Abstract
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.
Highlights
The Large Hadron Collider (LHC) will provide two countercirculating proton beams with colliding energies of nominally 14 TeV in the center of mass, requiring superconducting bending magnets operating in superfluid helium at 1.9 K
Its radiated power induces a heat load of 0.2 Wm per beam and may (i) stimulate gas desorption of weakly and tightly bound gases from the walls of the vacuum system either directly by photons or mediated by electrons [1,2], (ii) create photoelectrons which can be accelerated, to an average energy of 380 eV [3], towards the opposite wall by the positive space charge of the bunched beam leading to additional gas desorption and heat loads on the cryogenic system, (iii) create secondary electrons which may contribute to electron multipacting [2]
The results obtained are divided into seven sections, each one addressing a particular key point: namely, (A) the photon energy dependence and angular distributions of the energy distribution curves (EDCs) of Au, (B) the use of monochromatic synchrotron radiation (SR) to understand the effect of white light (WL) irradiations and surface conditioning effects, (C) the photon energy dependence of the photoelectron yield (PY) (CFS), (D) the measured WL exposed PY of the as-received samples obtained from the sample drain current, (E) the measured WL exposed PY of the conditioned samples obtained from the sample drain current, (F) the EDC of such WL exposed samples, (G) the generation of “low dose” WL EDC, (H) the energy distribution of the secondary electrons in the LHC
Summary
The Large Hadron Collider (LHC) will provide two countercirculating proton beams with colliding energies of nominally 14 TeV in the center of mass, requiring superconducting bending magnets operating in superfluid helium at 1.9 K. Its radiated power induces a heat load of 0.2 Wm per beam and may (i) stimulate gas desorption of weakly and tightly bound gases from the walls of the vacuum system either directly by photons or mediated by electrons [1,2], (ii) create photoelectrons which can be accelerated, to an average energy of 380 eV [3], towards the opposite wall by the positive space charge of the bunched beam leading to additional gas desorption and heat loads on the cryogenic system, (iii) create secondary electrons which may contribute to electron multipacting [2].
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