Study of SEY degradation of amorphous carbon coatings (original) (raw)
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Increase of secondary electron yield of amorphous carbon coatings under high vacuum conditions
Vacuum, 2013
Electron cloud (e-cloud) is one of the major limitations for beam quality in modern particle accelerators. The macroscopic property which governs this phenomenon is the secondary electron yield (SEY) of a surface, defined as the number of emitted electrons per incident electron. SEY of inner surface walls must be less than 1.3 to prevent the formation of an e-cloud. Although most pure metals possess an SEY within this range, technical surfaces (i.e. those resulting from the necessary machining to produce vacuum parts) typically display much higher SEY. An elegant and effective solution to this problem is to deposit the carbon coating on these surfaces by magnetron sputtering. However, the first measurements performed at CERN revealed an increase of the SEY as a function of long term air exposure. Furthermore, we observed a rapid increase of the SEY of these samples whilst under high vacuum conditions. In order to determine the contaminant responsible for the observed ageing, as well as the ageing mechanism, the samples were exposed to various gases and vapours such as water vapour, H 2 , rotary pump oil vapour, etc. The results confirm that the vapour of rotary pump oils is responsible for exceptionally fast sample ageing. We also observed that high SEY samples usually have an increased surface concentration of oxygen. The possible ageing mechanisms are discussed.
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 chambers with a material of low SEY, which does not require bake-out and is robust against air exposure. For such a purpose amorphous carbon (a-C) coatings were produced by magnetron sputtering of graphite targets. They exhibit maximum SEY between 0.95 and 1.05 after air transfer to the measuring instrument. After 1 month of air exposure the SEY rises by 10 - 20 % of the initial values. Storage in desiccator or by packaging in Al foil makes this increase negligible. The coatings have a similar X-ray photoelectron spectroscopy (XPS) C1s spectrum for a large set of deposition parameters and exhibit an enlarged linewidth compared to HOPG graphite. ...
Amorphous carbon coatings for the mitigation of electron cloud in the CERN Super Proton Synchrotron
Physical Review Special Topics - Accelerators and Beams, 2011
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.
Characterization of Carbon Coatings with Low Secondary Electron Yield
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 samples. The initial level of oxygen on the surface of the various samples does not seem to be related to the initial maximum SEY value. However, the increase of the SEY with air exposure time on each individual sample is related to the amount of oxygen containing adsorbates. Storage in different environments has been investigated (static vacuum, aluminium foil, dry nitrogen and desiccators), and shows significant differences in the “aging” behaviour. Aging is very moderate when storing samples wrapped in aluminium foil in air. Samples which have undergone aging due to inappropriate storage can be recovered nearly to the initial value of their...
Carbon coatings with low secondary electron yield
Vacuum, 2013
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 Microscopy (SEM), Nuclear Reaction Analysis (NRA) and X-ray Photoelectron Spectroscopy (XPS). For more than 100 carbon thin films prepared by sputtering the average maximum SEY is 0.98+/-0.07 after air transfer. The density of the films is lower than the density of Highly Ordered Pyrolytic Graphite (HOPG), a fact which partially explains their lower SEY. XPS shows that magnetron sputtered samples exhibit mainly sp 2 type bonds. The intensity on the high binding energy side of C1s is found to be related to the value of the SEY. Instead the initial surface concentration of oxygen has no influence on the resulting SEY, when it is below 16%. The thin films produced by PECVD have a much higher maximum SEY of 1.49+/-0.07. Storage conditions in air, namely wrapping in aluminium foil, preserves the low SEY by more than one year. Such coatings have already been applied successfully in accelerators and multipacting test benches.
Amorphous Carbon Coating in SPS
2021
Within the LHC Injector Upgrade (LIU) project, the Super Proton Synchrotron (SPS) needs to be upgraded to inject into the LHC higher intensity and brighter 25-ns bunch spaced beams. To mitigate the Electron Multipacting (E.M.) phenomenon, a well-known limiting factor for high-intensity positively charged beams, CERN developed carbon coatings with a low Secondary Electron Yield (SEY). During the 2016 2017 year-end technical stops, such coatings were deposited on the inner wall of the vacuum chambers of some SPS quadrupole and dipole magnets by a dedicated in-situ setup. A much larger scale deployment was implemented during the Long Shutdown 2 (2019-2020) to coat all beam pipes of focussing quadrupoles (QF) and their adjacent short straight sections. In this contribution, we remind the motivation of the project, and present the results and the quality control of the carbon coating campaign during the latter phase of implementation.
Photodesorption and Electron Yield Measurements of Thin Film Coatings for Future Accelerators
2015
The performance of future accelerators could be limited by electron cloud phenomena and high photodesorption yields. For such a reason, the study of secondary electron and photodesorption yields of vacuum materials is essential. The eradication or mitigation of both secondary electron and molecule desorption could strongly reduce the beam scrubbing time and increase the availability of nominal beams for experiments. Surface modifications with the desired characteristics can be achieved by thin-film coatings, in particular made of amorphous carbon and non-evaporable getters (NEG). In the framework of a new collaboration, CERN’s vacuum group has manufactured several vacuum chambers with a geometry similar to the beam pipe of future accelerators, and has applied different coatings on each of them. The samples were then irradiated at KEK’s Photon Factory (PF) with synchrotron radiation of 4 keV critical energy during several days, allowing the measurement of the photodesorption yield as...
Nine years of carbon coating development for the SPS upgrade: achievements and heritage
2015
CERN has succeeded in producing carbon coatings that eradicate electron cloud in accelerators without any beam conditioning or in situ heating. Investing about 1 MCHF and dedicating 31 FTE (28 manyears, staff and associate members) in 9 years in the framework of LIU-SPS, CERN has increased: technological competences in thin-film coatings, worldwide visibility in surface characterisation, and capability in electron cloud measurement. Future projects are still benefiting from such an investment. Summary This document describes the development performed at CERN in the period 2007-2015 aiming at producing carbon coatings with low Secondary Electron Yield (SEY) in the framework of the LIU-SPS project. Initiated by a request of the Proton Accelerators for the Future committee in autumn 2007, the development was started as a future mean to mitigate electron cloud in the SPS. Very low SEY, close to 1, were achieved within few months on laboratory samples. Particle generation and outgassing ...
Secondary electron yield measurements from thin surface coatings for NLC electron cloud reduction
2004
In the beam pipe of the positron damping ring of the Next Linear Collider, electrons will be created by beam interaction with the surrounding vacuum chamber wall and give rise to an electron cloud. Several solutions are possible for avoiding the electron cloud, without changing the bunch structure or the diameter of the vacuum chamber. Some of the currently available solutions for preventing this spurious electron load include reducing residual gas ionization by the beam, minimizing beam photon-induced electron production, and lowering the secondary electron yield (SEY) of the chamber wall. We will report on recent SEY measurements performed at SLAC on TiN coatings and TiZrV non-evaporable getter thin films.