Electron Cloud Effects Research Articles

Sub-THz Characterization of Technical Surfaces for Particle Accelerator Vacuum Chambers.

Coatings play a crucial role in the functionality of vacuum chambers in particle accelerators, serving a dual goal by efficiently facilitating pumping and mitigating electron cloud effects. However, their impact on the surface impedance of the chamber walls raises concerns, potentially affecting the machine performance and imposing limitations on achievable energies and currents. Therefore, an electromagnetic characterization is essential for a comprehensive study of accelerator structures, particularly in the context of the next-generation machines where the demand for extremely short particle bunches accentuates the importance of evaluating material responses in the very-high-frequency region. We present a technique for probing the sub-THz response of coating materials by measuring pulsed signals passing through a specifically designed waveguide, in which is placed a slab with the deposited material under test. The proposed methodology allows for a comprehensive exploration of the electromagnetic properties of the most used technical surfaces (substrate plus coatings) in accelerators under realistic conditions, providing valuable insights into their behavior in the sub-THz frequency range. The experimental data of three different Non-Evaporable Getter coating samples, prepared on a copper substrate at the CERN deposition facilities under different sputtering conditions, are discussed. The findings contribute to a deeper understanding of the complex interactions between coatings and accelerator structures, with the aim of optimizing performance and efficiency in the evolving landscape of particle acceleration technologies. The limitations and advantages of the technique are also reported.

Study on the vacuum properties of laser-etched oxygen-free copper

The performance of operating particle accelerators has been seriously affected by the electron cloud (e-cloud) effect. The secondary electron emission (SEE) and the e-cloud can be effectively suppressed through laser-etching the inner surface of the vacuum chamber. Oxygen-free copper (OFC) has become the first choice for the vacuum chambers of modern accelerators due to its high electric and thermal conductivity and effective radiation shielding property. It is necessary to study the vacuum properties of the laser-etched OFC for the application in the particle accelerators. In this paper, the photon stimulated desorption (PSD) yield and the outgassing rate of the laser-etched OFC were measured. The results show that the laser-etched OFC presents lower PSD yield compared to the untreated OFC, while the outgassing rates of the laser-etched and unetched samples are similar.

Quadrupolar 23Na+ NMR relaxation as a probe of subpicosecond collective dynamics in aqueous electrolyte solutions

Nuclear magnetic resonance relaxometry represents a powerful tool for extracting dynamic information. Yet, obtaining links to molecular motion is challenging for many ions that relax through the quadrupolar mechanism, which is mediated by electric field gradient fluctuations and lacks a detailed microscopic description. For sodium ions in aqueous electrolytes, we combine ab initio calculations to account for electron cloud effects with classical molecular dynamics to sample long-time fluctuations, and obtain relaxation rates in good agreement with experiments over broad concentration and temperature ranges. We demonstrate that quadrupolar nuclear relaxation is sensitive to subpicosecond dynamics not captured by previous models based on water reorientation or cluster rotation. While ions affect the overall water retardation, experimental trends are mainly explained by dynamics in the first two solvation shells of sodium, which contain mostly water. This work thus paves the way to the quantitative understanding of quadrupolar relaxation in electrolyte and bioelectrolyte systems.

Role of surface chemistry in conditioning of materials in particle accelerators

For the vacuum scientists and the accelerator community, finding solutions to mitigate pressure rises induced by electron, photon and ion desorption, and also beam instabilities induced by ion and electron clouds is a major issue. Along the time, changes in the surface chemistry of vacuum chambers are observed during beam operations, leading to modifications of: outgassing rates, stimulated desorption processes and secondary emission yields (SEY). To understand the role of the surface chemistry of air exposed materials in the electron conditioning process, typical air exposed materials used in particle accelerators : thin film coatings (NEG, i.e a Ti-Zr-V alloy, and carbon), copper (and its oxides Cu2O and CuO) and niobium were conditioned by low energy electron irradiation for a better understanding of electron-cloud effect. First, SEY was measured to understand the changes of surface conditioning upon particle irradiation; then, surface chemistry evolution after electron irradiation was investigated by both XPS and TOF-SIMS analyses using the ANDROMEDE facility at IJCLab. Finally, the relationship between the surface chemistry and the conditioning phenomenon will be discussed.

Suppression of electron cloud using copper thermal spray coating in SuperKEKB

The electron cloud effect (ECE) has posed serious challenges in recent high-intensity proton and positron rings. An aluminum beam pipe coated with copper thermal spray film (T.S. coating), which had a low secondary electron yield (SEY), was developed to suppress electron clouds in accelerators. After 72 h conditioning, the lowest δmax (the maximum SEY within scanning) reached ∼0.7. In addition, to check the applicability of T.S. coating in accelerators, its outgassing rate and surface resistance were measured as a reference. Finally, an aluminum beam pipe with a T.S. coating was produced and installed in the positron ring of SuperKEKB to measure the electron density. The results show that the electron density in the finished beam pipe was comparable with that of the TiN-coated beam pipe; however, the relatively high surface resistance of T.S. coating must be considered when applied to accelerators.

Mitigation of electron cloud effects in the FCC-ee collider

Electron clouds forming inside the beam vacuum chamber due to photoemission and secondary emission may limit the accelerator performance. Specifically, the electron clouds can blow up the vertical emittance of a positron beam, through a head-tail-type single-bunch instability, if the central electron density exceeds a certain threshold value, that can be estimated analytically. Using the codes PyECLOUD and VSim, we carried out detailed simulations of the electron-cloud build up for the main arcs and the damping ring of the FCC-ee collider, in order to identify the effective photoemission rate and secondary emission yield required for achieving and maintaining the design emittance. To this end, we present the simulated electron density at the centre of the beam pipe for various bunch spacings, secondary emission yields, and photoemission parameters, in the damping ring and in the arcs of the collider positron ring. To gain further insight into the underlying dynamics, the obtained spatial and energy distributions of the cloud electrons are illustrated as a function of time. In addition, we compare results obtained for two different secondary emission models (“Furman–Pivi” and “ECLOUD”), thereby indicating the uncertainty inherent in this type of study, without any prototype vacuum chambers yet available. We also point out a few situations where the two secondary-emission models yield similar density values. Finally, based on our simulation results for two different design variants, we conclude that the new parameter baseline of the FCC-ee will facilitate electron-cloud mitigation.

Non-monotonic dependence of heat loads induced by electron cloud on bunch population at the LHC

Electron cloud effects are among the main performance limitations for the operation of the Large Hadron Collider with 25 ns bunch spacing. Electrons impacting on the beam screens of the superconducting magnets induce a significant heat load reaching values close to the full cooling capacity available from the cryogenic system in some LHC sectors. To better understand this performance limitation, numerical simulations with the PyECLOUD code were performed to study the dependence of the heat load on different beam and machine parameters, in particular the bunch population, which is foreseen to be considerably increased with the impending HL-LHC upgrade. The simulations predict a complex, non-monotonic behavior of the heat load with bunch population which has important implications in defining the upgrade of the cryogenic system required for coping with HL-LHC beam intensities. An in-depth analysis of the simulation results shows that the non-monotonic dependence of the heat load on the bunch population is driven by an interplay between the spectrum of the impacting electrons and the shape of the Secondary Electron Yield curve. Experimental data were collected at the LHC during normal operation and dedicated experiments in order to validate the simulation model and confirm the expected non-monotonic behavior. The simulation results are found to reproduce very well the measurement data.

High intensity proton beam impact at 440 GeV/c on Mo and Cu coated CfC/graphite and SiC/SiC absorbers for beam intercepting devices

Beam Intercepting Devices (BIDs) are essential protection elements for the operation of the Large Hadron Collider (LHC) complex. The LHC internal beam dump (LHC Target Dump Injection or LHC TDI) is the main protection BID of the LHC injection system; its main function is to protect LHC equipment in the event of a malfunction of the injection kicker magnets during beam transfer from the SPS to the LHC. Several issues with the TDI were encountered during LHC operation, most of them due to outgassing from its core components induced by electron cloud effects, which led to limitations of the injector intensity and hence had an impact on LHC availability. The absorbing cores of the TDIs, and of beam intercepting devices in general, need to deal with high thermo-mechanical loads induced by the high intensity particle beams. In addition, devices such as the TDI — where the absorbing materials are installed close to the beam, are important contributors to the accelerator impedance budget. To reduce impedance, the absorbing materials that make up the core must be typically coated with high electrical conductivity metals. Beam impact testing of the coated absorbers is a crucial element of development work to ensure their correct operation.In the work covered by this paper, the behaviour of several metal-coated absorber materials was investigated when exposed to high intensity and high energy proton beams in the HiRadMat facility at CERN. Different coating configurations based on copper and molybdenum, and absorbing materials such as isostatic graphite, Carbon Fibre Composite (CfC) and Silicon Carbide reinforced with Silicon Carbide fibres (SiC-SiC), were tested in the facility to assess the TDI's performance and to extract information for other BIDs using these materials. In addition to beam impact tests and an extensive Post Irradiation Examination (PIE) campaign to assess the performance of the coatings and the structural integrity of the substrates, extensive numerical simulations were carried out.

Study on the anisotropy of the secondary electron yield and resistance of the laser-etched copper

The performance of modern accelerators has been seriously affected by the electron cloud effect (e-cloud) caused by the accumulation of secondary electrons. Such problem can be effectively eliminated by laser-etching the inner surface of the vacuum chambers of the modern accelerators. In this work, surfaces of oxygen-free copper (OFC) were laser etched following either parallel lines or square-grid pattern with different scanning speeds. The secondary electron yields (SEYs), resistance, and roughness of the etched OFC samples were measured from various directions. The surfaces of etched OFC samples have been proven to form some regularly arranged grooves or cones, the geometries of which are directly related to the etching parameters. As a result, the parallelly etched samples exhibit apparent anisotropy. The sample etched along parallel lines, present higher SEYs, resistance and roughness when measured in the direction perpendicular to the groove-lines. But the sample etched following square-grid have little difference in the two directions and have a lower SEY value than the parallelly etched samples. Moreover, low-speed etching is more favorable to producing large aspect ratio and porosity structures, which is conducive to obtain lower SEY values.

Numerical simulations of electron cloud build-up in circular accelerators in the presence of multimode distribution beams

Electron cloud effects have become one of the main performance limitations for circular particle accelerators operating with positively charged beams. Among other machines worldwide, the CERN Super Proton Synchrotron (SPS) as well as the Large Hadron Collider (LHC) are affected by these phenomena. Intense efforts have been devoted in recent years to improving the understanding of electron cloud (EC) generation with the aim of finding efficient mitigation measures. In a different domain of accelerator physics, non-linear resonances in the transverse phase space have been proposed as novel means of manipulating charged particle beams. While the original goal was to perform multi-turn extraction from the CERN Proton Synchrotron (PS), several other applications have been proposed. In this paper, the study of EC generation in the presence of charged particle beams with multimode horizontal distribution is presented. Such a peculiar distribution can be generated by different approaches, one of which consists in splitting the initial Gaussian beam distribution by crossing a non-linear resonance. In this paper, the outcome of detailed numerical simulations is presented and discussed.

FIELDS OF AN ULTRA-RELATIVISTIC BEAM OF CHARGED PARTICLES BETWEEN PARALLEL PLATES. EXACT TWO-DIMENSIONAL SOLUTIONS BY THE METHOD OF IMAGES AND APPLICATIONS TO THE HL-LHC

Exact 2D analytic expressions for E and B fields and their potentials created by a linear beam of relativistic charged particles between infinite perfectly conductive plates and ferromagnetic poles are derived. The solutions are obtained by summing an infinite sequence of fields from linear charge-images and current-images in complex space. Knowledge of the normal component of the E field on the conductor surface makes it possible to calculate the induced electric charge surface density. In addition, we derive within an improved linear approximation new analytical expressions for fields near the beam in the case of an arbitrary beam offset from the median plane. The mathematical features of exact solutions and limitations for the applicability of linear approximations are specified.The primary goals of the future high-luminosity p-p and heavy-ion LHC programme are the search for yet unobserved effects of physics beyond the SM, searches for rare or low-sensitivity processes in the Higgs sector, and probing in more detail the mechanism of EW symmetry breaking. This programme relies on the stable operation of the accelerator. However, as the beam luminosity increases, a number of destabilizing phenomena occur, in particular field emission, enhancing the electron cloud effect. For the case of a proton beam, we apply the exact 2D solution for estimating the intensity of electron field emission activated by the electric field of the beam in collimators of the future high-luminosity LHC. Calculation shows that the field emission intensity is very sensitive to a collimator surface roughness. In addition, with a relatively small and accidental beam displacement from the median path, about 20% of the collimator half-gap, the emission intensity increases by a factor of 1.E+7. This will partially neutralize the beam space charge, violating acceleration dynamics and enhancing instability effects.

CEPC and FCCee parameters from the viewpoint of the beam–beam and electron cloud effects

The parameters of CEPC and FCC-ee are being fixed. Despite the same circumference and energy of the two colliders, their strategies of the target luminosity are different. The key issues for the target luminosity are the beam–beam and electron cloud effects. We present studies on the beam–beam and electron cloud effects for the two colliders and discuss the difference of the strategies of the two colliders.

Charge exchange injection into accelerators and storage rings

The development of charge exchange injection technology at the Budker Institute of Nuclear Physics, Siberian Branch of the Russian Academy of Sciences, mastering charge exchange injection at research centers worldwide, and the experimental observation, explanation, and damping of electron–proton instability (electron cloud effect) are overviewed. Techniques for preparing a circulating proton beam whose intensity is an order of magnitude higher than the space-charge limit are considered. Problems with stripping-foil stability in the process of injection of intense beams and laser ionization of accelerated atoms are also discussed.

Mitigating the electron cloud effect in the SuperKEKB positron ring

SuperKEKB is an electron-positron collider with asymmetric energies located at the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan. After more than five years of upgrading work on KEKB, phase-1 commissioning commenced in February 2016 and ended in June of the same year. Following a 20-month shutdown for the installation of a particle detector, BELLE II, phase-2 commissioning commenced in March 2018 and ended in July of the same year. This paper describes one major issue faced by SuperKEKB: the electron cloud effect (ECE) in the positron ring, which was observed during phase-1 commissioning. In the high-beam-current region, electron clouds, i.e., the source of the ECE, existed in the beam pipes at drift spaces of the ring, which had antechambers and a titanium nitride (TiN) film coating as countermeasures against the ECE. Permanent magnets and solenoids used to generate magnetic fields in the beam direction were attached to the beam pipes as additional countermeasures before the next commissioning phase commenced. Consequently, during phase-2 commissioning, experiments showed that the threshold of the current linear density for exciting the ECE increased by a factor of at least 2 compared to that during phase-1 commissioning. While the countermeasures were strengthened, the effectiveness of the antechambers and TiN film coating of the real beam pipe was reevaluated. Through various simulations and dedicated experiments during phase-2 commissioning, the antechambers were found to be less effective than anticipated with regard to reducing the number of photoelectrons in the beam channel. In contrast, the TiN film coating had a low secondary electron yield, as expected.

SuperKEKB main ring vacuum system status until the end of Phase-2 commissioning

SuperKEKB is an electron–positron collider with asymmetric energies at KEK aiming at a high luminosity of 8.0 × 1035 cm–2 s–1. The main ring (MR) consists of two rings: a 4 GeV positron ring (low-energy ring, LER) and a 7 GeV electron ring (high-energy ring, HER). During the Phase-1 commissioning of the MR from February to June 2016 and the Phase-2 commissioning from March to July 2018, the vacuum system worked well overall. By the end of Phase-2, the total beam doses (integrated beam currents) were 1113 and 1002 Ah, and the maximum beam currents were approximately 1010 and 870 mA for the LER and HER, respectively. The increase in pressure per unit beam current decreased steadily. The photodesorption coefficients reached approximately 1 × 10−6 and 7 × 10−8 molecules photon−1 with photon doses of 5.88 × 1024 and 9.27 × 1024 photons m−1 for the LER and HER, respectively. The beam lifetime was limited by the Touschek effect in both rings. Various new vacuum components, including those installed after the Phase-1 commissioning, such as the beam pipes for the Belle II particle detector, beam collimators, and so on, have been functioning well so far. The electron cloud effect was observed in the LER during Phase-1, but it was suppressed by additional countermeasures applied before Phase-2. Regarding the pressure bursts accompanied with beam losses, which were frequently observed during Phase-1, the frequency of the pressure bursts drastically decreased during Phase-2, but the longer operation time with lower beam currents during Phase-2 than Phase-1 should be taken into account. Several new problems, such as the heating of stainless-steel beam pipes due to the synchrotron radiation emitted from the final focusing quadrupole magnets at the collision point, were found during Phase-2, and countermeasures are being prepared for the Phase-3 commissioning, which will commence in March 2019. SuperKEKB has been accumulating various data and information that will facilitate the design and operation of the vacuum systems in future high-current accelerators.

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.

Beam scrubbing of beam pipes during the first commissioning of SuperKEKB

The first (Phase-1) commissioning of SuperKEKB—an electron-positron collider with asymmetric energies located at KEK, in Tsukuba, Japan—started in February 2016, after more than five years of upgrading work on KEKB, and successfully ended in June of the same year. This paper describes one major task of Phase-1 commissioning: beam scrubbing the surface of the beam pipes, to prepare them for a sufficiently long beam lifetime and low background noise in the next commissioning, when a new particle detector will be installed. The pressure rises per unit beam current (dP/dI [Pa A−1]) were continuously monitored, and the coefficient of photon-stimulated desorption (PSD), η [molecules photon−1], was evaluated in the arc sections. The value of η decreased steadily with the beam dose, as expected. For arc sections in the positron ring, where most of the beam pipes were newly fabricated, the decrease in η against the photon dose (D) was similar to that previously reported; that is: η∝D−0.5∼0.8. At high storage beam currents, the evolution of η was affected by gas desorption resulting from the multipacting of electrons—that is, the electron cloud effect (ECE), which is a phenomenon particular to high-intensity positron rings. For the arc sections in the electron ring, η also decreased smoothly with the photon dose D, approximately as ∝ D−0.8. Given that most of these beam pipes were reused from KEKB, the value of η was much lower than that of the positron ring, and also lower than that of the electron ring of KEKB from the early stages of D. This implies that the surface of the reused beam pipes remembered the conditions in the KEKB, which is a known memory effect. The results obtained for η are compared with those obtained in various other accelerators.

Vacuum system of positron damping ring for SuperKEKB

To satisfy the requirements of high beam quality for positron injection into the SuperKEKB electron-positron collider, a new damping ring (DR) is constructed in an upgraded injector system. The DR has two arc sections and two straight sections. In the arc sections, the beam pipes have beam position monitor blocks, pumping ports, radio-frequency-shielded bellows, and special connection flanges to fit narrow spaces between closely located magnets. The beam pipes have antechambers on both sides of a beam channel to deal with synchrotron radiation (SR) which irradiates both sides of the beam pipes. The antechambers are also effective in reducing the electron cloud in the beam channel. To remove the heat caused by SR irradiation, a water cooling system is also required in the arc sections. In the straight sections on the other hand, an antechamber structure and a water cooling system are not necessary, and the beam pipes are much simpler than those in the arc sections. Required beam lifetime due to residual gas scattering is longer than 1000 s, and the average pressure should be lower than 1 × 10−5 Pa. Nonevaporable getter (NEG) pumps are mainly used with auxiliary ion pumps, and the average effective pumping speed in the arc sections is approximately 0.013 m3 s−1 m−1 (for CO), just after NEG activation. The material of the beam pipes is aluminum alloy. As additional countermeasures against the electron cloud effect, titanium nitride coating and grooved surfacing are adopted. These reduce the secondary electron yield of the inner surfaces of the beam pipes.

Achievements and problems in the first commissioning of superKEKB vacuum system

The first (phase-1) commissioning of SuperKEKB, an asymmetric-energy electron–positron collider at High Energy Accelerator Research Organization, known as KEK, Japan, began in February 2016 and successfully ended in June 2016. Three major tasks of the phase-1 commissioning were as follows: (1) vacuum scrubbing of the new beam pipes in anticipation of a sufficiently long beam lifetime in the next commissioning, (2) checking the stabilities of various new vacuum components at high beam currents of approximately 1 A, and (3) beam tuning to study stability and achieve low emittance. The coefficient of photon stimulated desorption rate, η (molecules photon−1), decreased steadily with increasing photon dose, as expected. The temperature increases of the bellows chambers, connection flanges, etc., were less than 5 °C at 1 A. The effectiveness of an antechamber, a TiN coating, and a clearing electrode to suppress the electron cloud effect (ECE) was confirmed. However, the ECE in the Al-alloy bellows chambers without TiN coating was observed. The use of permanent magnets to create an axial magnetic field successfully suppressed this effect. However, the ECE was still exhibited at higher linear current densities (=bunch current/bunch spacing). Furthermore, the attachment of permanent magnets or solenoids even to the beam pipes with antechambers and TiN coating at drift spaces is under consideration before the next (phase-2) commissioning.

Simulating synchrotron radiation in accelerators including diffuse and specular reflections

Accurate modeling of synchrotron-radiation photon flux inside the vacuum chamber is important for mitigating electron-cloud effects at future accelerators.