Room 402, 4th floor, central library (entrance via 2nd floor sherfield)
Dr Mauro Taborelli, CERN
Abstract::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. 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 for zones operating at cryogenic temperatures there is an additional heat load.
Operating the accelerator for sufficiently long time partially reduces the intensity of the electron cloud. The reduction is induced by the irradiation of the surface by the electrons and the mechanism of the phenomenon is still under study at laboratory level from the point of view of surface analysis. The presentation will illustrate the effect of the electron dose on the electron yield and the 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, solutions are under study to mitigate and suppress the electron cloud, thus preserving a high quality proton beam and avoiding complex and expensive modifications of the cryogenic plant. Surface modifications, either by changing the chemical composition of the surface or its topography, were considered. CERN developments focused on low secondary electron yield coatings based on carbon, but alternative treatments with potential for electron cloud mitigation will also be discussed. 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.