A mode of coherent longitudinal dipole motion of the beam in the National Synchrotron Light Source (NSLS) vacuum ultraviolet (vuv) ring with small frequency is observed by Biscardi et al. [Rev. Sci. Instrum. 66, 1856 (1995)]. We point out in this paper that the coherent motion observed is, although a solution to the well-known secular equation for Robinson instability, a distinct mode of beam dipole motion from what is normally known as the Robinson mode. In this paper the mode’s properties and its relationships with the usual Robinson mode are developed. The collision of the two modes is understood analytically and observed experimentally in the NSLS vuv ring.

• Received 6 October 1997

DOI:https://doi.org/10.1103/PhysRevE.57.3461