Electron Clouds in High Energy Hadron Accelerators

The formation of electron clouds in accelerators operating with positrons and positively charge ions is a well-known problem. Depending on the parameters of the beam the electron cloud manifests itself differently. In this thesis the electron cloud phenomenon is studied for the CERN Super Proton Synchrotron (SPS) and Large Hadron Collider (LHC) conditions, and for the heavy-ion synchrotron SIS-100 as a part of the FAIR complex in Darmstadt, Germany. Under the FAIR conditions the extensive use of slow extraction will be made. After the acceleration the beam will be debunched and continuously extracted to the experimental area. During this process, residual gas electrons can accumulate in the electric field of the beam. If this accumulation is not prevented, then at some point the beam can become unstable. Under the SPS and LHC conditions the beam is always bunched. The accumulation of electron cloud happens due to secondary electron emission. At the time when this thesis was being written the electron cloud was known to limit the maximum intensity of the two machines. During the operation with 25 ns bunch spacing, the electron cloud was causing significant beam quality deterioration. At moderate intensities below the instability threshold the electron cloud was responsible for the bunch energy loss. In the framework of this thesis it was found that the instability thresholds of the coasting beams with similar space charge tune shifts, emittances and energies are identical. First of their kind simulations of the effect of Coulomb collisions on electron cloud density in coasting beams were performed. It was found that for any hadron coasting beam one can choose vacuum conditions that will limit the accumulation of the electron cloud below the instability threshold. We call such conditions the "good" vacuum regime. In application to SIS-100 the design pressure 10e−12 mbar corresponds to the good vacuum regime. The transition to the bad vacuum regime can happen for pressures larger than 10e−11 mbar. For the SPS and LHC conditions the simulations of the electron cloud wake fields were performed. The wake fields calculated using VORPAL were compared with the results of the 2D electrostatic code for the first time. In the latter the beam-cloud interaction is purely transverse. It was found that the results of the simplified code agree very well with the results of 3D electromagnetic simulations in VORPAL. Moreover, based on the kick approximation an analytical expression for the longitudinal electron cloud wake field was derived. The analytical expression gives a good agreement with the simulation result for very low bunch intensities