  NORMA is a design for a normal-conducting race track fixed-field
alternating-gradient accelerator (FFAG) for protons from 50 to 350 MeV. In this
article we show the development from an idealised lattice to a design
implemented with field maps from rigorous two-dimensional (2D) and
three-dimensional (3D) FEM magnet modelling. We show that whilst the fields
from a 2D model may reproduce the idealised field to a close approximation,
adjustments must be made to the lattice to account for differences brought
about by the 3D model and fringe fields and full 3D models. Implementing these
lattice corrections we recover the required properties of small tune shift with
energy and a sufficiently-large dynamic aperture. The main result is an
iterative design method to produce the first realistic design for a proton
therapy accelerator that can rapidly deliver protons for both treatment and for
imaging at up to 350 MeV. The first iteration is performed explicitly and
described in detail in the text.
