Next Generation Of Particle Accelerators Research Articles

Towards high-field applications: high-performance, low-cost iron-based superconductors

ABSTRACT High magnetic fields play a crucial role in advancing basic sciences, fusion energy, and magnetic resonance imaging systems. However, the widespread use of high-field magnets requires affordable high-temperature superconducting wires that can carry large supercurrents. Iron-based superconductors offer an economically attractive solution to push forward important yet costly scientific programs, such as nuclear fusion reactors and next-generation particle accelerators. In this review, we start with the fabrication of iron-based superconducting wires and tapes and continue to discuss several key factors governing the current transport properties. State-of-the-art wires and tapes are introduced with emphasis on grain boundary characteristics, flux pinning, and anisotropy. The architecture of flexible conductors enables low cost, high mechanical strength, and high thermal stability. Recent progress in practical applications, including superconducting joints and insert coils, is also reviewed. Finally, we propose several key questions faced by iron-based superconductors in future practical applications.

Conceptual Design of 20 T Hybrid Accelerator Dipole Magnets

Hybrid magnets are currently under consideration as an economically viable option towards 20 T dipole magnets for next generation of particle accelerators. In these magnets, High Temperature Superconducting (HTS) materials are used in the high field part of the coil with so-called insert coils, and Low Temperature Superconductors (LTS) like Nb3Sn and Nb-Ti superconductors are used in the lower field region with so-called outsert coils. The attractiveness of the hybrid option lays on the fact that, on the one hand, the 20 T field level is beyond the Nb3Sn practical limits of 15-16 T for accelerator magnets and can be achieved only via HTS materials; on the other hand, the high cost of HTS superconductors compared to LTS superconductors makes it advantageous exploring a hybrid approach, where the HTS portion of the coil is minimized. We present in this paper an overview of different design options aimed at generating 20 T field in a 50 mm clear aperture. The coil layouts investigated include the Cos-theta design (CT), with its variations to reduce the conductor peak stress, namely the Canted Cos-theta design (CCT) and the Stress Management Cos-theta design (SMCT), and, in addition, the Block-type design (BL) including a form of stress management and the Common-Coil design (CC). Results from a magnetic and mechanical analysis are discussed, with particular focus on the comparison between the different options regarding quantity of superconducting material, field quality, conductor peak stress, and quench protection.

Flux Creep in a Bi-2212 Rutherford Cable for Particle Accelerator Applications

Bi-2212 superconducting cables are being considered for use in the high field magnets needed for the next generation of particle accelerators. Magnetization in these cables and the decay of that magnetization lead to field error and field-error drift, respectively, which need to be compensated. To study this, a segment of the winding pack was extracted from a racetrack coil made from Bi-2212 Rutherford cable. Using a Hall probe measurement technique, we measured the response of the cable's magnetization and its magnetization decay to changes in the applied magnetic field. The effect of adjustments to the cycling of the magnetic field was studied, intended to simulate the preinjection cycles of an accelerator magnet. Three <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> vs. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> loops were constructed by sweeping the magnetic field applied to the sample from 0 to 2.5 T, then to a preinjection field “x” (where x = 0, 0.25, 0.75 T), and finally up to 1 T. The applied field was then held at 1 T for 1500 s, and the magnetization decay was measured. The decay was found to vary from 8% to 14% after 1500 s, depending on the preinjection field cycle.

High-temperature superconducting CORC® wires with record-breaking axial tensile strain tolerance present a breakthrough for high-field magnets

Cuprate high-temperature superconductors (HTS), such as RE-Ba2Cu3O7−δ (REBCO, RE = rare earth), (Bi,Pb)2Sr2Ca2Cu3O10−x and Bi2Sr2CaCu2O8−x , have enabled the development of high-field superconducting magnets capable of generating magnetic fields far exceeding 20 T. The brittle nature of HTS requires elaborate means to protect them against the high stresses and strains associated with high-field magnet operation, and so far, has prevented reliable high-field HTS magnets from becoming a reality. Here we report a more than tenfold increase in the irreversible strain limit under axial tension (ϵ irr) to over 7% in optimized high-current conductor on round core (CORC®) conductors, compared to the REBCO tapes from which the CORC® conductor is wound. Minimizing the tape winding pitch of the helical wind mechanically decouples the brittle REBCO film from the overall conductor. The REBCO tapes behave as springs, limiting the rate at which applied strain is transferred to the ceramic film. In addition, high-strength alloy cores allow the critical stress (ϵ crit) under axial tension at which initial degradation of CORC® conductors occurs to exceed 600 MPa, making them one of the strongest superconductors available. Mechanically decoupling the ceramic REBCO films from the overall CORC® conductor allows effective protection against the high operating stresses in high-field magnets. This breakthrough presents a monumental shift for HTS magnet technology, bringing reliable high-field superconducting magnets for compact fusion machines, the next generation of particle accelerators, and 40–60 T research solenoids within reach.

Temperature-dependent near-surface interstitial segregation in niobium

Niobium’s superconducting properties are affected by the presence and precipitation of impurities in the near-surface region. A systematic wide-temperature range x-ray diffraction study is presented addressing the effect of low temperatures (108 K–130 K) and annealing treatments (523 K in nitrogen atmosphere, 400 K in UHV) on the near-surface region of a hydrogen-loaded Nb(100) single-crystal. Under these conditions, the response of the natural surface oxides (Nb2O5, NbO2, and NbO) and the changes in the subsurface concentration of interstitial species in Nb are explored, thereby including the cryogenic temperature regime relevant for device operation. The formation and suppression of niobium hydrides in such conditions are also investigated. These treatments are shown to result in: (i) an increase in the concentration of interstitial species (oxygen and nitrogen) occupying the octahedral sites of the Nb bcc lattice at room temperature, both in the near-surface region and in the bulk. (ii) A decrease in the concentration of interstitials within the first 10 nm from the surface at 130 K. (iii) Hydride formation suppression at temperatures as low as 130 K. These results show that mild annealing in nitrogen atmosphere can suppress the formation of superconducting-detrimental niobium hydrides, while subsurface interstitial atoms tend to segregate towards the surface at 130 K, therefore altering the local concentration of impurities within the RF penetration depth of Nb. These processes are discussed in the context of the improvement of niobium superconducting radio-frequency cavities for next-generation particle accelerators.

Cryogenic Liquid Jets for High Repetition Rate Discovery Science

This protocol presents a detailed procedure for the operation of continuous, micron-sized cryogenic cylindrical and planar liquid jets. When operated as described here, the jet exhibits high laminarity and stability for centimeters. Successful operation of a cryogenic liquid jet in the Rayleigh regime requires a basic understanding of fluid dynamics and thermodynamics at cryogenic temperatures. Theoretical calculations and typical empirical values are provided as a guide to design a comparable system. This report identifies the importance of both cleanliness during cryogenic source assembly and stability of the cryogenic source temperature once liquefied. The system can be used for high repetition rate laser-driven proton acceleration, with an envisioned application in proton therapy. Other applications include laboratory astrophysics, materials science, and next-generation particle accelerators.

Laser Processed Oxygen-Free High-Conductivity Copper with Ti and Ti–Zr–V–Hf Films Applied in Neutron Tube

The laser processing method has proven to produce surfaces while ensuring a low secondary electron yield of oxygen-free high-conductivity copper (OFHC) samples, making it attractive for electron cloud mitigation in next-generation particle accelerators and neutron tubes. In this work, the laser processing method is proposed to OFHC targets for the first time, aiming to reduce the secondary electrons in the neutron tube. The secondary electron yields (SEYs) and the thermal conductivities of Ti film and quaternary Ti–Zr–V–Hf films with unprocessed and laser processed OFHC substrates are investigated. Our results highlight that the thermal conductivity of Ti film with laser processed OFHC substrates is in proximity to the cleaned bare OFHC sample, especially at high temperatures. Moreover, the SEY of coated OFHC substrates are higher than that of coated laser processed substrates, which indicates the better secondary electron suppression capability of coated laser processed substrates. Therefore, the thermal conductivity and SEY results illustrate that the application of Ti and Ti–Zr–V–Hf coated laser processed OFHC can be considered to improve the neutron yield in neutron tubes in the future.

Single-shot electrons and protons time-resolved detection from high-intensity laser–solid matter interactions at SPARC_LAB

Laser–plasma interactions have been studied in detail over the past twenty years, as they show great potential for the next generation of particle accelerators. The interaction between an ultra-intense laser and a solid-state target produces a huge amount of particles: electrons and photons (X-rays and $\unicode[STIX]{x03B3}$ -rays) at early stages of the process, with protons and ions following them. At SPARC_LAB Test Facility we have set up two diagnostic lines to perform simultaneous temporally resolved measurements on both electrons and protons.

Secondary electron emission characteristics of graphene films with copper substrate**Supported by the National Natural Science Foundation of China (11475166) National Natural Science Foundation of China (11205155) and National Natural Science Foundation of China (11575214).

For modern and future circular accelerators, especially high-intensity proton synchrotrons or colliders, the electron cloud effect is a key issue. So, in order to reduce the electron cloud effect, exploring very low secondary electron yield (SEY) material or coating used in vacuum tubes becomes necessary. In this article, we studied the SEY characteristics of graphene films with different thicknesses which were deposited on copper substrates using chemical vapor deposition. The SEY tests were done at temperatures of 25 °C and vacuum pressure of (2 − 6) × 10−9 torr. The properties of the deposited graphene films were investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The SEY curves show that the number of graphene layers has a great effect on the SEY of graphene films. The maximum SEY of graphene films decreases with the increase of the number of layers. The maximum SEY of 6–8 layers of graphene film is 1.25. These results have a great significance for next-generation particle accelerators.

Persistent-Current Magnetization Effects in High-Field Superconducting Accelerator Magnets

Magnetic field changes in superconducting filaments are shielded by the so-called persistent currents. These currents produce field errors, which are particularly important at low energy level, where the effect is stronger compared to the field generated by the transport current. For the next generation of particle accelerators, magnetic fields higher than 10 T are needed, requiring the use of Nb3Sn as superconductor. Due to the larger filament size (typically ten times larger than in the Nb–Ti Large Hadron Collider dipoles) and higher current density, strand magnetization effects shall be carefully studied. This paper presents an analysis on the expected field errors due to persistent currents in a 16-T dipole magnet. We briefly discuss the method and compare with experimental results on the 11-T Nb3Sn dipole for the Hi-Luminosity upgrade. The model is then used to predict the persistent-current effects for different coil layouts and strand characteristics in a 16-T dipole.

Hybrid High-Field Cosine-Theta Accelerator Magnet R&amp;amp;D With Second-Generation HTS

Next-generation particle accelerators, such as the proposed Large Hadron Collider (LHC) energy upgrade (highenergy LHC), will require very-high-field (> 20 T) superconducting magnets. This paper describes the progress toward this goal made to date as a part of a collaborative work between Particle Beam Lasers, Inc., and Brookhaven National Laboratory. To reduce the cost, high-temperature superconductors (HTSs) are used in a hybrid design with conventional low-temperature superconductors (LTSs) Nb3Sn and NbTi. The focus of this paper is on using a second generation (2G) ReBCO HTS tape in cosine-theta coil geometry. The complex ends of the cosine-theta geometry are particularly challenging for a brittle HTS tape. We report the construction and 77-K test results, one with a 4-mm and another with a 12-mm ReBCO tape, neither showing measurable degradation. This paper also presents the first successful use of Kapton CI on an HTS tape, which offers many advantages. Future plans include the construction and 4-K testing of a full cosine-theta HTS coil (first in a stand-alone mode and then in a hybrid structure with LTS coils) and the modeling and measurements of magnetization. This paper is a part of comprehensive research and development toward eventually building a high-field accelerator-quality dipole magnet.

An ultracold low emittance electron source

Ultracold atom-based electron sources have recently been proposed as an alternative to the conventional photo-injectors or thermionic electron guns widely used in modern particle accelerators. The advantages of ultracold atom-based electron sources lie in the fact that the electrons extracted from the plasma (created from near threshold photo-ionization of ultracold atoms) have a very low temperature, i.e. down to tens of Kelvin. Extraction of these electrons has the potential for producing very low emittance electron bunches. These features are crucial for the next generation of particle accelerators, including free electron lasers, plasma-based accelerators and future linear colliders. The source also has many potential direct applications, including ultrafast electron diffraction (UED) and electron microscopy, due to its intrinsically high coherence. In this paper, the basic mechanism of ultracold electron beam production is discussed and our new research facility for an ultracold, low emittance electron source is introduced. This source is based on a novel alternating current Magneto-Optical Trap (the AC-MOT). Detailed simulations for a proposed extraction system have shown that for a 1 pC bunch charge, a beam emittance of 0.35 mm mrad is obtainable, with a bunch length of 3 mm and energy spread 1%.

Results on Ultra-Precise Magnet Yoke Sectors Assembly Tests

Due to beam dynamics requirements and to reduce the cost and power consumption for the next generation of particle accelerators, their magnet systems need to be more and more compact and precise. As a consequence, the required tolerances on machining and assembling of components like the electro-magnet iron yoke sectors are becoming tighter. R&D and prototypes procurement for future projects like the Compact Linear Collider require quadrupole yoke quadrants machined with micrometric precision, and assembled with an overall precision in the range of ±10 μm. A test program was launched at CERN in order to investigate the feasibility of such tight assembly tolerances. The program consists in testing three different types of assembly methods for ultra-precise dummy quadrants produced by electro discharge machining. The aim is to identify the best performing assembly procedures for magnet iron yokes. In this paper the first relevant results of this test program are presented and discussed.

In Situ Study of Nb Oxide and Hydride for SRF Cavity Applications Using Aberration-Corrected STEM and Electron Energy Loss Spectroscopy

Nb is the preferred material for superconducting radio-frequency (SRF) cavities in next-generation particle accelerators, but also exhibits a high solubility for oxygen and hydrogen, especially within the RF-field penetration depth. Using electron diffraction, we observe superlattice reflections near the SRF cavities surface at 300 K and 95 K, which are related to the ordering of oxygen and hydrogen atoms in the niobium crystal. High-resolution scanning transmission electron microscopy images allow direct observation of light atoms, such as hydrogen, carbon and oxygen, and combined with EELS, we can reveal the element composition at grain boundaries and precipitates near the cavity surface. These changes in local composition will be related to the different cavity processing methods.

The future is fibre accelerators

Could massive arrays of thousands of fibre lasers be the driving force behind next-generation particle accelerators? The International Coherent Amplification Network project believes so and is currently performing a feasibility study. The challenge of producing the next generation of particle accelerators, for both fundamental research at laboratories such as CERN and more applied tasks such as proton therapy and nuclear transmutation, has been taken up by the high-intensity laser community.

Direct Atomic-Scale Imaging of Hydrogen and Oxygen Interstitials in Pure Niobium Using Atom-Probe Tomography and Aberration-Corrected Scanning Transmission Electron Microscopy

Imaging the three-dimensional atomic-scale structure of complex interfaces has been the goal of many recent studies, due to its importance to technologically relevant areas. Combining atom-probe tomography and aberration-corrected scanning transmission electron microscopy (STEM), we present an atomic-scale study of ultrathin (~5 nm) native oxide layers on niobium (Nb) and the formation of ordered niobium hydride phases near the oxide/Nb interface. Nb, an elemental type-II superconductor with the highest critical temperature (T(c) = 9.2 K), is the preferred material for superconducting radio frequency (SRF) cavities in next-generation particle accelerators. Nb exhibits high solubilities for oxygen and hydrogen, especially within the RF-field penetration depth, which is believed to result in SRF quality factor losses. STEM imaging and electron energy-loss spectroscopy followed by ultraviolet laser-assisted local-electrode atom-probe tomography on the same needle-like sample reveals the NbO(2), Nb(2)O(5), NbO, Nb stacking sequence; annular bright-field imaging is used to visualize directly hydrogen atoms in bulk β-NbH.

Factorization-based sparse solvers and preconditioners

Efficient solution of large-scale, ill-conditioned and highly-indefinite algebraic equations often relies on high quality preconditioners together with iterative solvers. Because of their robustness, the factorization-based algorithms could play a significant role when they are combined with iterative methods, particularly in the development of scalable solvers. We present our recent work in using the direct solver SuperLU code base to develop a new supernode-based ILU preconditioner and a domain-decomposition hybrid solver. Our ILU preconditioner is a modification of the classic ILUTP approach, incorporating a number of techniques to improve robustness and performance, which include new dropping strategies that accommodate the use of supernodal structure in the factored matrix. Our hybrid solver is based on the Schur complement method. We use parallel graph partitioning to obtain hierarchical interface/domain decomposition, and multiple parallel direct solvers to solve the subdomain problems simultaneously, and parallel preconditioned iterative solvers to solve the interface problem. We will demonstrate the effectiveness of our new techniques by applying them to two SciDAC applications, modeling next-generation particle accelerators and fusion devices.

Development of High Performance ${\rm Nb}_{3}{\rm Sn}$ Conductor for Fusion and Accelerator Applications

While the highest field performance is a primary goal of the conductor in the next generation of particle accelerators, emphasis has been focused mainly on the increase in current carrying capacity. The conductor needed for fusion application, however, requires much lower magnetization limit along with a critical current as high as possible. The upgraded strand specifications for the International Thermonuclear Experimental Reactor (ITER) and particle accelerator applications require further improvement of current density at little expense of ac loss or effective filament diameter. A conductor designed to meet such requirements has been fabricated. The effect of heat treatment condition on its properties was investigated to develop an optimized reaction condition. Significant efforts have been made to explore the sub-element design and configuration of the strand to ensure minimum hysteresis of the wire. An attempt was also made to scale up the restack assembly unit for mass production.

Magnetostrictive Fast Tuners for SRF Cavities

Energen, Inc. has demonstrated new tuners based on magnetostrictive actuators to address SRF cavity fast tuning requirements for next-generation particle accelerators. Prototype fast tuners were fabricated and delivered to DESY, Argonne National Lab and Jefferson Lab. Results are reported from tests at Jefferson Laboratory, which were done under realistic operating conditions. In these tests, Energen's fast tuner was found to have a tuning range double the required 1200 Hz and provided precise, nanometer-resolution motion. Slow tuner results have been reported in earlier papers.

Comparison Study of Electromagnet and Permanent Magnet Systems for an Accelerator Using Cost-Based Failure Modes and Effects Analysis

The next generation of particle accelerators will be one-of-a-kind facilities, and to meet their luminosity goals they must have guaranteed availability over their several decade lifetimes. The Next Linear Collider (NLC) is one viable option for a 1 TeV electron-positron linear collider, it has an 85% overall availability goal. We previously showed how a traditional failure modes and effects analysis (FMEA) of a SLAC electromagnet leads to reliability-enhancing design changes. Traditional FMEA identifies failure modes with high risk but does not consider the consequences in terms of cost, which could lead to unnecessarily expensive components. We have used a new methodology, "life cost-based FMEA", which measures risk of failure in terms of cost, in order to evaluate and compare two different technologies that might be used for the 8653 NLC magnets: electromagnets or permanent magnets. The availabilities for the two different types of magnet systems have been estimated using empirical data from SLAC's accelerator failure database plus expert opinion on permanent magnet failure modes and industry standard failure data. Labor and material costs to repair magnet failures are predicted using a Monte Carlo simulation of all possible magnet failures over a 30-year lifetime. Our goal is to maximize up-time of the NLC through magnet design improvements and the optimal combination of electromagnets and permanent magnets, while reducing magnet.