade

The High-Luminosity upgrade of the Large Hadron Collider (HL-LHC) will increase the accelerator’s luminosity by a factor 10 beyond its original design value, giving rise to more collisions and generating an intense flow of debris. A new beam screen has been designed for the inner triplets that incorporates tungsten alloy blocks to shield the superconducting magnets and the 1.9 K superfluid helium bath from incoming radiation. These screens will operate between 60 K and 80 K and are designed to sustain a nominal head load of 15 Wm−1, over 10 times the nominal heat load for the original LHC design. Their overall new and more complex design requires them and their constituent parts to be characterised from a thermal performance standpoint. In this paper we describe the experimental parametric study carried out on two principal thermal components: a representative sample of the beam screen with a tungsten-based alloy block and thermal link and the supporting structure composed of an assembly of ceramic spheres and titanium springs. Results from both studies are shown and discussed regarding their impact on the baseline considerations for the thermal design of the beam screens.

Operational experience of the upgraded LHC injection kicker magnets during Run 2 and future plans

During Run 1 of the LHC, one of the injection kicker magnets caused occasional operational delays due to beam induced heating with high bunch intensity and short bunch lengths. In addition, there were also sporadic issues with vacuum activity and electrical flashover of the injection kickers. An extensive program of studies was launched and significant upgrades were carried out during Long Shutdown 1 (LS 1). These upgrades included a new design of beam screen to reduce both beam coupling impedance of the kicker magnet and the electric field associated with the screen conductors, hence decreasing the probability of electrical breakdown in this region. This paper presents operational experience of the injection kicker magnets during the first years of Run 2 of the LHC, including a discussion of faults and kicker magnet issues that limited LHC operation. In addition, in light of these issues, plans for further upgrades are briefly discussed.

Beam screen cryogenic control improvements for the LHC run 2

This paper presents the improvements made on the cryogenic control system for the LHC beam screens. The regulation objective is to maintain an acceptable temperature range around 20 K which simultaneously ensures a good LHC beam vacuum and limits cryogenic heat loads. In total, through the 27 km of the LHC machine, there are 485 regulation loops affected by beam disturbances. Due to the increase of the LHC performance during Run 2, standard PID controllers cannot keeps the temperature transients of the beam screens within desired limits. Several alternative control techniques have been studied and validated using dynamic simulation and then deployed on the LHC cryogenic control system in 2015. The main contribution is the addition of a feed-forward control in order to compensate the beam effects on the beam screen temperature based on the main beam parameters of the machine in real time.

Investigation and performance assessment of hydraulic schemes for the beam screen cooling for the Future Circular Collider of hadron beams

The international study at CERN of a possible future circular collider (FCC) considers an option for a very high energy hadron-hadron collider located in a quasi-circular underground tunnel of about 100 km of length. The technical segmentation of the collider foresees continuously cooled sections of up to 10.4 km; throughout the entire section length, more than 600 kW of heat mainly generated by the beam synchrotron radiation must be removed from the beam screen circuits at a mean temperature of 50 K. The cryogenic system has to be designed to extract the heat load dependably with a high-efficiency refrigeration process. Reliable and efficient cooling of the FCC beam screen in all possible operational modes requires a solid basic design as well as well-matched components in the final arrangement. After illustrating the decision making process leading to the selection of an elementary hydraulic scheme, this paper presents preliminary conceptual designs of the FCC beam screen cooling system and compares the different schemes regarding the technical advantages and disadvantages with respect to the exergetic efficiency.