Abstract
For the future HL-LHC or FCC study, the understanding of the beam interactions with the vacuum chamber is fundamental to provide solutions to mitigate the pressure rises induced by electronic, photonic and ionic molecular desorption. The proton beam circulating in the LHC vacuum chamber ionizes the residual gas producing electrons as well as positive ions. In-situ measurements were carried out, on the LHC Vacuum Pilot Sector during the LHC RUN II, to monitor the dynamic pressure, and to collect the electrical signals due to the electron cloud and to the ions interacting with the vacuum chamber walls. Experimental measurements of electrical signals recorded by copper electrodes were compared to calculations taking into account both the Secondary Electron Yield of copper and electron energy distribution. Finally, it seems that copper electrodes were not fully conditioned and an ion current could be estimated.
Highlights
Ultra-High Vacuum is an essential requirement to reach design performances in high-energy particle colliders
The aim of this paper is to report the first investigation on the ion behaviour in the Vacuum Pilot Sector (VPS) installed in the LHC [2]
After the injection a slight decrease of beam intensity is observed due to proton losses along their path. (ii) Energy ramp-up: evolution of measurements during this step depends on two main effects; first, pressure and electrical signal variations are related to modifications of energy spread due to RF noise injected to mitigate longitudinal beam instability; from 2.8 TeV, the main contribution comes from photoelectrons interacting with the residual gas and the chamber walls. (iii) During Stable Beam, proton intensity decreases still due to proton losses; (iv) Beginning of proton-proton collisions
Summary
Ultra-High Vacuum is an essential requirement to reach design performances in high-energy particle colliders. The behaviour of ions, created by ionisation of the residual gas by both the proton beam and the electron cloud, isn’t widely known. These ions (e.g. H2+ or CO+ [1]) are accelerated away from the beam and reach the vacuum chamber wall. (ii) Energy ramp-up: evolution of measurements during this step depends on two main effects; first, pressure and electrical signal variations are related to modifications of energy spread (depending on both the bunch length and the RF) due to RF noise injected to mitigate longitudinal beam instability; from 2.8 TeV, the main contribution comes from photoelectrons interacting with the residual gas and the chamber walls. (iii) During Stable Beam, proton intensity decreases still due to proton losses; (iv) Beginning of proton-proton collisions From this time, electrical signals decrease with the pressure
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