EPFL, Lausanne
CERN, Geneva
BNL, Upton, Long Island, New York
Since 2003, single bunches of protons with high intensity and low longitudinal emittance have been observed to suffer from heavy losses in less than one synchrotron period after injection in the CERN Super Proton Synchrotron (SPS) when the vertical chromaticity is corrected. This fast instability does not limit the current performance of the SPS, but would be a major limitation in case of an anticipated upgrade of the SPS, which requires bunches of 4·1011 protons (p). Besides, the characteristics of this instability are also complementary indicators of the value of the SPS beam coupling impedance. MOSES analytical calculations, HEADTAIL macroparticle tracking simulations, as well as several measurement campaigns in the SPS indicate that this instability may be due to a coupling between transverse modes ‘-2’ and ‘-3’. The aim of this contribution is to report improvements of the SPS impedance model used by HEADTAIL simulations, and to find out more characteristics of the measured instability in order to assess whether the observed instability in the SPS is indeed a Transverse Mode Coupling Instability (TMCI).
UMAN, Manchester
CERN, Geneva
EPFL, Lausanne
The largest contribution to the LHC transverse resistive wall impedance is given by the graphite collimators. Such a contribution is predicted by analytical calculations. A series of laboratory measurements were performed to experimentally validate the analytical results in the case of small gaps and in a low frequency regime where the skin depth becomes comparable to the collimator thickness. The measurement method consists in determining the dependence of a probe coil input impedance on the surrounding materials and was applied to sample graphite plates, stand alone LHC collimator jaws and a full collimator assembly. After reviewing the measurement procedures, problematics and stages, the results are compared to analytical predictions and numerical simulations.