On limitations on low-/spl alpha/ rings performance due to /spl nabla/Z-instabilities

We analyze collective instabilities of a bunch due to the radial gradient of the longitudinal impedance. One of the specific features of these collective effects is that decrements of collective synchrotron modes do not depend on the value of momentum compaction factor of the ring. This effect can limit the performance of the so-called low-/spl alpha/ storage rings. It can be used for the damping of unstable modes of the synchrotron coherent oscillations as well. >