CERN, Geneva
As one of the pre-injectors for the Large Hadron Collider, the CERN Proton Synchrotron must reliably deliver a wide range of beam parameters. The large variety of bunch spacings from 25 to 150 ns at extraction requires the acceleration of small, high-density bunches as well as highly intense ones. Above a threshold bunch density, longitudinal coupled-bunch instabilities are observed after transition crossing and the main accelerating cavities have been identified as part of the impedance driving them. Transient beam loading causes asymmetries of the various bunch splittings used to establish the required bunch spacing, compromising beam quality at the head of the bunch train delivered. Recent measurements and longitudinal limitations of beams for the LHC are presented, together with possible cures and options for future hardware improvements.
EPFL, Lausanne
CERN, Geneva
Crossing transition energy in the CERN PS is critical for the stability of high intensity beams, even with the use of a second order gamma transition jump scheme. The intense single bunch beam used for the neutron Time-of-Flight facility (n-ToF) needs a controlled longitudinal emittance blow-up at the flat bottom to prevent a fast single-bunch vertical instability from developing near transition. This instability is believed to be a Transverse Mode Coupling (TMCI) type. A series of measurements performed in 2009 and 2010 aims at using this TMCI observed on the ToF beam at transition, as a tool for estimating the transverse global impedance of the PS. For this purpose, we compare the measurement results with the predictions of the HEADTAIL code and find the matching parameters. This will allow predicting the stability of the high brightness LHC beam near transition. The final goal is to study the feasability of a possible cure to the fast vertical instability measured on the ToF beam by applying an improved gamma transition jump scheme instead of compromising the longitudinal density.
