Observation of quadrupole wake-field effects in the KEKB linac

The characteristics of quadrupole wake-field effects were investigated using a single-bunch, high-intensity primary electron beam for positron production at the KEKB linac. X-Y coupling phenomena in transverse motion, which are inherent in the instability due to quadrupole wake-field effects in linacs, were examined; their dependence on the initial beam kicks was measured as a quadratic nature. The results are in good agreement with a simple calculation based on a two-particle model.