ABSTRACT
The multi-disciplinary accelerator based facilities at iThemba LABS are used intensively for
nuclear physics experiments, radiotherapy and the production of radioisotopes. To increase the
beam intensity for radioisotope production and to improve the beam quality of the 66 MeV
proton beam, a double-gap horizontal half-wave flat-topping resonator has been developed for
the separated-sector cyclotron to operate at the associated fixed frequency. This type of flattopping
resonator has never before been implemented in a cyclotron and this study is the first
to show that it can be done, featuring a special characteristic not offered by other types. The
resonator is reviewed against the other types of resonators that are already in use at other
institutes around the world.
The flat-topping voltage of the selected type has a sinusoidal half-wave distribution along a
radial line in each of its acceleration gaps with the nodal points located on the injection and
extraction orbits. The flat-topping voltage therefore progressively increases from zero at both
of the two most critical orbits in a cyclotron to a maximum at about halfway between them. As
a result will this resonator, apart from its basic function to reduce the energy spread in the
beam, not decrease the orbit separation at the injection and extraction orbits in the cyclotron,
as is the case with other types of flat-topping resonators. This advantageous feature implies
that the beam pattern in the cyclotron is not affected in the regions of the delicate injection and
extraction components and therefore will these components or the operational control of the
beam not require any modification to accommodate the resonator.
In order to design a resonator that will meet our requirements, the theory of the beam
dynamics and resonator characteristics were studied to ascertain the expected improvement in
beam quality and beam intensity when a flat-topping resonator is implemented with the
cyclotron. All resonator types were considered and studied in terms of their power dissipation,
voltage distribution, harmonic number, space requirements and influence on the beam. The
horizontal half-wave resonator type, with two acceleration gaps, was selected as the most
suitable for our application, because of its preferred electromagnetic characteristics and its
geometric shape that permits the installation inside an existing vacuum chamber through an
existing flange. Initially a half-scale resonator model was build to test the feasibility of such a
resonator and also to verify the calculation methods.
Transmission line methods and numerical field analysis in 3D were applied to determine the
resonator characteristics. In the former method a computer program, POISSON, was used to
calculate curvilinear squares on sections through a triangular-shaped transmission line and in
the latter method a commercial computer program, SOPRANO, was used. SOPRANO is part
of an internationally acclaimed suite of programs and the acquired knowledge and skill to use
this state-of-the-art software for the studying and designing of such and other electromagnetic
devices also put the institute amongst the front-runners in the world.
The calculated characteristics of the half- and full-scale resonator models, the study of the
different electromagnetic modes that resonate in close proximity to the required frequency, the
heat transport modelling and the theory and implementation of the coupling and tuning
devices are all in good agreement with their respective measured results and are reported in
this document.
This study lead the way to have the first-ever double-gap horizontal half-wave flat-topping
resonator in a cyclotron successfully commissioned at iThemba LABS and the first tests with
beam report very stable operation. Accelerator physicists now have another option to utilize for the establishing of flat-topped acceleration voltages.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/5949 |
| Date | 20 January 2009 |
| Creators | De Villiers, John Garrett |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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