  The present Fermilab proton Booster is an early example of a rapidly-cycling
synchrotron (RCS). Built in the 1960s, it features a design in which the
combined-function dipole magnets serve as vacuum chambers. Such a design is
quite cost-effective, and it does not have the limitations associated with the
eddy currents in a metallic vacuum chamber. However, an important drawback of
that design is a high impedance, as seen by a beam, because of the magnet
laminations. More recent RCS designs (e.g. J-PARC) employ large and complex
ceramic vacuum chambers in order to mitigate the eddy current effects and to
shield the beam from the magnet laminations. Such a design, albeit very
successful, is quite costly because it requires large-bore magnets and
large-bore RF cavities. In this article, we will consider an RCS concept with a
thin-wall metallic vacuum chamber as a compromise between the chamber-less
Fermilab Booster design and the large-bore design with ceramic chambers.
