  An understanding of collective effects is of fundamental importance for the
design and optimisation of the performance of modern accelerators. In
particular, the design of an accelerator with strict requirements on the beam
quality, such as a free electron laser (FEL), is highly dependent on a
correspondence between simulation, theory and experiments in order to correctly
account for the effect of coherent synchrotron radiation (CSR), and other
collective effects. A traditional approach in accelerator simulation codes is
to utilise an analytic one-dimensional approximation to the CSR force. We
present an extension of the 1D CSR theory in order to correctly account for the
CSR force at the entrance and exit of a bending magnet. A limited range of
applicability to this solution, in particular in bunches with a large
transverse spot size or offset from the nominal axis, is recognised. More
recently developed codes calculate the CSR effect in dispersive regions
directly from the Lienard-Wiechert potentials, albeit with approximations to
improve the computational time. A new module of the General Particle Tracer
(GPT) code was developed for simulating the effects of CSR, and benchmarked
against other codes. We experimentally demonstrate departure from the commonly
used 1D CSR theory for more extreme bunch length compression scenarios at the
FERMI FEL facility. Better agreement is found between experimental data and the
codes which account for the transverse extent of the bunch, particularly in
more extreme compression scenarios.
