Cold/sticky systems

The understanding of complex and/or large vacuum systems operating at cryogenic temperatures requires a specific knowledge of vacuum science at such temperatures. At room temperature, molecules with a low binding energy to a surface are not pumped. However, at cryogenic temperatures, their sojourn time is significantly increased, thanks to the temperature reduction, which allows a ‘cryopumping’. This pumping mechanism is described by different regimes. Sticking probabilities, capture factor and thermal transpiration concepts are also used to characterize the pumping mechanism. At cryogenic temperature, a gas load into a vacuum system turns into an increase of the surface coverage and of its associated vapour pressure. Some adsorption isotherms of H2 and He which differ with key parameters such as surface nature and temperature are also presented. As an application of this field of vacuum technology, the vacuum system of the CERN Large Hadron Collider is introduced. The implementation of cryosorbers and the consequences of He leaks in the accelerator beam tube are reported

Measurement of the primary phodesorption yield at 4.2 K, 77 K and room temperature in a quasi-closed geometry

In the context of the Large Hadron Collider project, the normal incidence photodesorption yield of neutral gases from a stainless steel surface has been measured at 4.2 K, 77 K and room temperature. The yields were measured using a synchrotron radiation photon beam with a critical energy of 45.3 eV, which is very near that to be expected in the LHC. It has been shown that the primary photodesorption yield decrease with decreasing temperature. The gases desorbed were H2, CH4, CO and CO2. At 4.2 K and 77 K the H2O primary photodesorption yield was practically zero. At room temperature the primary photodesorption yields were 5 10-4, 1.6 10-5, 2.5 10-4 and 2.2 10-4 molecules photon-1 respectively for H2, CH4, CO and CO2. At 77 K the primary photodesorption yields of H2, CH4, CO and CO2 were reduced by factors of 2, 4, 17 and 32 respectively with respect to room temperature. At 4.2 K, these corresponding reduction factors were 14, 20, 42 and 31

CERN SPS electron cloud heat load measurements and simulations

A calorimeter, WAMPAC, operating at room temperature has been designed and installed into the SPS to measure directly the electron cloud induced heat load due to the Large Hadron Collider (LHC)-type proton beam. Theoretical behavior, calibrations, measurement protocols, preliminary results, and simulation benchmarking are presented. Scaling of the results to the LHC indicated a linear heating power in a LHC dipole of about 500  mW m^{-1} for 5×10^{10}   protons/bunch^{-1} for a copper surface which is not fully conditioned (maximum of secondary electron yield ∼1.9)

Radiation monitors as a vacuum diagnostic in the room temperature parts of the LHC straight sections

In the absence of collisions, inelastic interactions between protons and residual gas molecules are the main source of radiation in the room temperature parts of the LHC long straight sections. In this case the variations in the radiation levels will reflect the dynamics of the residual pressure distribution. Based on the background simulations for the long straight section of the LHC IP5 and on the current understanding of the residual pressure dynamics, we evaluate the possibility to use the radiation monitors for the purpose of the vacuum diagnostic, and we present the first estimates of the predicted monitor counts for different scenarios of the machine operation

VUV photoemission studies of candidate Large Hadron Collider vacuum chamber materials

In the context of future accelerators and, in particular, the beam vacuum of the Large Hadron Collider (LHC), a 27 km circumference proton collider to be built at CERN, VUV synchrotron radiation (SR) has been used to study both qualitatively and quantitatively candidate vacuum chamber materials. Emphasis is given to show that angle and energy resolved photoemission is an extremely powerful tool to address important issues relevant to the LHC, such as the emission of electrons that contributes to the creation of an electron cloud which may cause serious beam instabilities and unmanageable heat loads on the cryogenic system. Here we present not only the measured photoelectron yields from the proposed materials, prepared on an industrial scale, but also the energy and in some cases the angular dependence of the emitted electrons when excited with either a white light (WL) spectrum, simulating that in the arcs of the LHC, or monochromatic light in the photon energy range of interest. The effects on the materials examined of WL irradiation and /or ion sputtering, simulating the SR and ion bombardment expected in the LHC, were investigated. The studied samples exhibited significant modifications, in terms of electron emission, when exposed to the WL spectrum from the BESSY Toroidal Grating Monochromator beam line. Moreover, annealing and ion bombardment also induce substantial changes to the surface thereby indicating that such surfaces would not have a constant electron emission during machine operation. Such characteristics may be an important issue to define the surface properties of the LHC vacuum chamber material and are presented in detail for the various samples analyzed. It should be noted that all the measurements presented here were recorded at room temperature, whereas the majority of the LHC vacuum system will be maintained at temperatures below 20 K. The results cannot therefore be directly applied to these sections of the machine until measurements at cryogenic temperatures, i.e., in the presence of cryosorbed gas layers, are obtained. However, these results are directly relevant to all the warm regions of the LHC vacuum system, such as the experimental vacuum chambers and warm element vacuum chambers in the insertion regions

Photoelectron Yield and Photon Reflectivity from Candidate LHC Vacuum Chamber Materials with Implications to the Vacuum Chamber Design

Studies of the photoelectron yield and photon reflectivity at grazing incidence (11 mrad) from candidate LHC vacuum chamber materials have been made on a dedicated beam line on the Electron Positron A ccumulator (EPA) ring at CERN. These measurements provide realistic input toward a better understanding of the electron cloud phenomena expected in the LHC. The measurements were made using synchrotro n radiation with critical photon energies of 194 eV and 45 eV; the latter corresponding to that of the LHC at the design energy of 7 TeV. The test materials are mainly copper, either, i) coated by co- lamination or by electroplating onto stainless steel, or ii) bulk copper prepared by special machining. The key parameters explored were the effect of surface roughness on the reflectivity and the pho toelectron yield at grazing photon incidence, and the effect of magnetic field direction on the yields measured at normal photon incidence. The implications of the results on the electron cloud phenom ena, and thus the LHC vacuum chamber design, is discussed

VUV photoemission studies of candidate LHC vacuum chamber materials

In the context of future accelerators and, in particular, the beam vacuum of the LargeHadron Collider (LHC), a 27 km circumference proton collider to be built at CERN, VUVsynchrotron radiation (SR) has been used to study both qualitatively and quantitatively candidatevacuum chamber materials. Emphasis is given to show that angle and energy resolvedphotoemission is an extremely powerful tool to address important issues relevant to the LHC, suchas the emission of electrons that contribute to the creation of an electron cloud which may causeserious beam instabilities. Here we present not only the measured photoelectron yields (PY)from the proposed materials, prepared on an industrial scale, but also the energy and, in some cases,the angular dependence of the emitted electrons when excited with either a white light (WL)spectrum, simulating that in the arcs of the LHC or monochromatic light in the photon energy rangeof interest. The effects on the materials examined of WL irradiation and/or ion sputtering,simulating the SR and ion bombardment expected in the LHC, were investigated. The studiedsamples exhibited significant modifications, in terms of electron emission, when exposed to the WLspectrum from the BESSY TGM7 beamline. Moreover, annealing and ion bombardment alsoinduce substantial changes to the surface thereby indicating that such surfaces would not have aconstant electron emission during machine operation. Such characteristics may be an importantissue to define the surface properties of the LHC vacuum chamber material and are presented indetail for the various samples analysed