Beam Instabilities Observed at the CEA

Single beam instabilities have been observed at the Cambridge Electron Accelerator during accumulation and storage of electron and positron beams. In the filling mode, the instability appears at about 3 mA peak current. It develops structure in the beam sausage, makes the beam horizontally larger with increasing currents, shortens the decay time, and limits the maximum accumulated peak current to 40 mA for electrons and 10 mA for positrons. The instability involves an interaction over several bunches. Coherent horizontal betatron and phase oscillations have been observed in connection with the instability. The threshold could be increased with a high frequency quadrupole powered at a harmonic of the circumference frequency. It gives different bunches different υ values, which reduces the bunch-bunch interaction. The instability associated with the phase oscillation was reduced with a cavity oscillating at a different harmonic number from the RF. Neighboring bunches now see different RF-voltage gradients and have different synchrotron oscillation frequencies. Peak currents of 70 mA for electrons and 25 for positrons were achieved. During the storage mode, vertical and horizontal single bunch instabilities have been observed. Their threshold can be increased with octupole fields. The beam-beam interaction during accumulation and without separation has been investigated. The threshold seems to be independent of the number of bunches (or the υ-value shift if there were more than 25 bunches in each beam).