Modified octupoles for damping coherent instabilities

The introduction tune spread in circular e/sup +/e/sup -/ accelerators with modified octupoles to reduce the loss of dynamic aperture is discussed. The new magnet design features an octupole field component on-axis and a tapered field structure off-axis to minimize loss of dynamic aperture. Tracking studies show that the modified octupoles can produce the desired tune spread in SPEAR without compromising confinement of the beam. The technique for designing such magnets is presented, together with an example of magnets that give the required field distribution.<>