Optical effects of energy degraders on the performance of fragment separators

Next-generation facilities for the production of exotic beams require large acceptance fragment separators to separate and transmit rare isotopes. Fragment separators require energy degraders in order to achieve high purity separation of these rare species. The introduction of the degrader into an aberration-free optical design of a separator induces aberrations at the achromatic image. These may be completely eliminated by shaping the degrader appropriately. We have shown that, in order to eliminate these aberrations, some aberrations must be nonzero at the dispersive image, where the degrader is placed. A second order design with robust optics is presented.