Multipacting and Surface Conditioning

• Amedeo Bellunato (Politecnico di Milano (IT))

Room: 30/7-010 ABSTRACT The electron cloud build-up induced by the beam is one the major limitations inside the high luminosity project for the LHC. Multipacting is an RF resonant electron discharge sustained by secondary emission, inside accelerators cavities it induces pressure rises, heat loads and beam instabilities. It is ruled by the beam parameters, the magnetic field and the surfaces' SEY (secondary electron yield). To ful_ll the targets of the HL and LIU projects, it is required a reduction of the beam-pipes' yield in the SPS. The SEY is reduced by either by thin films coatings or beam scrubbing (beam conditioning). The amorphous carbon coatings seem a promising solution, with an SEY≈1. The beam scrubbing, instead, uses the beam induced multipacting to irradiate the pipes with energetic electrons. These electrons scrub the surface by electron stimulated desorption (ESD) and surface graphitization. Two RF test-benches are made of MBB or MBA profiled chambers and crossed by a W wire, they work as resonators and are surrounded by dipoles. The wire develops the electric field to ignite the multipacting, which is detected by the ESD peaks and the perturbation of the RF modes of the resonators. One of the set-up is provided even of an electron cloud monitor to measure the electrons current. The test-benches stimulate the multipacting with a defined frequency for long time periods to condition the surfaces. The present work focuses on the influence of the magnetic field on the multipacting, and the use of the electron scrubbing to suppress the electron cloud. In addition, a leak valve for the injection of hydrocarbon gases is installed on one of the systems to achieve a faster SEY reduction, using, for instance, Acetyelene or Dodecane. Lastly, the efficiency of an a-C coating in suppressing the multipacting it is investigated in the present test-benches.”