|Titolo||Mitigation of electron cloud in modern particle accelerators|
|Dove||Aula A, Via della Vasca Navale 84|
|Oratore||Dr. Mauro Taborelli, TE-VSC, European Organisation for Nuclear Research, CERN, Geneva, Switzerland|
Electron multiplication, so-called multipacting or electron cloud, is one of the major limitations for the intensity of modern particle accelerators with positively charged beams. Synchrotron radiation and beam collisions with the residual gas generate electrons, which are accelerated by the beam potential toward the vessel surfaces. Low energy secondary electrons are thus emitted and, for a sufficiently large yield, the process leads to a multiplication. The effect for the accelerator is a degradation of the beam quality and increased heat load in the cryogenic temperature sections. Operating the accelerator for sufficiently long time partially reduces the intensity of the electron cloud and laboratory tests demonstrate that the irradiation of the surface by the electrons decreases the secondary electron yield. The presentation will illustrate the measurements and evolution of the secondary electron yield, the effect of the electron dose on the surface and the resulting chemical modifications as identified by X-ray Photoemission Spectroscopy. In view of the increase of intensity of the Large Hadron Collider (LHC) towards the High Luminosity era, the necessary solutions to mitigate and suppress the electron cloud were developed, in order to preserve a high quality proton beam and avoiding complex and expensive interventions on the cryogenic plant. This can be achieved by surface modifications, either by changing the chemical composition of the surface or its topography. CERN developments focused on low secondary electron yield coatings based on carbon. Carbon coatings exhibit a yield close to unity and an excellent robustness toward air exposure. Their characterization will be presented together with the implementation process, starting from the preparation of small laboratory samples to the devices for coating ‘in situ’ in various parts of the underground accelerator complex. In a staged approach, such coatings are being applied to the Super Proton Synchrotron, the last injector before LHC. During the next long maintenance period it is envisaged to treat LHC regions close to the interactions points.