Study of collective effects in a low-emittance PEP lattice

Single-bunch current limits were measured for the low-emittance tunes in PEP (Positron-Electron Project) and found to be limited by the transverse mode coupling instability to less than 2.5 mA for RF voltages up to 10.5 MV. The horizontal and vertical transverse impedances appear to be Z_x=1.05 MΩ/m and Z_y=4.2 MΩ/m, respectively, assuming that the average beta functions are equal to those in the RF cavities. At the maximum single-bunch currents, considerable coherent motion of the bunch was observed, probably resulting from the large wakefields caused by residual closed-orbit distortions. The multiple-bunch operation showed large longitudinal and transverse coherent oscillations when bunches were filled to the single-bunch limit. Total currents in excess of 30 mA were obtained by filling widely spaced bunches to less than 1/3 of the single bunch limit. With this method the coherent beam motion was quite small in all dimensions. The results obtained indicate that the lower momentum compaction factor and large beta function variations of the low-emittance lattice (a common feature of all proposed high-energy radiation sources) contribute to enhanced collective effects, limiting the early achievable beam intensity to less than the desired 100 mA