The main applications and techniques of thin film coating and plating on vacuum components for particle accelerators are reviewed through significative examples.

Laser-induced surface roughening is a technique that facilitates the reduction of secondary electron emission (SEE) from materials, which is crucial for mitigating electron cloud (EC) formation in particle accelerators, that operate with positively charged species, such as the large hadron collider (LHC). This study focuses on the development of a selective laser surface treatment of the inner copper surface of beam screens (BS) within superconducting (SC) magnets. Several technical challenges linked to laser processing exist including the reduction of treatment time and the control of ablation depth. Based on the found correlations between laser treatment parameters and materials properties, and considering all technical constraints for execution of such a process in SC magnets, a tailored laser processing strategy is developed, which includes creation of a rough Cu surface with trenches of 15-20 mu m depth and an initial secondary electron yield maximum of 1.4-1.5, only in the most relevant regions of the BS. Resulting material properties are characterized such as the surface resistance and related beam impedance, as well as the SEE at both room temperature and cryogenic conditions. The efficiency to mitigate EC formation and thus improve beam quality is demonstrated via EC simulations and electron-induced conditioning experiments. This study also explores under which circumstances the risk of particulate detachment from the surface, which could lead to critical beam interaction, can be minimized.

Laser-induced periodic surface structures (LIPSS) attract considerable attention due to the manifold applications enabled by these self-organised structures ranging from optical colouring to bio-mimicking or wetting effects. The mechanism of LIPSS-formation at metal surfaces includes laser-ablation processes that results in phase transitions, explosive material removal and partial redeposition of the ablation products in the form of nanoscopic debris or even sub-micrometre-sized spherical particles in case of melt ejection. For some applications, these particulates, debris, and microscopic features on top of the periodic surface structures are disadvantageous. We studied wet-chemical cleaning approaches to remove redeposited material from LIPSS to enhance their applicability. LIPSS on copper with a lateral periodicity of similar to 365 nm, that were fabricated by ultrashort pulse laser exposure (lambda: 515 nm, t(p): 260 fs), were cleaned with different liquids ranging from solvents to microemulsions. The surface morphologies were characterised by scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM) to study the surface topography, the size and density of surface particulates as well as other surface contaminations before and after wet cleaning. The LIPSS surface composition was analysed by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and Energy-dispersive x-ray spectroscopy (EDX) at the FIB-cross sections. The different cleaning approaches are classified with respect to their capability to remove particles and contaminations as well as to their influence on the morphology and shape of LIPSS pattern, whereby substantial differences are found. At least two wet-chemical solutions enable the removal of nanoparticles with only minor modification of the LIPSS topography. The main effect of wet cleaning is the detachment of particulates including sub-micrometre-sized spheres due to a gentle and selective etching of the interface region between the LIPSS and the redeposited material that comprise of modified copper and copper oxides.

Understanding the effect of radiation on the functional properties of epoxy resins is crucial for their application in future particle accelerators like the Future Circular Collider (FCC). We compare the irradiation induced aging rates of six epoxy resin systems that can be used for the vacuum impregnation of magnet coils. Aging is assessed based on Dynamical Mechanical Analysis (DMA), 3-point bending and outgassing tests. DMA storage and loss moduli evolutions reveal the effect of the competing influence of cross-linking and chain scission on the glass transition temperature ( Tg ). The same proton and gamma irradiation dose has a similar effect on the thermomechanical epoxy resin properties. Aging rates differ strongly for the different resins, and the fastest aging is observed for the MY750 resin system, which Tg decreases with a rate of about minus 9 °C/MGy.

Next-generation gravitational wave detectors (GWDs) like the Cosmic Explorer and Einstein Telescope require extensive vacuum tubing, necessitating cost-effective materials. This study explores the viability of mild steel as an alternative to austenitic stainless steel for ultrahigh vacuum beampipes, focusing on outgassing rates and surface chemistry after low-temperature bakeouts. Mild steels exhibit significantly lower hydrogen outgassing rates, below 10−14 mbar l s−1 cm−2 after bakeouts at 80 °C for 48 h. While water vapor is the primary residual gas after such low-temperature bakeouts, repeated treatments reduce its outgassing rate and modify surface conditions so that such benefit is preserved after at least six months of exposure to laboratory air. These findings position mild steel as an economical and efficient material for future GWD beampipes.

The Large Hadron Collider (LHC) Long Shutdown 2 (2019-2021), following LHC Run 2, was primarily dedicated to the upgrade of the LHC Injectors but it included also a significant amount of activities aimed at consolidation of the LHC machine components, removal of known limitations and initial upgrades in view of the High -Luminosity LHC (HL-LHC) to favour the intensity ramp -up during Run 3 (2022-2025). An overview of the major modifications to the accelerator and its systems is followed by a summary of the results of the superconducting magnet training campaign to increase the LHC operation energy beyond the maximum value of 6.5 TeV reached during Run 2. The LHC configuration and the scenarios for proton and ion operation for Run 3 are presented considering the expected performance of the upgraded LHC Injectors and the proton beam intensity limitations resulting from the heat load on the cryogenic system due to beam -induced electron cloud and impedance.

To achieve the vacuum quality required for the operation of particle accelerators, the surface of the vacuum vessels must be clean from hydrocarbons. This is usually achieved by wet chemistry processes, e.g., degreasing chemical baths that, in case of radioactive vessels, must be disposed accordingly. An alternative way exploits the oxygen plasma produced by a downstream RF plasma source. This technique offers the possibility of operating in-situ, which is an advantageous option to avoid the handling of voluminous and/or fragile components and a more sustainable alternative to large volume disposable baths. In this work, we test a commercial plasma source in dedicated vacuum systems equipped with quartz crystal microbalances (QCMs). The evolution of the etching rates of amorphous carbon (a-C) thin films deposited on the QCMs to mimic contamination are studied as function of operating parameters. We present the results of the plasma cleaning process applied to the real case of a hydrocarbons-contaminated large vacuum vessel. The studies are complemented by transport simulations and surface contamination monitoring by X-ray photoelectron spectroscopy (XPS) analysis. The evaluation of the vessel cleanliness, which is performed via residual gas analysis (RGA) measurements, is based on CERN’s outgassing acceptance criteria and agrees with both simulations and XPS results.

Laser-induced surface structuring is a promising method to suppress electron mulitpacting in the vacuum pipes of particle accelerators. Electrons are scattered inside the rough surface structure, resulting in a low Secondary Electron Yield (SEY) of the material. However, laser processing of internal pipe surfaces with a large aspect ratio is technologically challenging in terms of laser beam guidance and focusing. We present a 532 nm ultrashort-pulse laser setup to process the inner parts of 15 m long beam vacuum tubes of the Large Hadron Collider (LHC). Picosecond pulses at a repetition rate of 200 kHz are guided through an optical fiber toward an inchworm robot traveling inside the beam pipe. The system was installed, characterized, and tested for reliability. First surface treatments achieved the required scan precision. Cu2O-dominated nano-features were observed when processing at high average laser power (5 W) and slow scanning speed (5 mm s-1) in nitrogen flow, and the maximum SEY of copper was decreased from 2.1 to 0.7.

We have demonstrated out-of-focus ultrafast pulsed laser processing of copper with a variable working distance, without the need for mechanical movement. This was achieved by employing a diffractive optical element. The method has been demonstrated in a practical application to reduce the secondary electron yield (SEY) of copper to below 1.3. We show that using an extended focus element not only increases the consistency of processing across a range of working distances, but also changes the topography of the produced structures, reducing the SEY. This presented approach shows promise in facilitating the Large Hadron Collider’s (LHC’s) upcoming high luminosity upgrade by preventing electron clouds.

Ultrashort-pulse laser processing of copper is performed in air to reduce the secondary electron yield (SEY). By UV (355 nm), green (532 nm), and IR (1064 nm) laser-light induced surface modification, this study investigates the influence of the most relevant experimental parameters, such as laser power, scanning speed, and scanning line distance (represented as accumulated fluence) on the ablation depth, surface oxidation, topography, and ultimately on the SEY. Increasing the accumulated laser fluence results in a gradual change from a Cu2O to a CuO-dominated surface with deeper micrometer trenches, higher density of redeposited surface particles from the plasma phase, and a reduced SEY. While the surface modifications are less pronounced for IR radiation at low accumulated fluence (<1000 J/cm2), analogous results are obtained for all wavelengths when reaching the nonlinear absorption regime, for which the SEY maximum converges to 0.7. Furthermore, independent of the extent of the structural transformations, an electron-induced surface conditioning at 250 eV allows a reduction of the SEY maximum below unity at doses of 5×10-4 C/mm2. Consequently, optimization of processing parameters for application in particle accelerators can be obtained for a sufficiently low SEY at controlled ablation depth and surface particle density, which are factors that limit the surface impedance and the applicability of the material processing for ultrahigh vacuum systems. The relations between processing parameters and surface features will provide guidance in treating the surface of vacuum components, especially beam screens of selected magnets of the Large Hadron Collider or of future colliders.

The irradiation of metals with ultrashort laser pulses enables the rapid and cost-effective production of nanostructured surfaces with a wide range of industrial applications. The laser-induced surface roughening modifies the interaction processes upon electron impact, leading to a modification of the secondary electron emission. In this study, the nanostructuring as well as the secondary electron yield (SEY) variation of polycrystalline copper surfaces was investigated by irradiation with 1030 nm infrared ultrashort laser pulses at a constant repetition rate of 100 kHz. The influence of varying the pulse duration between 238 fs and 10 ps, the laser power and the number of laser pulses per unit area (induced by varying the scanning speed) on the surface topography and the SEY was investigated. Irrespective of the pulse duration, irradiation with low scan speed (v <= 20 mm/s) and high laser power (P >= 2.6 W) results in the formation of a surface with compact nanostructures and a very low maximum SEY delta max < 0.7. The delta max increased slightly with increasing pulse duration at similar laser parameters. Increasing the pulse duration also resulted in a slight decrease in the ablation threshold and volume. The observed SEY dependence is probably explained by the pulse duration dependence of the ablation. The results suggest that nanostructured copper surfaces with very low SEY can be produced with ultrashort laser pulses over a wide range of pulse durations.

The formation of a fissured copper surface by picosecond pulsed laser irradiation is combined with functional coatings consisting of Ti and amorphous carbon layers or a Ti–Zr–V compound film to fabricate surfaces with the maximum of the secondary electron yield being as low as 0.4. By structural and spectroscopic analysis of the formed surfaces it is demonstrated that both coatings enclose the nanostructures generated by redeposition of metal structures from the laser‐induced plasma plume, keeping the initial topography intact. This allows an efficient elimination of secondary electron emission by combining the benefits from structural surface modification and adaption of electronic surface properties to efficiently dissipate the energy of impinging electrons. Thermal activation tests of the Ti–Zr–V nonevaporable getter films revealed that for films on nanostructured substrates, which have a much higher effective surface, a slight diminution of surface activation occurs at 160 and 200 °C, while this effect is completely compensated when heating up to 250 °C indicating promising pumping capabilities. Both examples highlight the benefits from combining 3D substrate patterning with classical 2D deposition technologies.

Amorphous carbon (a-C) films, having low secondary electron yield (SEY), are used at CERN to suppress electron multipacting in the beam pipes of particle accelerators. It was already demonstrated that hydrogen impurities increase the SEY of a-C films. In this work, a systematic characterization of a set of a-C coatings, deliberately contaminated by deuterium during the magnetron sputtering deposition, by scanning electron microscopy, ion beam analysis, secondary ion mass spectrometry, and optical absorption spectroscopy was performed to establish a correlation between the hydrogen content and the secondary electron emission properties. In parallel, the mechanisms of contamination were also investigated. Adding deuterium allows resolving the contributions of intentional and natural contamination. The results enabled us to quantify the relative deuterium/hydrogen (D/H) amounts and relate them with the maximum SEY (SEYmax). The first step of incorporation appears to be formation of D/H atoms in the discharge. An increase in both the flux of deposited carbon atoms and the discharge current with a D2 fraction in the gas discharge can be explained by target poisoning with deuterium species followed by etching of CxDy clusters, mainly by physical sputtering. For overall relative D/H amounts between 11% and 47% in the discharge gas, the SEYmax increases almost linearly from 0.99 to 1.38. An abrupt growth of SEYmax from 1.38 to 2.12 takes place in the narrow range of D/H relative content of 47%–54%, for which the nature of the deposited films changes to a polymer-like layer.

Laser-processing the inner surfaces of 15 m long vacuum pipes installed in the LHC aims to create a nanostructured functional surface with low Secondary Electron Yield (SEY). The experimental system to treat the vacuum pipes in-situ, including a 532 nm picosecond-laser, a 15 m long optical fiber, and an inchworm robot, will be presented. The laser-induced generation of micro- and nanostructures reduces the SEY of the surface. To optimize the surface treatment, the processing parameters were varied, and different scanning patterns applied. The variation in ablation depth, surface topography and composition correlate well with changes of the SEY.

All over Run 2, the LHC beam-induced heat load on the cryogenic system exhibited a wide scattering along the ring. Studies ascribed the heat source to electron cloud build-up, indicating an unexpected high Secondary Electron Yield (SEY) of the beam screen surface in some LHC regions. The inner copper surface of high and low heat load beam screens, extracted during the Long Shutdown 2, was analysed. On the low heat load ones, the surface was covered with the native Cu 2 O oxide, while on the high heat load ones CuO dominated at surface, and it exhibited a very low carbon coverage. Such chemical modifications increase the SEY and inhibit a proper conditioning of the affected surfaces. Following this characterisation, the mechanisms for CuO build-up in the LHC beam pipe were investigated on a newly commissioned cryogenic system allowing electron irradiation, surface chemical characterisation by X-ray Photoelectron Spectroscopy and SEY measurements on samples held below 15 K. In parallel, curative solutions against the presence of CuO in the LHC beam screens were explored, which could be implemented in-situ to recover a proper conditioning and lower the beam-induced heat load.

Nanostructured surfaces exhibit remarkable chemical, physical and microbiological properties and have therefore various technical and industrial applications. The ultrashort laser pulse irradiation (wavelength λ = 355 nm, pulse duration Δtp = 12 ps, repetition rate f = 100 kHz) of copper samples with appropriate laser parameters results in the formation of a micro- and nanostructured surfaces. The influence of these hierarchically textured surfaces on the secondary electron yield (SEY) was studied especially with regard to their morphological and geometrical properties. Specific SEY changes are caused by both, the shape and the depth of the microstructures, as well as the morphology of the formed nanostructures; that can be either compact flower head-like nanostructures, non-compact filament-shaped nanostructures, molten and resolidified spherical structures, or minor modified surfaces. The measured SEY as a function of the primary electron energy is correlated with the surface topography that forms upon laser irradiation. The SEY decreases with increasing accumulated laser fluence and ablated volume, respectively. Especially flower-head-like nanostructures can be produced at high laser power (P ≥ 400 mW) and low scanning velocity (v ≤ 10 mm/s) and represent a surface with strongly reduced SEY maximum as low as 0.7.

Nanostructured surfaces exhibit outstanding properties and enable manifold industrial applications. In this study the laser surface processing of polycrystalline, flat copper surfaces by 532 nm picosecond laser irradiation for secondary electron yield (SEY) reduction is reported. The laser beam was scanned in parallel lines across the sample surface in order to modify large surface areas. Morphology and SEY are characterized in dependence of the process parameters to derive correlations and mechanisms of the laser-based SEY engineering process. The nano- and microstructure morphology of the laser-modified surface was characterized by scanning electron microscopy and the secondary electron yield was measured. In general, an SEY reduction with increasing accumulated laser fluence was found. In particular, at low scanning speed (1 mm/s – 10 mm/s) and "high" laser power (∼ 1 W) compact nanostructures with a very low SEY maximum of 0.7 are formed.

The authors report in this paper the possibility to control the thickness profile of a thin film deposited by High Power Impulse Magnetron Sputtering (HiPIMS). It is shown that the combination between a HiPIMS discharge, an unbalanced magnetic configuration and the application of a negative bias onto the surface to coat enables tailoring on demand the coating thickness profile. This effect is hereafter used to coat complex shapes such as low-beta accelerating cavities with a niobium layer. The authors first present the magnetic design proposed to obtain an unbalanced cylindrical sputtering source. Numerical simulations are then used to predict the electron density and energy spatial distributions that can subsequently be correlated to the ionization region shape. Finally, the authors present the effect of such technique comparing Direct Current Magnetron Sputtering (DCMS), HiPIMS and biased HiPIMS using, respectively, a balanced and an unbalanced magnetic configuration, as well as detailing the effect of modifying either the magnetic field lines distribution or the magnetic strength.

We present a study of radio frequency properties of niobium films deposited on copper by two different approaches based on High Power Impulse Magnetron Sputtering, namely with a DC voltage biased substrate and with bipolar target voltage. Such approaches enable the synthesis of dense superconducting (SC) layers. The SC radio frequency losses of these films are characterized as a function of the applied RF magnetic field using a dedicated calorimetric method. We report on a significant reduction of the Q-slope phenomenon and the residual surface resistance in the characterized films, achieving similar values as those obtained on bulk niobium surfaces qualified with the same technique. Our results pave the way towards the realization of Nb/Cu coated accelerating cavities featuring a surface resistance 2-3 times lower than the state-of-the-art values at working frequencies of 400 and 800 MHz, making this technology even more appealing for future particle accelerators and colliders.

The durability of polymeric materials is closely linked to their degradation under specific operating conditions when different stressors-general or specific, such as high temperature, sunlight or ionizing radiation, solvents, or mechanical stresses-act simultaneously, causing degradation. In the case of electrical cables, the durability of the electrically insulating materials used in their construction is an important parameter to ensure their operational security. In this work, we studied the degradation state of various types of electrical insulating materials from cables used in particle acceleration systems under European Organization for Nuclear Research (CERN) conditions (e.g., Super Proton Synchrotron, SPS) as a function of time and irradiation dose. A simple kinetic model was proposed based on the exponential decrease in the antioxidant amount in polymeric insulations. The onset oxidation time (OIT) values, used as an indicator of antioxidant concentration, were obtained from isothermal differential scanning calorimetry (DSC) and chemiluminescence (CL) measurements. Fourier transform infrared (FTIR) measurements were used to assess the degradation state and identify polymeric materials. The practical applicability of such a model in diagnosing degradation and in the subsequent evaluation of the remaining service life is of interest, as it can be adapted to a broad range of operating conditions and materials.