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This article describes a method for measurement of the photodesorption yield and sticking probability for a vacuum chamber coated with nonevaporable getter (NEG). This method is based on a three-gauge method of measurement which was developed and used on a specially designed and assembled setup for the photodesorption yield measurements of test chambers. It has been modified for photodesorption measurements of a vacuum chamber with sorbing walls. Although a short description of the installation and the results of the photodesorption measurements with the NEG-coated vacuum chamber have been published [V. V. Anashin, I. R. Collins, R. V. Dostovalov, N. V. Fedorov, A. A. Krasnov, O. B. Malyshev, and V. L. Ruzinov, The 8th European Particle Accelerator Conference, Paris, France, 2002, p. 2550; Vacuum 75, 155 (2004)], the details of the method, the analysis, and the experimental setup are also described in this article. Special attention is paid to define the sensitivity limit of the method and the setup and to reduce electron- and photon-stimulated desorption in the measurement port.This article describes a method for measurement of the photodesorption yield and sticking probability for a vacuum chamber coated with nonevaporable getter (NEG). This method is based on a three-gauge method of measurement which was developed and used on a specially designed and assembled setup for the photodesorption yield measurements of test chambers. It has been modified for photodesorption measurements of a vacuum chamber with sorbing walls. Although a short description of the installation and the results of the photodesorption measurements with the NEG-coated vacuum chamber have been published [V. V. Anashin, I. R. Collins, R. V. Dostovalov, N. V. Fedorov, A. A. Krasnov, O. B. Malyshev, and V. L. Ruzinov, The 8th European Particle Accelerator Conference, Paris, France, 2002, p. 2550; Vacuum 75, 155 (2004)], the details of the method, the analysis, and the experimental setup are also described in this article. Special attention is paid to define the sensitivity limit of the method and the setup and t...

A new carbon fibre material was developed at the Institute of Solid State Chemistry and Mechanochemistry at the Siberian Branch of the Russian Academy of Science (SB RAS) to meet the requirements of a cryosorber for the large hadron collider (LHC) vacuum chamber. The material must have a large sorbing capacity, a certain pumping speed, a working temperature range between 5 and 20 K, a low activation temperature (below room temperature), a certain size in order to fit into the limited space available and it should be easy to mount. The vacuum parameters of the LHC vacuum chamber prototype with a carbon fibre cryosorber mounted onto the beam screen were studied in the beam screen temperature range from 14 to 25 K at the Budker Institute of Nuclear Physics SB RAS. This carbon fibre material has shown sufficient sorption capacity for hydrogen at operational temperatures of the beam screen in the LHC long straight sections. It is also very important that this material does not crumble and makes a convenient fixation onto the beam screen in comparison to the widely used granulated charcoal. The problem of fluff and ways of reducing the quantity of fluff in the beam channel were studied. The results of these studies show that the carbon fibre material is a possible cryosorber-candidate for use in the LHC and other long vacuum system at cryogenic temperatures. The experimental set-up and results of measurements of the H 2 cryosorption capacity of this carbon fibre material are presented in this paper.

In the vacuum science community, it is now commonly accepted that, for the present and next generation of accelerators, the surface electronic properties of the vacuum chamber material have to be studied in detail. Moreover, such studies are of valuable help to define the cleaning procedures of the chosen materials and to identify the most efficient vacuum commissioning. In the case of the large hadron collider (LHC) the proton beam stability, in the presence of an electron cloud, is analysed using beam induced electron multipacting (BIEM) simulations requiring a number of surface related properties, such as photon reflectivity, electron and photon induced electron emission, heat load, etc. and their modification during machine commissioning and operation. Such simulation codes base their validity on the completeness and reliability of the aforementioned input data. In this work we describe how a surface science approach has been applied to measure, total electron yield (SEY) as well as energy distribution curves excited by a very low-energy electron beam (0–320 eV), from the industrially prepared Cu co-laminated material, the adopted LHC beam-screen material, held at cryogenic temperatures (about 9 K). The data show that the SEY converges to unity at zero primary electron energy and that the ratio of reflected to secondary electrons increases for decreasing energy below about 70 eV, and becomes dominant below electron energies of about 20 eV. These observations lead to the notion of long-lived low-energy electrons in the accelerator vacuum chamber, which could be an issue for the LHC, damping rings and future accelerators.

A three-gauge method of measurements (Nucl. Instrum. Methods A 359 (1995) 110) is described and used to study the photodesorption yields and sticking probabilities of a stainless steel test chamber coated with approximately 3 μm of TiZrV getter material. Using the measured sticking probability for hydrogen of 0.007 for the TiZrV coated chamber and the known sticking probabilities of CH4, CO and CO2 of 0, 0.5, 0.5, respectively (Vacuum 60 (2001) 57), the photodesorption yields are deduced. After activation, the H2, CH4, CO, and CO2 desorption yields are found to be 1.5×10−5, 2×10−7, These results are compared to those from stainless steel test chamber measurements performed for the same geometry under identical conditions. After saturation with CO and during continuous CO injection, the effect of the irradiation is to reduce the pressure, contrary to the normal dynamic behaviour of a conventional vacuum system. This observation is explained in terms of photon induced pumping of the CO by the TiZrV coating.

The cold bore experiment installed in the super proton synchrotron has been used to study the performance of a vacuum system operating at cryogenic temperatures in the presence of a large hadron collider (LHC) type proton beam. The ∼2 m long cryostat, which can be cooled below 3 K, is fitted with an actively cooled beam screen which can be temperature controlled between 5 and 100 K. Molecular desorption and deposited heat load measurements, with or without gas pre-condensation, have been performed. Implications to the LHC design and operation will be discussed.

The vacuum chamber inside the cryogenic magnets in the LHC Long Straight Sections will have a beam screen at a temperature between 5 and 20 K to protect the cold bore against the synchrotron radiation, electron and ion exposure. The desorbed molecules of H 2 will leave the inner part of the beam screen through the pumping slots on the beam screen and eventually condense on the cryosorber, which is mounted on the shadowed (outer) part of the beam screen for magnets operating at 4.5 K. The design of the experimental set-up, the results of the adsorption capacity measurements for charcoal, the pumping speed and the capture factor of the beam screen with charcoal for a proposed LHC vacuum chamber configuration are described in this paper.

In particle accelerators with intense positively charged bunched beams, an electron cloud may induce beam instabilities and the related beam induced electron multipacting (BIEM) can result in an undesired pressure rise. In a cryogenic machine such as the large hadron collider (LHC), the BIEM will introduce additional heat load. When present, synchrotron radiation (SR) may generate a significant number of photoelectrons, that may play a role in determining the onset and the detailed properties of the electron cloud related instability. Since electrons are constrained to move along field lines, those created on the accelerator equator in a strong vertical (dipole) field cannot participate in the e-cloud build-up. Therefore, for the LHC there has been a continuous effort to find solutions to absorb the photons on the equator. The solution adopted for the LHC dipole beam screens is a saw-tooth structure on the illuminated equator. SR from a bending magnet beamline at ELETTRA, Italy (BEAR) has been used to measure the reflectivities (forward, back-scattered and diffuse), for a flat and a saw-tooth structured Cu co-laminated surface using both white light SR, similar to the one emitted by LHC, and monochromatic light. Our data show that the saw-tooth structure does reduce the total reflectivity and modifies the photon energy distribution of the reflected photons. The implications of these results on the LHC arc vacuum system are discussed. © 2004 Elsevier B.V. All rights reserved.