| Paper | Title | Page |
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| THPAB290 | Evolution of the LHC Beam Screen Surface Conditioning Upon Electron Irradiation | 4370 |
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| For the vacuum scientists and the accelerator community, finding solutions to mitigate pressure rises induced by electron, photon, and ion desorption, and also beam instabilities induced by ion and electron clouds is a major issue. Moreover, it is worth noting that the OFE copper beam screen of the LHC is initially cleaned with standard industrial processes, leading to residual chemical contamination. Along the time, changes in the surface chemistry of vacuum chambers are observed during beam operations, leading to modifications of outgassing rates, stimulated desorption processes, and secondary emission yields (SEY). The impact of ions on molecule desorption and electron production was investigated to identify their influence on the global pressure rises and to quantify the ion conditioning effect on copper surfaces: (i) SEY evolution was measured to understand the changes of surface conditioning upon particle irradiation; (ii) surface chemistry evolution after electron irradiation was investigated by both XPS and TOF-SIMS analyses using the ANDROMEDE facility at IJCLab. Finally, the relationship between surface chemistry and the conditioning phenomenon will be discussed. | ||
| DOI • | reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-THPAB290 | |
| About • | paper received ※ 19 May 2021 paper accepted ※ 14 July 2021 issue date ※ 10 August 2021 | |
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| THPAB291 | DYVACS (DYnamic VACuum Simulation) Code: Gas Density Profiles in Presence of Electron Cloud in the LHC | 4373 |
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The computation of residual gas density profiles in particle accelerators is an essential task to optimize beam pipes and vacuum system design. In a hadron collider such as the LHC, the beam induces dynamic effects due to ion, electron, and photon-stimulated gas desorption. The well-known VASCO* code developed at CERN in 2004 is already used to estimate vacuum stability and density profiles in steady-state conditions. Nevertheless, some phenomena are not taken into account such as the ionization of residual gas by the electron clouds and the evolution of the electronic density related to the electron cloud build-up. Therefore, we propose an upgrade of this code by introducing electron cloud maps** to estimate the electron density and the ionization of gas by electrons leading to an increase of induced desorption. The pressure evolution computed with DYVACS reproduces with good accuracy the experimental pressure recorded in the VPS beam pipes sector*** of the LHC from the proton beam injection to the stable beam period. Additionally, DYVACS can also be used as a predictive tool to compute the pressure evolution in the beam pipes for Future Circular Colliders (FCC-hh or -ee).
* A. Rossi, Tech. Report, LHC Project Note 341 ** T. Demma et al Phys. Rev. Acceler. and Beams 10, 114401 (2007) *** B. Henrist et al, Proc. IPAC2014, Dresden |
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| DOI • | reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-THPAB291 | |
| About • | paper received ※ 19 May 2021 paper accepted ※ 02 August 2021 issue date ※ 31 August 2021 | |
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| THPAB292 | Dynamic Pressure in the LHC: Detection of Ions Induced by Ionization of Residual Gas by the Proton Beam and by the Electron-Cloud | 4377 |
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Ultra-High Vacuum is an essential requirement to achieve design performances and high luminosities in high-energy particle colliders. Consequently, the understanding of the dynamic pressure evolution during accelerator operation is fundamental to provide solutions to mitigate pressure rises induced by multiple effects leading to beam instabilities. For the LHC, the appearance of instabilities may be due to the succession of several phenomena: (i) the induced desorption of gases adsorbed on the surfaces leading to pressure rises; (ii) the creation of secondary particles (ions, electrons); (iii) the production of the so-called Electron Cloud build-up by multipacting effect. This work aims to investigate some fundamental phenomena which drive the dynamic pressure in the LHC, namely the effects induced by electrons and ions interacting with the copper surface of the beam screens. Electron and ion currents, as well as pressure, were recorded in situ in the Vacuum Pilot Sector (VPS*) located on the LHC ring during the RUN II. By analyzing the results, more ions than expected were detected and the interplay between electrons, ions, and pressure changes was investigated.
* The LHC Vacuum Pilot-Sector Project, B. Henrist, V. Baglin, G. Bregliozzi, and P. Chiggiato, CERN, Geneva, Switzerland, Proceedings of IPAC2014, Dresden, Germany |
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| DOI • | reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-THPAB292 | |
| About • | paper received ※ 19 May 2021 paper accepted ※ 01 July 2021 issue date ※ 27 August 2021 | |
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