Change of the Energy Distribution in an Electron Storage Ring by a Dipole-Octupole Wiggler

A dipole-octupole wiggler is proposed to change the energy distribution of the electrons in a storage ring so that the peak current and the related collective effects are reduced. With a dipole-quadrupole wiggler the radiation damping partition can be changed and with a dipole-octupole wiggler this partition becomes dependent on the phase oscillation amplitude. These wigglers can be operated so that the longitudinal damping is negative for small amplitudes but increases for larger amplitudes. The particle distribution will peak around the amplitude which gives neither damping nor antidamping. This leads to a longitudinal blow-up of the bunch core without affecting very much the tails or the quantum lifetime. The distribution has been calculated analytically with an appropriate Fokker-Planck equation and by computer simulation. The results allow the optimization of the wiggler parameters. Due to the sum rule for the damping partitions this wiggler also affects the distribution in horizontal betatron amplitudes.