SPACE-CHARGE EFFECTS ON THE LONGITUDINAL OSCILLATION MODES OF ELECTRON-COOLED BUNCHED BEAMS

The longitudinal time structure and oscillation modes of very cold rf- bunched proton beams were experimentally studied in the IUCF electron cooling ring. Both longitudinal bunch density and quadrupole oscillati on frequency measurements indicate a high degree of space charge domin ance. The non-linear (sinusoidal) rf focusing field normally causes an incoherent synchrotron frequency spread leading to rapid decoherence of a coherent oscillation. In this regime of very cold beams, however, the small oscillations instead remain coherent and a coherent shift i n the frequency of small amplitude dipole synchrotron oscillations is observed. This phenomenon of coherent space-charge tune shift has not previously been observed, nor predicted.