The 50 MEZ Transverse Feedback System in the CERN ISR

One of the intensity limits in the ISR is imposed by the transverse coherent instability. The intensity at which the instability occurs may be raised by Landau damping, i.e. increasing the tune spread of the stacked beam. However, due to non linear resonances, the maximum tune spread is limited. A vertical feedback system which detects and damps the first eight modes of the instability has succeeded in increasing the intensity limit by a factor of ~1.6. Recently this factor has once again become insufficient due to the increase in intensity resulting from improvements in the longitudinal density and vacuum system. A new 50 MHz feedback system has now been installed which damps the first 166 modes of the instability. Hence at low chromaticity values the critical intensity is raised by around another factor of two. The electronic gain of this system decreases with frequency in order to limit the blow-up of vertical oscillations due to electronic noise and to reduce phase errors at high frequencies. The 50 MHz system has been used to explore the ISR vacuum limit to intensities of 39 A.