Future Particle Accelerators Research Articles

Azimuthal decomposition study of a realistic laser profile for efficient modeling of Laser WakeField Acceleration

The advent of ultra short high intensity lasers has paved the way to new and promising, yet challenging, areas of research in the laser-plasma interaction physics. The success of constructing petawatt femtosecond lasers, for instance the Apollon laser in France, will help understanding and designing future particle accelerators and next generation of light sources. Achieving this goal intrinsically relies on the combination between experiments and massively parallel simulations. So far, Particle-In-Cell (PIC) codes have been the ultimate tool to accurately describe the laser-plasma interaction especially in the field of Laser WakeField Acceleration (LWFA). Nevertheless, the numerical modelling of laser plasma accelerators in 3D can be a very challenging task. This is due to the large dispersity between the scales involved in this process. In order to make such simulations feasible with a significant speed up, we need to use reduced numerical models which simplify the problem while retaining a high fidelity. Among these models, Fourier field decomposition in azimuthal modes for the cylindrical geometry [1] is a promising reduced model especially for physical problems that have close to cylindrical symmetry which is the case in LWFA. This geometry has been implemented in the open-source code SMILEI [2] in Finite Difference Time Domain (FDTD) discretization scheme for the Maxwell solver. In this paper we will study the case of a realistic laser measurement from Apollon facility, the ability of this method to describe it correctly and the determination of the necessary number of modes for this purpose. We will also show the importance of higher modes inclusion in the case of realistic laser profiles to insure fidelity in simulation.

Thermal desorption of cryopumped gases from laser treated copper

Recently, laser processing of copper samples has been demonstrated to produce rough surfaces whose nanostructuring ensures unquestionable advantages for electron cloud mitigation in future particle accelerators. The actual application of laser treated surfaces in accelerators implies that this new material is compliant with many issues, going from impedance vacuum properties to many others. A significant experimental effort is therefore ongoing to study and optimize their various properties of interest. Here we analyze their vacuum behavior versus temperature. To this end, we studied thermal programmed desorption from CO, ${\mathrm{CH}}_{4}$ and ${\mathrm{H}}_{2}$ once cryosorbed on laser treated copper substrate and on its flat counterpart. These molecules are typically present in the residual vacuum of any accelerator. The results show that the desorption of such gases from the laser treated substrates occurs in a much broader and higher temperature range with respect to what is observed from the flat substrate. We also show that, at equal doses, treated samples adsorb/desorb significantly more gas than their flat counterpart. These findings can be ascribed to their nanostructured porous morphology. A quantitative analysis is given, allowing to properly estimate fluctuations of the number of molecules during unavoidable temperature variations of the cryogenic vacuum system.

Vortex pinning properties at dc and microwave frequencies of YBa2Cu3O7-x films with nanorods and nanoparticles

YBa2Cu3O7−x (YBCO) nanocomposites for wire applications need to operate in a broad range of frequencies, ranging from dc in magnets to GHz in cavities and screenings of future particle accelerators. We have investigated the in-field and angular vortex pinning performance in dc and at 50 GHz of two types of nanocomposites, pulsed laser deposition (PLD) YBCO with mixed Ba2YNbO6 + Ba2YTaO6 (BYNTO) nanorods and chemical solution deposited (CSD) YBCO with BaHfO3 (BHO) nanoparticles (NPs), and the pristine counterpart films, grown on top of single-crystalline substrates. Transport measurements performed up to 9 T between 5 and 77 K show that CSD nanocomposites exhibit a smooth field decay and increased single-to-collective crossover field H* compared to pristine samples, associated to the enhanced isotropic pinning contribution induced by the NPs, while PLD films exhibit unchanged H* and superior critical current densities up to higher irreversibility fields, associated to the anisotropic contribution introduced by the rods. Microwave in-field measurements of the pinning constant kp revealed CSD NCs exhibit a qualitatively similar, but smoother kp(H) than pristine samples, whereas for PLD samples, a growing kp(H) dependence is observed as a result of the increased relevance of the stiffness of the fluxons pinned by nanorods.

Investigation of charge multiplication in irradiated p-type silicon strip sensors designed for the ATLAS phase II tracking detector

Future particle accelerator experiments, for example the High Luminosity-LHC (HL-LHC) in 2025, will demand a much higher radiation hardness of silicon strip sensors due to high fluences and long operation times. New p-type strip sensors have been developed and their charge collection behavior as well as their electrical performance have been investigated. Additionally the long-term behavior under operation has to be investigated and understood. In previous studies of the annealing behavior, the phenomenon of charge multiplication appeared in highly irradiated and/or long-term annealed sensors. This phenomenon is now thoroughly investigated within this study for ATLAS12 strip sensors irradiated to fluences ranging from 5⋅1014neq∕cm2 to 2⋅1015neq∕cm2. Therefore systematic charge collection, leakage current and electric field measurements have been carried out at several annealing temperatures between 23°C and 80°C to investigate the dependence on temperature and fluence. The stability, temperature effects and the maximum amount of multiplication reachable were investigated. Severe changes in the signal pulses in time were observed with increasing amount of annealing. The pulse duration started to increase for enhanced charge multiplication, exceeding the readout shaping time of the ALIBAVA system.

A contingent valuation experiment about future particle accelerators at CERN.

Investment in basic science is mainly supported by government funding, but little is known about citizens' willingness to pay for large-scale projects. A survey to a representative sample of French taxpayers, designed as a contingent valuation experiment about a future particle accelerator for CERN, reveals that citizens' willingness to pay is correlated with education, income, age, and-crucially-previous awareness, attitudes and interest in science. A (slim) majority of the participants would accept paying more in taxes for CERN. The estimated willingness to pay is higher than the current implicit per capita tax burden of French citizens. The experimental setting is novel and replicable for empirically assessing social attitudes towards science for other research infrastructures and countries.

Angular Dependence of Copper Surface Damage Induced by an Intense Coherent THz Radiation Beam

In this work, we show the damage induced by an intense coherent terahertz (THz) beam on copper surfaces. The metallic surface was irradiated by multiple picosecond THz pulses generated by the Free Electron Laser (FEL) at the ISIR facility of the Osaka University, reaching an electric field on the sample surface up to ~4 GV/m. No damage occurs at normal incidence, while images and spectroscopic analysis of the surface point out a clear dependence of the damage on the incidence angle, the electric field intensity, and polarization of the pulsed THz radiation. Ab initio analysis shows that the damage at high incidence angles could be related to the increase of the absorbance, i.e., to the increase of the temperature around or above 1000 °C. The experimental approach we introduced with multiple fast irradiations represents a new powerful technique useful to test, in a reproducible way, the damage induced by an intense electric gradient on copper and other metallic surfaces in view of future THz-based compact particle accelerators.

Generation and Characterization of Attosecond Microbunched Electron Pulse Trains via Dielectric Laser Acceleration

Dielectric laser acceleration is a versatile scheme to accelerate and control electrons with the help of femtosecond laser pulses in nanophotonic structures. We demonstrate here the generation of a train of electron pulses with individual pulse durations as short as 270±80 attoseconds (FWHM), measured in an indirect fashion, based on two subsequent dielectric laser interaction regions connected by a free-space electron drift section, all on a single photonic chip. In the first interaction region (the modulator), an energy modulation is imprinted on the electron pulse. During free propagation, this energy modulation evolves into a charge density modulation, which we probe in the second interaction region (the analyzer). These results will lead to new ways of probing ultrafast dynamics in matter and are essential for future laser-based particle accelerators on a photonic chip.

Non-Evaporable Getter Ti-V-Hf-Zr Film Coating on Laser-Treated Aluminum Alloy Substrate for Electron Cloud Mitigation

For improving the vacuum and mitigating the electron clouds in ultra-high vacuum chamber systems of high-energy accelerators, the deposition of Ti-V-Hf-Zr getter film on a laser-treated aluminum alloy substrate was proposed and exploited for the first time in this study. The laser-treated aluminum surface exhibits a low secondary electron yield (SEY), which is even lower than 1 for some selected laser parameters. Non-evaporable getter (NEG) Ti-V-Hf-Zr film coatings were prepared using the direct current (DC) sputtering method. The surface morphology, surface roughness and composition of Ti-V-Hf-Zr getter films were characterized and analyzed. The maximum SEY of unactivated Ti-V-Hf-Zr getter film on laser-treated aluminum alloy substrates ranged from 1.10 to 1.48. The X-ray photoelectron spectroscopy (XPS) spectra demonstrate that the Ti-V-Hf-Zr coated laser-treated aluminum alloy could be partially activated after being heated at 100 and 150 °C, respectively, for 1 h in a vacuum and also used as a pump. The results were demonstrated initially and the potential application should be considered in future particle accelerators.

Impact of Flux Jumps on High-Precision Powering of Nb3Sn Superconducting Magnets

Nb3Sn superconducting magnets represent a technology enabler for future high-energy particle accelerators. A possible impediment, though, comes from flux jumps that, so far, could not be avoided by design unlike for NbTi technology. However, the impact of flux jumps on the magnet powering has not been properly investigated to date. Flux jumps appear during current ramps at relatively low value of current and tend to disappear towards nominal current. They are usually detected as voltage jumps between different magnet coils but they might also produce overall voltage jumps across the magnet electrical terminals. Such jumps might perturb the power converter feedback control loop and therefore potentially jeopardize its precision performance during energy ramps. This work aims at: (i) presenting preliminary experimental test results on some HL-LHC Nb3Sn model and prototype magnets, and (ii) attempting to build a simplified electrical model of the flux jumps, with focus only at its interaction with the power converter feedback control loop. Such a work is a starting point for outlining possible power converters control strategies able to minimize flux jumps impact on high-precision powering of Nb3 Sn superconducting magnets.

Applications of silicon strip and pixel-based particle tracking detectors

As the review of the European particle physics strategy is underway and planned future particle accelerators are being discussed worldwide, new, improved silicon tracking detector concepts are currently being studied for use at the proposed new facilities. This Technical Review outlines the current state of the art in silicon tracking detectors for particle physics along with examples of some recent applications in other areas. The various technologies currently available in particle physics are listed and their relative merits for different uses and environments are discussed. The silicon detector research programmes to allow optimal scientific returns from future particle physics experiments are described along with their current status. Advances in semiconductor technologies have enabled the development of numerous designs of silicon tracking detectors in particle physics. This Technical Review outlines the current state-of-the-art technologies and discusses challenges, future directions and some of the recent applications outside particle physics.

Purification of Selective Laser Melting Additive Manufactured Niobium for Superconducting RF-Applications Transactions on Applied Superconductivity

Superconducting radio frequency (SRF)-cavities in the field of particle accelerators are mostly manufactured from bulk or sheet niobium (Nb). Complex shapes of SRF-components like high order mode-couplers or cavities working at high fields are high in cost and lead-time. The fabrication of these parts via additive manufacturing promises a more effective production and higher flexibility in geometries. At CERN, additive manufacturing with pure Nb by selective laser melting (SLM) is under development for future particle accelerator components. In comparison with conventionally produced Nb SRF-components, made of highly pure Nb, the SLM-manufactured Nb material contains considerable impurities due to raw material processing and the SLM-process conditions itself. Contaminations with light elements, such as oxygen and nitrogen, introduced into the material's structure by the powder and the SLM process, contribute to an important lowering of the residual resistance ratio (RRR). In order to increase the RRR-values for conventionally manufactured SRF-cavities, titanium (Ti) gettering-heat treatments were investigated since the 1980s. This paper introduces the first representative Nb-material realized at CERN by additive manufacturing by SLM and the current studies on the post-treatment of sample material by vacuum firing and Ti-gettering-heat treatments. The samples are analyzed by mass spectroscopy and RRR-measurements in correlation with different process parameters. First trials with Ti-gettering heat treatments demonstrate an increase in the RRR from 7 to above 100, whereas the oxygen content could be reduced from 600 to 17 ppm. These results show that a purification post-treatment of SLM-manufactured Nb-components is feasible and pave the way for functioning SRF-components made by additive manufacturing.

Magnetization and Creep in YBCO Tape and CORC Cables for Particle Accelerators: Value and Modification Via Preinjection Cycle

High-temperature superconductors (HTSs), including YBCO coated conductors, are being considered for high field dipole and quadrupole magnets for future particle accelerators. The quality of the magnetic field generated by the magnet and its temporal stability are important considerations. The persistent current magnetization of superconductors can degrade the field quality of the magnetic field generated by magnets made from them by causing error fields, and flux creep in the superconductor may lead to drift in the value of the magnetic field during magnetic field ramp holding sequences, including at particle injection. HTS composites possess larger persistent current magnetizations and larger flux creep than low-temperature superconductor composites, and therefore, the error fields and any temporal change of them are typically larger. In this paper, the 4.2 K magnetization of a small segment of coated conductor was measured in a vibrating sample magnetometer in perpendicular fields up to 14 T, and the decay was measured at 1 T. The magnetization and decay of a CORC cable were measured in a 12 T cryogen-free magnet. A Hall sensor, placed close to the sample, was used to measure the magnetization. The applied magnetic field was ramped through full flux penetration, and then, brought within one flux penetration depth of the target applied magnetic field before the decay measurement was started. Selective cycling of the magnetic field applied to the samples was explored as a technique to reduce the magnetization and its decay. Significant suppression of both the initial magnetization at 1 T and its decay were observed.

Energy Density Method: An Approach for a Quick Estimation of Quench Temperatures in High-Field Accelerator Magnets

Accelerator magnets for future particle accelerators are designed to work with as high energy densities as possible to achieve high fields and compact magnet designs. A key factor limiting the energy density is given by the protection in case of a quench: If a quench occurs, the stored energy must be first absorbed by the windings, and the magnet temperature shall not exceed a given limit. In this paper, we present a back-of-the-envelope method for estimating the magnet's maximum temperature after a quench based on its stored energy. The method combines the existing concepts of MIIT (Mega*current*current*time), time margin, and protection delay to allow for an easy and direct calculation of the hot-spot temperature. We apply the proposed method to several Nb 3 Sn dipole and quadrupole magnets developed for the high luminosity Large Hadron Collider and the Future Circular Collider for hadron-hadron collisions and compare the results to a more detailed simulation. The proposed energy density method is a useful tool for fast feedback in the early magnet design phase to ensure that the magnet is not impossible to protect.

On the compatibility of porous surfaces with cryogenic vacuum in future high-energy particle accelerators

Recently, pulsed laser processing of Cu samples has been demonstrated to produce rough surfaces whose structuring at the nanoscale ensures an impressive reduction of the secondary electron yield. This feature has an undoubted appealing for applications in future high energy particle accelerators. However, the effective application of such laser treated surfaces in this context requires a rigorous evaluation of their vacuum behavior, especially when used at cryogenic temperatures. To this aim, here, we compare thermal programmed desorption between 20 and 70 K by dosing Ar multilayers of different thicknesses on a laser treated copper substrate and on its flat counterpart. Our results highlight that the spongelike structural features confer to the laser treated sample's non-negligible effects due to the gas-substrate interaction. This results in a much vaster and higher desorption temperature range with respect to what is observed from the flat substrates. This evidence could render it very difficult to find temperature intervals for which detrimental vacuum transients could be avoided in the cryogenic beam pipes. On these bases, although the electron cloud mitigation efficiency has been settled, before definitely including porous surfaces in any cryogenic machine design, all the consequences of having a rough rather than a flat wall should be carefully evaluated.

Effect of resin impregnation on the transverse pressure dependence of the critical current in ReBCO Roebel cables

ReBCO Roebel cables are being developed as one of the promising routes towards much higher-field magnets for future particle accelerators. The Lorentz forces generated in such magnets lead to sizeable transverse pressure on the superconducting cables, in the present 15–20 T magnet designs of the order of 100–150 MPa. Contrary to single ReBCO coated conductors, unprotected Roebel cable due to their uneven surface, start to degrade already with 40 MPa as shown in earlier work. Previous publications also showed that proper impregnation with epoxy resin can dramatically improve the transverse pressure tolerance of a relatively simple model cable with 10 ReBCO strands, assembled from SuperPower tape. However, the epoxy resins used until now are not suitable for large-scale magnet vacuum impregnation. For this reason, an alternative impregnation method using CTD-101K with S2 glass fibers has been implemented. The present paper expands further on the effect of impregnation on the transverse pressure susceptibility of Roebel cables. Three cables, with an architecture that is directly relevant for the so-called EuCARD-2 accelerator demonstrator magnet, were investigated with a variable transverse mechanical load at 4.2 K in a 10.5 T perpendicular magnetic field. The new cables feature a relatively long transposition length of 226 mm and comprise 15 strands of either SuperPower or Bruker coated conductor. For reference, one of the new SuperPower tape based cables was impregnated with the same resin as the one used in the earlier publications. The other cables were impregnated using the new method. The critical current at 4.2 K of the Bruker tape Roebel cable is 2.5 to 3 times higher than that of the SuperPower tape ones with similar architecture. Remarkably no critical current degradation was observed for transverse pressure up to 440 MPa in the SuperPower tape cables and up to 370 MPa in the Bruker tape cable. These values by far satisfy the design requirements of presently envisaged 20 T class accelerator demonstrator magnets.

Record high-gradient SRF beam acceleration at Fermilab

Many modern and future particle accelerators employ high gradient superconducting RF (SRF) to generate beams of high energy, high intensity and high brightness for research in high energy and nuclear physics, basic energy sciences, etc. In this paper we report the record performance large-scale SRF system with average beam accelerating gradient matching the International Linear Collider (ILC) specification of 31.5 MV m−1. Design of the eight cavity 1.3 GHz SRF cryomodule, its performance without the beam and results of the system commissioning with high intensity electron beam at Fermilab Accelerator Science and Technology (FAST) facility are presented. We also briefly discuss opportunities for further beam studies and tests at FAST including those on even higher gradient and more efficient SRF acceleration, as well as exploration of the system performance with full ILC-type beam specifications.

Electrothermal Modeling of Quench in REBCO Roebel Cables

The possibility of REBCO Roebel cables to carry a large amount of current in high external magnetic fields makes them suitable for application in future particle accelerator magnets. The physical phenomena related to the initiation and propagation of a quench event in Roebel cables have not yet been fully analyzed. In particular, given the peculiar architecture of these conductors, the features of the current redistribution and normal zone propagation after quench in individual tapes still require experimental and theoretical investigation. In this paper, an innovative quasi-3D electrothermal FEM model is developed to analyze the effect of quench in HTS Roebel cables. The model is based on a reduced dimensionality approach, under the assumption of a negligible thickness of the individual REBCO tapes. While the tapes are meshed in an identical 1-D pattern, the noncontinuous electrical and thermal contacts among them are accounted for as a sources in the corresponding thermal and electrical equations. Notwithstanding these significant assumptions, the main physical aspects involved in the quench initiation and propagation are retained and correctly described by the reduced dimensionality approach. The quench energy and the normal zone propagation velocity are analyzed for different cable configurations.

Application of probabilistic modelling for the uncertainty evaluation of alignment measurements of large accelerator magnets assemblies

Micrometric assembly and alignment requirements for future particle accelerators, and especially large assemblies, create the need for accurate uncertainty budgeting of alignment measurements. Measurements and uncertainties have to be accurately stated and traceable, to international standards, for metre-long sized assemblies, in the range of tens of µm. Indeed, these hundreds of assemblies will be produced and measured by several suppliers around the world, and will have to be integrated into a single machine. As part of the PACMAN project at CERN, we proposed and studied a practical application of probabilistic modelling of task-specific alignment uncertainty by applying a simulation by constraints calibration method. Using this method, we calibrated our measurement model using available data from ISO standardised tests (10360 series) for the metrology equipment. We combined this model with reference measurements and analysis of the measured data to quantify the actual specific uncertainty of each alignment measurement procedure. Our methodology was successfully validated against a calibrated and traceable 3D artefact as part of an international inter-laboratory study. The validated models were used to study the expected alignment uncertainty and important sensitivity factors in measuring the shortest and longest of the compact linear collider study assemblies, 0.54 m and 2.1 m respectively. In both cases, the laboratory alignment uncertainty was within the targeted uncertainty budget of 12 µm (68% confidence level). It was found that the remaining uncertainty budget for any additional alignment error compensations, such as the thermal drift error due to variation in machine operation heat load conditions, must be within 8.9 µm and 9.8 µm (68% confidence level) respectively.

Photocathode laser based bunch shaping for high transformer ratio plasma wakefield acceleration

Beam driven plasma acceleration is one of the most promising candidates for future compact particle accelerator technologies. In this scheme a particle bunch drives a wake in a plasma medium. The fields inside of the wake can be used to accelerate a trailing witness bunch. To maximise the ratio between acceleration of the witness to deceleration of the drive bunch, the so called transformer ratio, several methods have been proposed. The ones yielding the most favorable results are based on shaped drive bunches that are long in terms of the plasma wavelength. We present here methods to create such drive bunches employing temporally shaped UV-laser pulses for the extraction of electron bunches from a photo-electron gun. Theoretical considerations, experimental results and possibilities for further improvements are discussed.

Investigations into dual-grating THz-driven accelerators

Advanced acceleration technologies are receiving considerable interest in order to miniaturize future particle accelerators. One such technology is the dual-grating dielectric structures, which can support accelerating fields one to two orders of magnitude higher than the metal RF cavities in conventional accelerators. This opens up the possibility of enabling high accelerating gradients of up to several GV/m. This paper investigates numerically a quartz dual-grating structure which is driven by THz pulses to accelerate electrons. Geometry optimizations are carried out to achieve the trade-offs between accelerating gradient and vacuum channel gap. A realistic electron bunch available from the future Compact Linear Accelerator for Research and Applications (CLARA) is loaded into an optimized 100-period dual-grating structure for a detailed wakefield study. A THz pulse is then employed to interact with this CLARA bunch in the optimized structure. The computed beam quality is analyzed in terms of emittance, energy spread and loaded accelerating gradient. The simulations show that an accelerating gradient of 348 ± 12 MV/m with an emittance growth of 3.0% can be obtained.