The Design of Helium Ion Accelerators for External Ion Beam Cancer Therapy

Currently protons and carbon ions are the only heavy charged particles used
for radiotherapy. There is particular interest in the use of helium ions for
therapy, as they present physical dose and clinical advantages over carbon
ions and protons. In this thesis, a Fixed Field Alternating Gradient (FFAG)
approach has been developed for helium ion therapy, which has never been
explored before. Two non scaling FFAG accelerators are designed, accelerating
He2+ ions to 900 MeV, the necessary energy to reach 30cm depth in
tissue.

Initial steps have characterised the beam optics in order to achieve isochronous
stable equilibrium orbits over the design energy range described above,
whilst maintaining the working point with few resonance crossings. The
desired energy range of the machine is split across two stages, of which
both were successfully optimised to have a time of
ight to within 1% of
each other. A common operating RF frequency has been identfied, and the
RF parameters that are currently achievable were assessed and suitability
for this design investigated. Having successfully chosen realistic RF parameters,
a beam has been accelerated and it has been demonstrated that there
is a sufficient dynamic aperture and extraction is possible.

The feasibility of a nsFFAG accelerator for the purposes of helium therapy
has been successfully investigated, and a helium ion beam was successfully
accelerated through simulation from 1 MeV to 900 MeV across two stages,
using fixed frequency acceleration. The design is compact, though splitting
the design into two machines has increased the footprint. However, the first
stage has the potential to be used as a standalone facility for research and
shallow tumours, and the accelerator could likely fit in a current radiotherapy
bunker. Further work will need in depth design studies to model the
magnetic field and RF cavity designs, to verify the design.