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Electron cloud ($e$-cloud) mitigation is an essential requirement for proton circular accelerators in order to guarantee beam stability at a high intensity and limit the heat load on cryogenic sections. Laser-engineered surface structuring is considered a credible process to reduce the secondary electron yield (SEY) of the surfaces facing the beam, thus suppressing the $e$-cloud phenomenon within the high luminosity upgrade of the LHC collider at CERN (HL-LHC). In this study, the SEY of Cu samples with different oxidation states, obtained either through laser treatment in air or in different gas atmospheres or via thermal annealing, has been measured at room and cryogenic temperatures and correlated with the surface composition measured by x-ray photoelectron spectroscopy. It was observed that samples treated in nitrogen display the lowest and more stable SEY values, correlated with the lower surface oxidation. In addition, the surface oxide layer of air-treated samples charges upon electron exposure at a low temperature, leading to fluctuations in the SEY.

International audience; Electron cloud is a critical phenomenon in particle accelerators operating with high intensity and positively charged beams, as it is responsible for beam instabilities, vacuum degradation, and heat load on cryogenic sections. Electron clouds provoke a conditioning of the beam pipe that is reflected on the reduction of its secondary electron yield (SEY). However, such a benefit is partially lost when vacuum sectors are vented for maintenance of accelerators; this phenomenon is called deconditioning. Samples removed from accelerators are also vented before surface analysis. Deconditioning amplifies the electron cloud at the resuming of beam operation and, on the other hand, hinders the understanding of the electron multipacting mechanism from surface analysis data. In this paper, copper deconditioning was studied for samples stored in a desiccator over months. Immediately after air exposure, an increase of the SEY is observed. This increase is driven by carbon recontamination and copper hydroxide growth on the conditioned surface as observed by x-ray photoelectron spectroscopy. After deconditioning, the differences of SEY present on the tested samples partially vanish, in particular, for surfaces conditioned to a maximum SEY below 1.45, limiting the level of accessible information when analyzing components extracted from accelerators. However, for a maximum SEY above 1.45, the differences remain visible for at least 8 weeks of storage. Among different storage conditions, vacuum efficiently stops the SEY increase over time. Besides, the memory effect of the conditioning is preserved over at least 4 months when closing the vacuum system on itself after venting with a clean and dry gas.

International audience; As a source of heat load on cryogenic sections, the electron cloud is currently a major limitation to the intensity of some modern particle accelerators such as the LHC and its high luminosity upgrade at CERN. During LHC operation, conditioning of the copper beam pipe surface occurs, leading to a decrease of the cloud intensity. To understand the role of the different chemical surface components of air exposed copper in the electron conditioning process, air exposed copper samples as well as specific model surfaces produced in the laboratory, namely sputter-cleaned copper and carbon-free cuprous oxide (Cu$_2$O), were conditioned by low energy electron irradiation. Conditioning of air exposed copper results in a decrease of the maximum secondary electron yield (SEY) below 1.1. Surface cleaning by electron stimulated desorption and carbon graphitization without increase of the carbon surface concentration are observed by x-ray photoelectron spectroscopy. After conditioning, the maximum SEY of both sputter-cleaned copper and Cu$_2$O remains higher than 1.1. No significant surface modification is observed by x-ray photoelectron spectroscopy during irradiation for these two surfaces. These results prove that neither an increase of the amount of surface carbon nor oxide modification is responsible for the SEY reduction observed during electron irradiation of air exposed copper. They confirm that graphitic carbon is required to decrease the maximum SEY of copper below 1.1.

In systems experiencing high temperature load, especially those under vacuum, radiative heat transfer plays a significant role. The key material parameter in heat transfer by radiation is spectral emissivity. The ability to engineer radiative response in a wide spectral region is therefore of paramount importance for radiation cooling of high temperature systems. Thermal emission control can be achieved via modification of surface topography and is currently realised with photonic crystal structures that can be tuned to enhance emissivity in a particular spectral region [1] as well as textured metallic coatings for a broadband emissivity control [2].

The surface resistance of state-of-the-art REBa2Cu3O7−x coated conductors has been measured at 8 GHz versus temperature and magnetic field. We show that the surface resistance of REBa2Cu3O7−x strongly depends on the microstructure of the material. We have compared our results to those determined by the rigid fluxon model. The model gives a very good qualitative description of our data, opening the door to unravel the effect of material microstructure and vortex interactions on the surface resistance of high temperature superconductors. Moreover, it provides a powerful tool to design the best coated conductor architecture that minimizes the infield surface resistance. We have found that the surface resistance of REBa2Cu3O7−x at 50 K and up to 9 T is lower than that of copper. This fact poses coated conductors as strong candidate to substitute copper as a beamscreen coating in CERN’s future circular collider. To this end we have also analyzed the secondary electron yield (SEY) of REBa2Cu3O7−x and found a compatible coating made of sputtered Ti and amorphous carbon that decreases the SEY close to unity, a mandatory requirement for the beamscreen chamber of a circular collider in order to prevent the electron-cloud phenomenon.

Effective thermal management is of paramount importance for all high-temperature systems operating under vacuum. Cooling of such systems relies mainly on radiative heat transfer requiring high spectral emissivity of surfaces, which is strongly affected by the surface condition. Pulsed laser structuring of stainless steel in air resulted in the spectral hemispherical emissivity values exceeding 0.95 in the 2.5–15 µm spectral region. The effects of surface oxidation and topography on spectral emissivity as well as high temperature stability of the surface structures were examined. High performance stability of the laser textured surfaces was confirmed after thermal aging studies at 320°C for 96 hours.

Within the CLIC (Compact Linear Collider) project, feasibility studies of a photoinjector option for the drive beam as an alternative to its baseline design using a thermionic electron gun (Geschonke et al. [1] ) are on-going. This R&D program covers both the laser and the photocathode side. Cesium antimonide cathodes were produced at CERN by co-deposition onto copper substrates and characterized by photoemission and by XPS (X-ray Photoemission Spectroscopy) analysis. A systematic study on newly produced and used photocathodes was conducted in order to correlate the surface composition to the photoemissive properties.

Surface degradation due to cyclic thermal loading plays a major role in the Accelerating Structures (AS) of the future Compact Linear Collider (CLIC). In this article results on surface degradation of thermally cycled polycrystalline copper as a function of the orientation of surface grains are presented. Samples with different grain sizes were subjected to thermal fatigue using two different methods and were then characterized using roughness measurements and Orientation Imaging Scanning-Electron-Microscopy (OIM-SEM). Samples fatigued by a pulsed laser show the same trend in the orientation-fatigue damage accumulation as the sample fatigued by pulsed Radio-Frequency-heating (RF). It is clearly shown that [1 1 1] surface grains develop significantly more damage than the surface grains oriented in [1 0 0] and three reasons for this behaviour are pointed out. Based on observations performed near grain boundaries, their role in the crack initiation process is discussed. The results are in good agreement with previous findings and can be interpreted on the basis of existing theories. Finally the fatigue effects induced by the two different applied types of thermal cycling are compared.

Electron cloud is a serious limitation for the operation of particle accelerators with intense positively charged beams. It occurs if the secondary electron yield (SEY) of the beam-pipe surface is sufficiently high to induce an electron multiplication. At low surface temperatures, the SEY is strongly influenced by the nature of the physisorbed gases and by the corresponding surface coverage. These conditions occur in many accelerators operating with superconducting magnets and cold vacuum sections such as the LHC and RHIC. In this work, we investigated the variation of the SEY of copper, aluminium and electro-polished copper as a function of physisorbed N2, CO, CO2, CH4, Kr, C2H6 at cryogenic temperatures. The conditioning by electron bombardment of the surface after the physisorption of H2O on electro polished copper will also be presented. The results of the various gases are compared in order to find a rationale for the behaviour of the secondary electrons for the various adsorbates.

Electron cloud buildup is a major limitation for high-energy particle accelerators such as the CERN Super Proton Synchrotron (SPS). Amorphous carbon thin films with low initial secondary electron yield (SEY ffi 1:0) have been applied as a mitigation material in the SPS vacuum chambers. This paper summarizes the experimental setups for electron cloud monitoring, coating procedures, and recent measurements performed with amorphous carbon coated vacuum chambers in the SPS. The electron cloud measured by dedicated monitors is completely suppressed for LHC-type beams. Even after more than one year’s exposure in the SPS with the machine in operation, the coating does not show any increase in the secondary electron yield. The study of coated vacuum chambers for the SPS dipole magnets is in progress; the correlation between electron cloud reduction and pressure rises is not yet fully understood. Some prototypes have already been installed in the accelerator and plans for the implementation of an optimized coating technique are under development.

The basic ideas underlying the use of SCRF cavities forthe detection of high frequency gravitational waves are dis-cussed. Experimental results on prototypes are presented.The outline of a possible detector design and its expectedsensitivity ae also shown.

The QT interval duration is an important non-invasive electrophysiological parameter that reflects ventricular depolarization and repolarization time. A knowledge of QT variability also provides specific information on the normal and pathological ventricular myocardium, and the influence of the autonomic nervous system. A numerical algorithm was developed for measuring QT interval and T-wave duration in the surface electrocardiogram. In comparison with numerous other approaches, this method calculates these durations without averaging of QRS complexes on a beat-to-beat basis. The main aim of this study was to assess the efficacy of this method for characterizing cardiac diseases. In 6 patients with hypertrophic cardiomyopathy and a control group of 5 healthy, age-matched subjects, different interval durations were calculated. In contrast to the mean QT interval duration, only the T-wave duration differed significantly between the two groups.

Discharge gas trapping in thin films produced by sputtering is known to be due to high-energy neutrals bouncing back from the cathode. Qualitatively, the phenomenon is enhanced by raising the discharge voltage and is strongly dependent on the atomic masses of the discharge gas and of the cathode material. In addition to these known effects it is shown that, for a given gas, the trapped amount decreases with increasing melting temperature of the deposited material. The results obtained both by sample melting and laser ablation are presented and discussed.

Electron cloud mitigation is an essential requirement for high-intensity proton circular accelerators. Among other solutions, laser engineered surface structures (LESS) present the advantages of having potentially a very low secondary electron yield (SEY) and allowing simple scalability for mass production. Two copper liners with LESS have been manufactured and successfully tested by monitoring the electron cloud current in a dipole magnet in the SPS accelerator at CERN during the 2016 run. In this paper we report on these results as well as the detailed experiments carried out on samples—such as the SEY and topography studies—which led to an optimized treatment in view of the SPS test and future possible use in the HL-LHC.

The recent discovery of AV_{3}Sb_{5} (A=K,Rb,Cs) has uncovered an intriguing arena for exotic Fermi surface instabilities in a kagome metal. Among them, superconductivity is found in the vicinity of multiple van Hove singularities, exhibiting indications of unconventional pairing. We show that the sublattice interference mechanism is central to understanding the formation of superconductivity in a kagome metal. Starting from an appropriately chosen minimal tight-binding model with multiple van Hove singularities close to the Fermi level for AV_{3}Sb_{5}, we provide a random phase approximation analysis of superconducting instabilities. Nonlocal Coulomb repulsion, the sublattice profile of the van Hove bands, and the interaction strength turn out to be the crucial parameters to determine the preferred pairing symmetry. Implications for potentially topological surface states are discussed, along with a proposal for additional measurements to pin down the nature of superconductivity in AV_{3}Sb_{5}.