The ASPUN project

A study of a pulsed spallation neutron source that could deliver fluxes in excess of 1 x 1017 N/cm2-s began at Argonne in 1981. A review of various accelerator concepts to act as an intense charged-particle source to generate spallation neutrons resulted in the selection of a Fixed-Field Alternating-Gradient (FFAG) accelerator as the preferred device. There are several features of an FFAG accelerator that make it well suited to be the charged-particle accelerator for a 1017 N/cm2-s pulsed source. Since the main magnets are dc operated, injection time can be long enough to provide for efficient injection and adiabatic capture, the rf system can be optimized for maximum use of the accelerating voltage, the repetition rate can reach hundreds of hertz and large transverse and momentum acceptances are possible. The large momentum acceptance and rapid acceleration helps avoid some of the instability problems. The conceptual design has evolved from a spiral-type FFAG with strong edge focussing to a radial-type FFAG. The radial-type FFAG allows easier installation of the rf cavities at the expense of higher magnetic fields. Since the magnets are dc operated, the higher field can be achieved with superconducting coils. Although the conceptual design presented in this paper is in principle able to deliver the required charge per pulse many features still need to be developed. (author)