Electron Cloud and Synchrotron Radiation characterization of technical surfaces with the Large Hadron Collider Vacuum Pilot Sector

Elena Buratin
2020
Thèse EPFL

This PhD thesis presents the experimental study on electron cloud (EC) and synchrotron radiation (SR) phenomena affecting the LHC storage ring performance. These phenomena are one of the major issues of the LHC storage ring, generating beam instabilities, pressure rise and heat loads on the beam pipes. This experimental study is carried out with an innovative system installed in a room temperature and field free section of the Large Hadron Collider (LHC), the Vacuum Pilot Sector (VPS). ln this system, several surfaces are simultaneously tested, in particular standard copper, amorphous carbon coating, NEG. Thanks to a wide spectrum of detectors, the EC and SR signals were studied as a function of beam parameters and beam pipe properties. The EC behaviour increases linearly with the number of bunches and bunch population above the multipacting threshold. This phenomenon increases linearly also with the bunch length for the Long Straight Sections of LHC (LSS). As expected, the bunch spacing has a crucial importance on the formation of EC dynamic: the 50 ns filling scheme avoids the multipacting effect at 450 GeV. At 6.5 TeV the photoelectron contribution is visible, with no multipacting. This scheme is not sufficient for the LHC collision studies and cannot be used as standard filling scheme. Instead, the 25 ns bunch spacing scheme suffers of EC; in this case this phenomenon can be drastically reduced only by bombarding the surface with electrons during dedicated scrubbing runs, or thanks to the installation of beam pipe surface with a low SEY, such as the amorphous-carbon coating. The measurements performed with pick-ups show the evolution of the EC dynamics for the first time in the LHC. The energy spectrum detectors and the gas analysers were also installed for the first time in the LHC history: the electron cloud spectrum and the gas composition could be measured during the machine operation. The electron energy spectrum evidences a peak at around 100 eV, corresponding to the accelerated electrons responsible for multipacting. The gas analysers measured the main gasses present in the LHC ultra-high vacuum system, such as methane, carbon dioxide and monoxide, water, hydrogen and traces of 2-Propynenitrile 3-fluoro (C3FN), probably linked to the effect of radiation into the kapton cables. It has been possible to estimate the Electron Stimulated Desorption (ESD) parameter of a copper surface as a function of the accumulated electron dose. The heat loads due to impedance, EC and SR were measured and, finally, the operation of HL-LHC machine has been estimated in terms of electron current, pressure and power deposition thanks to the results of this study. Doubling of the bunch population will increase the electron current and the pressure of one order of magnitude, while the deposited power due to EC will be smaller than 1 W/mchamber for a beam pipe SEY lower than 1.8.

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