Mauro Taborelli

CERN | CERN

Summary of interventions carried out during the LHC (CERN) long shutdown 2 to prepare for Run3 (Consolidations, HL upgrades...) and Run3 operational scenarios

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 fa...

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 p...

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...

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,...

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 o...

Within the LHC Injectors Upgrade (LIU) project, the Super Proton Synchrotron (SPS) underwent an upgrade programme to inject higher intensity and brighter 25-ns bunch spaced beams into the LHC. To mitigate Electron Multi-pacting (E.M.), a well-known limiting factor for high intensity positively charged beams, CERN developed carbon coatings with a lo...

All along the second run of the Large Hadron Collider (LHC), the beam induced heat loads in its cryogenic arcs exhibited strong scattering, with some arcs being close to their cooling capacity. Studies related the heat source to electron cloud, incriminating the surface properties of the arc beam screens, in particular their Secondary Electron Yiel...

A combination of picosecond 532 nm laser, 15 m long hollow-core fiber and a crawling robot is designed to roughen the inner vacuum pipe surfaces of selected LHC magnets and achieve secondary electron yield reduction.

Niobium thin film coated copper superconducting radio frequency elliptical cavities have demonstrated for many years their strong potential as an alternative to bulk niobium cavities. The thin film lower performance at high rf field is often attributed to the defects observed in the elaborated Nb layer, sometimes originated from defects inherited f...

Amorphous carbon thin films were prepared by direct current hollow cathode sputter deposition in Ar discharge with the injection of small amounts of H2 and/or N2. The influence of these additives on the film properties with particular focus on the application as a coating for electron cloud mitigation in particle accelerators is characterized by op...

Energy-resolved secondary electron spectroscopy has been performed on air-exposed standard Cu samples and modified Cu surfaces that are tested and possibly applied to efficiently suppress electron cloud formation in the high-luminosity upgrade of the Large Hadron Collider at CERN. The Cu samples comprise pristine oxygen-free, carbon-coated and lase...

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...

The production of a dense and void-free thin film on large and complex substrates is still a challenge in Physical Vapor Deposition. High Power Impulse Magnetron Sputtering (HiPIMS) with voltage inversion (positive pulse) after the main negative pulse is an attractive alternative to a negatively biased substrate to improve the film properties. In t...

In view of the High-Luminosity upgrade of the Large Hadron Collider (LHC) collimation system, a family of novel molybdenum-carbide graphite (MoGr) composites was developed to meet the challenging requirements of HL-LHC beam-halo collimation, in particular the electrical conductivity and thermo-mechanical performances. The Ultra-High Vacuum (UHV) be...

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 r...

The presence of hydrogen in electroformed copper from two different acidic copper sulfate solutions was evaluated: an additive-free solution and a solution including a sugar. D-xylose addition is found to inhibit H incorporation and allows the use of higher cathodic pulses before the copper diffusion limited range starts. TDS experiments show that...

The Future Circular Collider FCC-ee is a study toward a high luminosity electron-positron collider with a centre-of-mass energy from 91 GeV to 365 GeV. Due to the beam parameters and pipe dimensions, collective effects and electron cloud can be very critical aspects for the machine and can represent the main limitations to its performance. An estim...

First results concerning a new approach of TiZrV non-evaporable getter (NEG) thin films coating on very small diameter vacuum chambers are presented. This new process is based on the electroforming of a vacuum chamber around a sacrificial mandrel, which is precoated with a NEG thin film. Aluminum was selected as the material of the mandrel and magn...

This paper presents the first results obtained on DC magnetron sputtering of Nb3Sn thin films dedicated to superconducting radio frequency cavities (SRF). Nb/Sn ratio of 3.76 and 3.2 have been obtained for Ar coating pressures of respectively 1.10-3 mbar and 5.10-2 mbar. According to XRD analyses both coating pressures lead to amorphous Nb3Sn layer...

The thin films techniques applied to Superconducting RF (SRF) have a long history at CERN. A large panel of cavities has been coated from LEP, to LHC. For the current and future projects (HIE-ISOLDE, HL-LHC, FCC) there is a need of further higher RF-performances with focus on minimizing residual resistance Rres and maximizing quality factor Q0 of t...

The demanding beam performance requirements of the High Luminosity (HL-) LHC project translate into a set of requirements and upgrade paths for the LHC injector complex. In this paper the performance requirements for the SPS and the known limitations are reviewed in the light of the 2012 operational experience. The various SPS upgrades in progress...

Low Secondary Electron Yield (SEY) carbon thin films eradicate electron multipacting in accelerator beam pipes. Two magnetic cells of the SPS were coated with such material and installed. In total more than forty vacuum vessels and magnet interconnections were treated. The feasibility of the coating process was validated. The performance of the car...

The LHC injection kicker magnets include beam screens to shield the ferrite yokes against wakefields resulting from the high intensity beam. The screening is provided by conductors lodged in the inner wall of a ceramic support tube. The design of the beam screen has been upgraded to overcome limitations and permit LHC operation with increasingly hi...

Carbon thin films for electron cloud mitigation and anti-multipacting applications have been prepared by dc magnetron sputtering in both neon and argon discharge gases and by plasma enhanced chemical vapour deposition (PECVD) using acetylene. The thin films have been characterized using Secondary Electron Yield (SEY) measurements, Scanning Electron...

The Electron Multipacting (EM) phenomenon is a limiting factor for the achievement of high luminosity in accelerators for positively charged particles and for the performance of RF devices. At CERN, the Super Proton Synchrotron (SPS) must be upgraded in order to feed the Large Hadron Collider (LHC) with 25 ns bunch spaced beams. At such small bunch...

In high power RF devices for space, secondary electron emission appears as the main parameter governing the multipactor effect and as well as the e-cloud in large accelerators. Critical experimental activities included development of coatings with low secondary electron emission yield (SEY) for steel (large accelerators) and aluminium (space applic...

A common problem in all kinds of cavity-like structures in particle accelerators is the occurrence of RF-resonances. Typically, ferrite plates attached to the walls of such structures as diagnostic devices, kickers or collimators, are used to dampen those undesired modes. However, the heat transfer rate from these plates to the walls is rather limi...

The new synchrotron radiation facility of the MAX IV Laboratory is under construction and expected to deliver the first light beam in 2016. To cope with the small aperture, the intense photon bombardment and the lowpressure requirement, most of the beam pipes for the 3- GeV ring are going to be coated with Ti-Zr-V nonevaporable getter (NEG) thin fi...

To rectify the problems of electron clouds observed in RHIC and unacceptable ohmic heating for superconducting magnets that can limit future machine upgrades, we started developing a robotic plasma deposition technique for in-situ coating of the RHIC 316LN stainless steel cold bore tubes based on staged magnetrons mounted on a mobile mole for depos...

High secondary electron yield of metallic surfaces used in accelerator and also in space applications is of general concern. In addition to several well-known coating preparation techniques and microscopic or macroscopic mechanical roughness (grooves) which may significantly increase microwave losses the concept of magnetic surface roughness has be...

High secondary emission yield (SEY), and the subsequent build-up of a secondary electron cloud, may severely limit the stability of high-intensity particle beams inside particle accelerators. One of the best candidates of beam pipe coating for reduced SEY has been amorphous carbon (a-C) produced by direct current (D.C.) magnetron sputtering. Here w...

The CLIC Damping rings are designed to produce un-precedentedly low-emittances of 500 nm and 5 nm nor-malised at 2.86 GeV, with high bunch charge, necessary for the performance of the collider. The large beam bright-ness triggers a number of beam dynamics and technical challenges. Ring parameters such as energy, circumfer-ence, lattice, momentum co...

Obtaining basic knowledge of breakdowns is desirable in many fields of physics; however, often the energy absorbed by a breakdown is not known or can vary by large amounts. We have therefore investigated how processing and breakdown properties scale with available energy for two materials, Cu and Mo, in the energy range of about 1 mJ to 1 J. A cent...

Amorphous carbon thin films have been applied to the liners in the electron cloud monitors and to vacuum cham-bers of three dipole magnets in the SPS. The electron cloud is completely suppressed for LHC type beams in the lin-ers even after 3 months of air venting and no performance deterioration is observed after more than one year of SPS operation...

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 sub...

Understanding the mechanism of plasma build-up in vacuum arcs is essential in many fields of physics. A one-dimensional particle-in-cell computer simulation model is presented, which models the plasma developing from a field emitter tip under electrical breakdown conditions, taking into account the relevant physical phenomena. As a starting point,...

Understanding the mechanism of plasma build-up in vacuum arcs is essential in many fields of physics. A one-dimensional particle-in-cell computer simulation model is presented, which models the plasma developing from a field emitter tip under electrical breakdown conditions, taking into account the relevant physical phenomena. As a starting point,...

Amorphous carbon (a-C) coatings have been tested in electron cloud monitors (ECM) in the Super Proton Synchrotron (SPS) and have shown for LHC type beams a reduction of the electron cloud current by a factor 104 compared to stainless steel (StSt). This performance has been maintained for more than 3 years under SPS operation conditions. Secondary e...

The accelerating structures (AS) are one of the main components of the Compact LInear Collider (CLIC), under study at CERN. Each accelerating structure contains about 30 copper discs, which form the accelerating cavity. The requirements of different technical systems, such as vacuum and cooling, have to be considered during the engineering design....

Amorphous carbon (a-C) coatings can reliably be produced with a maximum secondary electron yield (SEY) close to 1 at room temperature. Measurements at low temperature (LHe) are in progress. Analysis by X-ray Photoemission Spectroscopy (XPS) shows a correlation between the lineshape of C1s spectrum in XPS and the maximum SEY of the investigated samp...

Electrical sparks and arcs are plasma discharges that carry large currents and can strongly modify surfaces. This damage usually comes in the form of micrometer-sized craters and frozen-in liquid on the surface. Using a combination of experiments, plasma and atomistic simulation tools, we now show that the observed formation of deep craters and liq...

Electron-cloud is one of the main limitations for particle accelerators with positively charged beams of high intensity and short bunch spacing, as the SPS at CERN. The Secondary Electron Yield (SEY) of the inner surface of the vacuum chamber is the main parameter governing the phenomenon. The effect could be eliminated by coating the vacuum chambe...

Optimizing the design and the manufacturing of the CLIC RF accelerating structures for achieving the target value of breakdown rate at the nominal accelerating gradient of 100 MV/m requires a detailed understanding of all the steps involved in the mechanism of breakdown. These include surface modification under RF fields, electron emission and neut...

The Cornell Electron Storage Ring (CESR) has been reconfigured as a test accelerator (CesrTA) for a program of electron cloud (EC) research at ultra low emittance. The instrumentation in the ring has been upgraded with local diagnostics for measurement of cloud density and with improved beam diagnostics for the characterization of both the low emit...

Amorphous carbon (a-C) thin films, produced in different coating configurations by using DC magnetron sputtering, have been investigated in laboratory for low secondary electron yield (SEY) applications. After the coatings had shown a reliable low initial SEY, the a-C thin films have been applied in the CERN Super Proton Synchrotron (SPS) and teste...

The CLIC damping rings should produce the ultra-low emittance with high bunch charge necessary for the luminosity performance of the collider. This large bunch density triggers a number of beam dynamics and technical challenges. Lattice studies have been focused on low emittance cells with optics that reduce Intra-beam scattering. Collective effect...

Electron cloud is a limitation to increasing the beam current in the CERN SPS in the frame of an intensity upgrade of the LHC complex. Coating the vacuum chamber with a thin amorphous carbon layer is expected to reduce the electron cloud build-up. Three SPS MBB magnets have been coated to study the performance of this carbon coating. The microwave...

Breakdowns occurring in rf accelerating structures will limit the ultimate performance of future linear colliders such as the Compact Linear Collider (CLIC). Because of the similarity of many aspects of dc and rf breakdown, a dc breakdown study is underway at CERN to better understand the vacuum breakdown mechanism in a simple setup. Measurements o...

RF accelerating structures of the Compact Linear Collider (CLIC) require a material capable of sustaining high electric field with a low breakdown rate and low induced damage. Because of the similarity of many aspects of DC and RF breakdown, a DC breakdown study is underway at CERN in order to test candidate materials and surface preparations, and...

In the context of the CLIC (Compact Linear Collider) project investigations of DC breakdown in ultra high vacuum are carried out in parallel with high power RF tests. From the point of view of saturation breakdown field the best material tested so far is stainless steel, followed by titanium. Copper shows a four times weaker breakdown field than...

In recent years evidence has been found that the maximum sustainable gradient in an accelerating structure depends on the rf power flow through the structure. The CLIC study group has consequently designed a new prototype structure for CLIC with a very low group velocity, input power and average aperture (<a/λ> = 0.13). The 18 cell structure has a...

For the production of the Compact Linear Collider (CLIC) RF accelerating structures, a material capable of sustaining high electric field, with a low breakdown rate and showing low damages after breakdowns is needed. A DC breakdown study is underway at CERN in order to test candidate materials and surface preparations. The saturated breakdown field...

The CLIC study group at CERN has built two X-band HDS (hybrid damped structure) accelerating structures for high-power testing in NLCTA at SLAC. These accelerating structures are novel with respect to their rf- design and their fabrication technique. The eleven-cell constant impedance structures, one made out of copper and one out of molybdenum, ar...

The CLIC power extraction and transfer structure (PETS) is a passive microwave device in which bunches of the drive beam interact with the impedance of the periodically loaded waveguide and excite preferentially the synchronous mode. The RF power produced (several hundred MW) is collected at the downstream end of the structure by means of the Power...

The selection of candidate materials for the accelerating cavities of the Compact Linear Collider (CLIC) is carried out in parallel with high power RF testing. The maximum DC breakdown field of copper, copper alloys, refractory metals, aluminium and titanium have been measured with a dedicated setup. Higher maximum fields are obtained for refractor...

The CLIC study is high power testing accelerating structures in a number of different materials and accelerating structure designs to understand the physics of breakdown, determine the appropriate scaling of performance and in particular to find ways to increase achievable accelerating gradient. The most recent 30 GHz structures which have been tes...

A comprehensive investigation of the effect of Ar, H2, air, and CO on the saturated breakdown field (Esat) of molybdenum, tungsten, and copper has been performed at intensive breakdown conditioning. A significant decrease in Esat is observed for molybdenum and tungsten when exposed to air. Specifically, at air pressures of ˜10-5mbar, the decrease i...

The oxides formed on the Sn–Ag coated Large Hadron Collider (LHC) superconducting cables during a 200 °C heat treatment in air are described and the oxide composition is compared with the interstrand contact resistance (RC). The analysis of more than 250 interstrand contact areas shows that the higher the average Cu content with respect to the Sn c...

The need for high accelerating gradients for the future 30 GHz multi-TeV e+e- Compact Linear Collider (CLIC) at CERN has triggered a comprehensive study of DC breakdown fields of metals in UHV. The study shows that molybdenum (Mo), tungsten (W), titanium (Ti) and TiVAl reach high breakdown fields, and are thus good candidates for the iris material...

The most commonly used quantity to characterize surface cleanliness through X-ray photoemission spectroscopy (XPS) measurements is the so-called relative atomic surface concentration of carbon (at.%C). We have investigated the relationship between at.%C values and the C 1s peak area on Cu and we find a nearly linear behaviour in the range 15–80at.%...

The CLIC study group at CERN has built two X-band accelerating structures to be tested at SLAC in NLCTA. The structures consist of copper cells with insert irises made out of molybdenum and tungsten, clamped together and installed in a vacuum tank. These structures are exactly scaled versions from structures tested previously at 30 GHz and with sho...