Experimental Studies of Carbon Coatings as Possible Means of Suppressing Beam Induced Electron Multipacting in the CERN SPS

Electron cloud build-up is a major limitation for the operation of the SPS with LHC beam above nominal intensity. These beams are envisaged in the frame of the LHC luminosity upgrade and will be available from the new injectors LPSPL and PS2. A series of studies have been conducted in order to identify possible means to suppress electron multipacting by coating the existing SPS vacuum chambers with thin films of amorphous carbon. After a description of the experimental apparatus installed in the SPS, the results of the tests performed with beam in 2008 will be presented

Update on Beam Induced RF Heating in the LHC

Since June 2011 the rapid increase of the luminosity performance of the LHC has come at the expense of both increased temperature and pressure of specific, near-beam, LHC equipment. In some cases, this beam induced heating has caused delays while equipment cool-down, beam dumps and even degradation of some devices. This contribution gathers the observations of beam induced heating, attributed to longitudinal beam coupling impedance, their current level of understanding and possible actions planned to be implemented during the 1st LHC Long Shutdown (LS1) in 2013-2014

LHC Impedance Model: Experience with High Intensity Operation in the LHC

The CERN Large Hadron Collider (LHC) is now in luminosity production mode and has been pushing its performance in the past months by increasing the proton beam brightness, the collision energy and the machine availability. As a consequence, collective effects have started to become more and more visible and have effectively slowed down the performance increase of the machine. Among these collective effects, the interaction of brighter LHC bunches with the longitudinal and transverse impedance of the machine has been observed to generate beam induced heating, as well as longitudinal and transverse instabilities since 2010. This contribution reviews the current LHC impedance model obtained from theory, simulations and bench measurements as well as a selection of measured effects with the LHC beam

Enhancement of the power factor of [Bi1.68Ca2O4](RS)[CoO2](1.69) - Ag composites prepared by the spray-drying method

[Bi1.68Ca2O4](RS)[CoO2](1.69) (BCCO) sample and Ag-BCCO composites (with 10, 20 or 30 wt% Ag) have been prepared by the spray-drying technique and uniaxially/isostatically packed. Scanning electron microscopy reveals that the Ag particles are well distributed in the BCCO cobaltite matrix at low Ag contents. The Ag particles have an important effect on densification and grain orientation of the samples, with a direct impact on their electrical conductivity. The electrical conductivity is higher for the uniaxial samples and increases with the Ag content up to 20% in weight, while the Seebeck coefficient is hardly affected. These features induce an improvement of the power factor, reaching a maximum value of 2.2 mu W K-2 cm(-1) at similar to 1050 K for the uniaxial sample with 20 wt% Ag. Our results suggest that the spray-drying technique is a promising method to obtain composites with a well-dispersed secondary phase. (C) 2010 Elsevier Masson SAS. All rights reserved

Characterisation of technical surfaces at cryogenic temperature under electron bombardment

The vacuum chambers of the LHC’s arcs operate in a temperature range between 1.9 K, i.e. the temperature of the superconducting magnets, and 20 K. At such low temperatures, most of the residual gas species are efficiently adsorbed on the cold surface. LHC’s proton beam emits synchrotron radiation inside its bending magnets and, consequently, electrons are extracted from the surrounding walls by the photoelectric effect. The successive proton bunches accelerates the photo-electrons, building up an “electron cloud” that generates gas desorption from the vacuum chamber and heat load for the cryogenic system. This phenomenon might become a limiting factor for the operation of the High Luminosity LHC upgrade, where more intense proton bunches will circulate. In order to study the electron interaction with gas adsorbed at cryogenic temperature, a new facility has been designed and built at CERN. It reproduces in the laboratory the typical conditions of a cryogenic ultra-high vacuum surface present in the accelerator. In this paper, the first results obtained with selected accelerator materials at different surface gas coverages are presented

Preliminary results obtained with the LHC Vacuum Pilot Sector

The Large Hadron Collider (LHC) is affected by electron cloud that reduces the quality of the beam, provokes instabilities, and increases the residual-gas pressure and heat load in the vacuum chambers. Synchrotron radiation, via photoelectron emission, plays also an important role in the electron cloud build-up. An innovative room temperature Vacuum Pilot Sector (VPS) was installed in a straight section of the LHC to investigate these phenomena in situ. The VPS is instrumented to monitor the electron cloud and its interaction with different surfaces. Currently the system is testing technical surfaces such as copper, amorphous carbon coating, and NEG thin films

Texture of alumina by neutron diffraction and SEM-EBSD

peer reviewedThe orientation distributions of alpha-Al2O3 textured ceramics are determined from neutron diffraction and SEM-EBSD. A curved position-sensitive detector coupled to a tilt angle (chi) scan allowed the whole neutron diffraction pattern treatment in the combined Rietveld-WIMV-Popa algorithm. Analyses from neutron and electron diffraction data gave similar results if EBSD data are smoothed to account for grain statistics. Four textured alumina ceramics were prepared by slip-casting under a high magnetic field and sintered at 800 degrees C, 1300 degrees C, 1400 degrees C and 1600 degrees C. The inverse pole figures and EBSD-mapping highlights the influence of the magnetic field and sintering temperature on the texture development. The inverse pole figures calculated for the fiber direction show a major (001) component for all the samples. With the increasing sintering temperature, the texture strength is enhanced and the c-axis distribution is sharper. The effectiveness of the combined approach for determining the crystallite size is also evident. As a global trend, the calculated crystallite size and observed grain size are similar and increase with the increasing sintering temperature. The mechanism of the texture development in the sintered specimens is certainly initiated from the preferred orientation of the green body after slip-casting under a high magnetic field. The basal texture is enhanced during sintering by selective anisotropic grain growth. We evidenced here the powerfulness of the Rietveld texture analysis correlated to SEM-EBSD calculation to provide a basis for the correlation of texture, microstructural parameters and anisotropic properties

The LHC Vacuum Pilot Sectors Project

The operation of the CERN Large Hadron Collider (LHC) at nominal beam parameters is expected for the next years. Increased synchrotron-radiation stimulated-desorption and electron-cloud build-up are expected. A deep understanding of the interactions between the proton beams and the beam pipe wall is mandatory to control the anticipated beam-induced pressure rise. A Vacuum Pilot Sector (VPS) has been designed to monitor the performance of the vacuum system with time. The VPS is installed along a double LHC room temperature vacuum sector (18 m long, 80 mm inner diameter beam pipes) and includes 8 standard modules, 1.4 m long each. Such modules are equipped with residual gas analysers, Bayard-Alpert gauges, photon and electron flux monitors etc. The chosen modular approach opens the possibility of studying different configurations and implementing future modifications. This contribution will describe the apparatus, the control system designed to drive measurements and possible applications during the LHC Run 2

Dynamic pressure evolution during the LHC operation

International audienceFor the accelerator community and the vacuum scientists, the understanding of the beam interactions with a vacuum chamber is fundamental to provide solutions to mitigate pressure rises induced by electron, photon, and ion molecular desorption. Moreover, beam instabilities induced by ion and electron clouds must be investigated in order to find solutions to reduce them. This study presents in situ measurements of pressure evolutions and electrical currents performed during the LHC RUN II (2018). The proton beam circulating in the LHC vacuum chamber ionizes the residual gas producing electrons as well as positive ions. These charged particles are accelerated away from the beam and reach the vacuum chamber wall, inducing, among other phenomena, stimulated desorption and secondary electron emission. Moreover, protons emit synchrotron radiations that also induce photodesorption and photoelectron production. Experimental measurements of the electrical signals recorded on copper electrodes were compared to calculations considering both the secondary electron yield of copper and the electron energy distribution. All measurements performed with the Vacuum Pilot Sector in the LHC ring show the importance of taking into account a large variety of phenomena in order to understand the pressure evolution in the LHC. Results show that the multipacting threshold, corresponding to an increase in the electron cloud density, strongly depends on the number of protons per bunch. Finally, the ion current was measured with a biased electrode lower than -500 V. It was much higher than expected, pointing its origin not only from simple beam-gas ionization but also from the ionization of the residual gas by the electron cloud