Conceptual Design Of Magic Chopper Used For 4SEASONS at J-PARC

4SEASONS is designed to utilize the multiple incident energies (MIE) selected by a special Fermi chopper and enhance the measurement efficiency. Chopper opening time determined by a conventional Fermi chopper is inevitably constant, and thus we cannot realize the optimal experimental condition for each incident energy. Therefore, a new concept of Fermi chopper is required for the success of MIE measurement at 4SEASONS. In this paper, we present a new concept of supermirror-coated Fermi chopper (MAGIC chopper) specialized for MIE measurement at 4SEASONS. MAGIC chopper increases the effective opening time at lower incident energies, and consequently the energy dependence of effective opening time gets close to that of moderator pulse width. In addition to the matching of experimental condition, MAGIC chopper also realizes almost constant energy resolution in the wide range of incident energy.