Coherent Synchrotron Relaxation Oscillation in an Electron Storage Ring

Detailed observations of the slow fluctuation of the stored beam in the NBS 280 MeV electron storage ring (SURF II) reveal a relaxation-type oscillation, in which brief intervals of dipole mode bunch oscillation alternate with relatively long recovery intervals free of coherent oscillations. A simple phase modulation model of the dipole oscillation shows that an infinite number of modulation sidebands develop around each RF harmonic of the beam. These vary in amplitude as the oscillation grows, giving the appearance of turbulence. The model reveals a self-limiting feedback mechanism which limits the growth of the oscillation. Graphic data illustrating this behavior will be presented.