Sidebands from longitudinal bunch oscillations in an electrostatic ion beam trap

Sidebands are observed in the Fourier transform of the pickup signal of an RF-bunched beam oscillating in an electrostatic ion beam trap. We show that the sidebands are dominated by a collective longitudinal oscillation of the center of the bunch around the synchronous ion as the bunch travels within the storage device. We present evidence for a linear dependence of the sideband frequency and for a seemingly chaotic behavior of the amplitude of the sideband peaks on the number of stored ions.