• E. Pozdeyev
  BNL, Upton, Long Island, New York
• F. Marti, R.C. York
  NSCL, East Lansing, Michigan
• J.A. Rodriguez
  CERN, Geneva

Studies of space charge effects in the Small Isochronous Ring (SIR) at Michigan State University revealed a fast longitudinal instability at and below the transition that could not be explained by the conventional negative mass instability. The observed beam behavior can be explained by the effect of the radial component of the coherent space charge force on the longitudinal motion. The transverse coherent space charge force effectively modifies the slip factor shifting the isochronous point and enhancing the negative mass instability. This paper presents results of numerical and experimental studies of the longitudinal beam dynamics in SIR and proposes an analytical model explaining the results.