Recent Observations on a Horizontal Instability in the DA φ NE Positron Ring

A strong horizontal instability limits the maximum positron current storable in the DAFNE Phi-Factory. A powerful feedback system makes it possible to store and put in collision more than 1300 mA of positron current in 105-109 bunches. Nevertheless, a much higher current (> 2.4A) has been successfully stored in the twin electron ring. Measurements have been carried out to understand the positron current limit and to characterize the behavior of the horizontal instability at high current with different bunch patterns. Grow/damp turn-by-turn data obtained by turning off the horizontal feedback have been acquired and analyzed. Spectral analysis and growth rates of the instability are shown. In particular, the -1 mode has strong evidence and fast growth rate. Its growth rate behavior is analyzed at different beam currents and bunch patterns.