Emittance increase due to transient beam loading

A stretcher/storage ring is filled with a 2.1 μs beam pulse delivered by a 700 MeV linear electron accelerator. The filling time of the accelerator waveguide structures is 1.3 μs. The large energy variations during the filling time due to transient beam loading are compensated for by a time delay of the RF power of some accelerator stations. In this way, the final energy spread can be minimized but along the accelerator large energy variations still exist. As a consequence the dispersive property of the accelerator mainly due to misalignments causes a substantial increase of emittance during the filling time. This effect showed to be dependent on the location of accelerator stations of which the RF power is delayed in time and on the beam focussing along the accelerator. A recipe has been developed to minimize the emittance increase. For this purpose computer simulations with a variety of alignment errors have been carried out for different beam focussing situations. Also timing imperfections (jitter, risetime) have been taken into account.