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New types of anodised aluminium, porous copper and charcoal-based materials are being developed as cryosorbing materials and have been studied in collaboration with a number of research institutes. The major aim was to find a suitable cryosorber with a working temperature in the range between 5 and 20 K that could be used in the LHC vacuum chamber inside the superconducting magnets at a temperature of 4.5 K and higher. The design of the experimental set-up, the results of cryosorption capacity measurements for porous copper, anodised aluminium and charcoal-based materials are described in this paper.

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.

A cryogenic vacuum system has been used to study both the surface pumping of gases and the molecular photodesorption yield from OFE copper in the temperature range from 5 to 300 K. The sticking coefficient on a ‘bare surface’ was found to increase with decreasing temperature of the substrate and below 20 K, to be in the range of a few per cent for all gas species studied. The molecular photodesorption yield for the ‘bare surface’ was found to decrease with decreasing temperature and below 20 K, after an accumulated photon dose of 3.1×10 22 photons m −1 , to be in the range of 10 −4 molecules photon −1 for H 2 and 10 −5 molecules photon −1 for the other gas species. These measured values are used to predict the evolution of the gas densities in the large hadron collider (LHC). Implications to the LHC are discussed.

The design of the large hadron collider (LHC) vacuum system requires a complete understanding of all processes which may affect the residual gas density in the cold bore of the 1.9 K cryomagnets. A wealth of data has been obtained which may be used to predict the residual gas density inside a cold vacuum system exposed to synchrotron radiation. In this study, the effect of cracking of cryosorbed molecules by synchrotron radiation photons has been included. Cracking of the molecular species CO 2 and CH 4 has been observed in recent studies and these findings have been incorporated in a more detailed dynamic gas density model for the LHC. In this paper, we describe the relevant physical processes and the parameters required for a full evaluation. It is shown that the dominant gas species in the LHC vacuum system with its beam screen are H 2 and CO. The important result of this study is that, while the surface coverage of cryosorbed CH 4 and CO 2 molecules is limited due to cracking, the coverage of H 2 and CO molecules may increase steadily during the long-term operation of the machine.

In a cryogenic proton accelerator, such as the LHC, the creation of an electron cloud and generated heat loads resulting from electron bombardment are strongly dependent on the azimuthal distribution of created photoelectrons. In this context, photon reflection and photoelectron yield measurements have been performed using a beam line on the VEPP-2M storage ring. Six electrodes, covering the complete vacuum chamber perimeter, were mounted such that they could be suitably biased, and while one electrode was irradiated with synchrotron radiation the resulting electron current of all the others could be measured. A detailed description of the experimental apparatus and the results of the measurements of photon reflection and the azimuthal distribution of generated photoelectrons are presented.

For the design of future accelerators, in particular, the beam vacuum of the large hadron collider (LHC), a 27 km circumference proton collider to be built at CERN, ‘white light’ (WL) and monochromatic VUV synchrotron radiation (SR) have been used to measure, both qualitatively and quantitatively, electron emission from candidate vacuum chamber materials. Emphasis has been placed on measuring the photoelectron yields (PY) and the energy distribution of the emitted electrons. These parameters are relevant to gas desorption, the LHC cryogenic cooling capacities and the possible creation of an electron cloud which may cause beam instabilities. Proposed materials, prepared on an industrial scale, such as 50 μm OFE Cu co-laminated onto a high Mn-content stainless steel, exhibit significant modifications when exposed to the WL spectrum from the BESSY TGM7 beamline. Surface cleaning treatments, such as sample annealing and ion bombardment, induce substantial changes to the electron emission which therefore indicate that such surfaces would not be constant, in terms of electron emission, during machine operation. Surfaces which are considered to be constant electron emitters, such as annealed TiZr alloys and commercial non-evaporable getters, were also investigated. These results and their implications for the choice of the material to be used for the LHC are discussed.

A detailed theoretical study of the filled outer-shell core levels of Ta(110) by bandstructure and photocurrent calculations of core level photoemission experiments is presented. The important question of the existence of bulk bandstructure effects in these levels is addressed. Bandwidths of 47 meV and over 1 eV are both shown to exist within the 4 f 7 2 and 5 p 3 2 levels, respectively, and to be important in the interpretation of the core level photoemission spectra. The implications of these effects within the core levels of Ta(110) are discussed.

Angle-resolved photoemission spectroscopy (ARPES), using HeI radiation, has been used to probe the occupied electronic states from a single domain of the Au-induced 5 × 2 reconstruction of Si(111), a quasi-one-dimensional (quasi-1D) metallic system. The ARPES spectra recorded parallel (perpendicular) to the Au chain structure show a signature of a metallic (non-metallic) system. These experimental results are intriguing since, for the first time, spectral intensity at the Fermi level is observed in a photoemission study of a quasi-1D metallic system.

Note: Univ liverpool,interdisciplinary res ctr surface sci,liverpool l69 3bx,merseyside,england. univ dublin trinity coll,dept pure & appl phys,dublin 2,ireland. ecole polytech fed lausanne,inst phys appl,ch-1015 lausanne,switzerland. univ liverpool,dept phys,liverpool l69 3bx,merseyside,england.ISI Document Delivery No.: RQ630 Reference LSE-ARTICLE-1995-006doi:10.1016/0039-6028(95)00438-6View record in Web of Science Record created on 2006-10-03, modified on 2017-05-12