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Centre of Physics and Technological Research
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The Role of Hydrogen Incorporation into Amorphous Carbon Films in the Change of the Secondary Electron Yield
Publication . Bundaleski, Nenad; Adame, Carolina F.; Alves, Eduardo; Barradas, Nuno P.; Cerqueira, Maria F.; Deuermeier, Jonas; Delaup, Yorick; Ferraria, Ana M.; Ferreira, Isabel M. M.; Neupert, Holger; Himmerlich, Marcel; Rego, Ana Maria M. B. do; Rimoldi, Martino; Teodoro, Orlando M. N. D.; Vasilevskiy, Mikhail; Costa Pinto, Pedro; DF – Departamento de Física; CeFITec – Centro de Física e Investigação Tecnológica; DCM - Departamento de Ciência dos Materiais; CENIMAT-i3N - Centro de Investigação de Materiais (Lab. Associado I3N); MDPI - Multidisciplinary Digital Publishing Institute
Over the last few years, there has been increasing interest in the use of amorphous carbon thin films with low secondary electron yield (SEY) to mitigate electron multipacting in particle accelerators and RF devices. Previous works found that the SEY increases with the amount of incorporated hydrogen and correlates with the Tauc gap. In this work, we analyse films produced by magnetron sputtering with different contents of hydrogen and deuterium incorporated via the target poisoning and sputtering of CxDy molecules. XPS was implemented to estimate the phase composition of the films. The maximal SEY was found to decrease linearly with the fraction of the graphitic phase in the films. These results are supported by Raman scattering and UPS measurements. The graphitic phase decreases almost linearly for hydrogen and deuterium concentrations between 12% and 46% (at.), but abruptly decreases when the concentration reaches 53%. This vanishing of the graphitic phase is accompanied by a strong increase of SEY and the Tauc gap. These results suggest that the SEY is not dictated directly by the concentration of H/D, but by the fraction of the graphitic phase in the film. The results are supported by an original model used to calculate the SEY of films consisting of a mixture of graphitic and polymeric phases.
Effect of the magnetic field on the operation of ionisation gauges
Publication . Silva, Ricardo A. S.; Bundaleski, Nenad; Teodoro, Orlando M. N. D.; DF – Departamento de Física; CeFITec – Centro de Física e Investigação Tecnológica; Elsevier
Hot cathode ionisation gauges are the only reliable pressure measurement devices suitable for both high and ultra-high vacuum measurement. These devices are characterized by low accuracy that is hardly better than 20%. The sources of loss of accuracy are documented in the literature although their quantification and details are not fully understood. In the present work, simulations of Bayard-Alpert and extractor gauges were performed under the influence of uniform magnetic fields up to 30 Gs. Results are in qualitative agreement with measurements taken previously with the Bayard-Alpert gauge. Both gauges are sensitive to low intensity magnetic fields which may be present in vacuum chambers. However, the extractor gauge proved to be more stable in the presence of a magnetic field due to its ring-shaped cathode.
Amorphous carbon thin films
Publication . Adame, C. F.; Alves, E.; Barradas, N. P.; Costa Pinto, P.; Delaup, Y.; Ferreira, I. M. M.; Neupert, H.; Himmerlich, M.; Pfeiffer, S.; Rimoldi, M.; Taborelli, M.; Teodoro, O. M. N. D.; Bundaleski, N.; DF – Departamento de Física; CeFITec – Centro de Física e Investigação Tecnológica; DCM - Departamento de Ciência dos Materiais; CENIMAT-i3N - Centro de Investigação de Materiais (Lab. Associado I3N); American Vacuum Society
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.
Cylindrical hot cathode ionisation gauge
Publication . Silva, Ricardo A. S.; Bundaleski, Nenad; Teodoro, Orlando M. D. N.; CeFITec – Centro de Física e Investigação Tecnológica; DF – Departamento de Física; Elsevier
A novel design of an ionisation vacuum gauge is presented, aiming to achieve predictable sensitivity and high accuracy in high and the ultra-high vacuum range. The proposed design features a belt-like electron beam emitted from a linear filament, following a circular trajectory between two cylindrical electrodes, resembling a cylindrical analyser. The proposed design offers several key upsides: a precisely defined electron beam trajectory with reduced susceptibility to path variations, effective electron collection in a Faraday cup able to contain secondary emissions and backscattered electrons, and the inclusion of a suppressor grid in front of the ion collector to eliminate ion-induced secondary electron emission. These features are expected to secure high stability of the gauge and the low pressure limit. An in-depth description of the design is presented, along with the discussions on simulations of the key components that provide the improved performance.
Cylindrical hot cathode ionisation gauge – Construction and testing
Publication . Silva, Ricardo A. S.; Bundaleski, Nenad; Jenninger, Berthold; Teodoro, Orlando M. N. D.; CeFITec – Centro de Física e Investigação Tecnológica; DF – Departamento de Física; Elsevier
Hot cathode ionisation gauges are indispensable in measuring high and ultra-high vacuum. In the present work the construction details and the metrological evaluation of a novel gauge design — the cylindrical ionisation gauge is presented. With the aim of enhancing accuracy and stability both in the short and long term, details of the mechanical assembling of electrodes and precautions to minimize leakage currents influencing ion current measurement are described. Performance evaluations demonstrate high short-term stability, with linearity error, reproducibility, and repeatability values consistently below 1 %, placing the gauge at the same level as the most stable ionisation gauges. Operating across a pressure range of at least 6 orders of magnitude, the gauge exhibits low linearity error from high to ultra-high vacuum ranges. Its capability to measure pressures below 10−9 mbar remains open and requires further investigation.
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Entidade financiadora
Fundação para a Ciência e a Tecnologia
Programa de financiamento
6817 - DCRRNI ID
Número da atribuição
UIDP/00068/2020