Focusing in the Houghton College Cyclotron

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Morrow, Sylvia
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Houghton College
The small cyclotron at Houghton College loses most of its beam current to collisions with the Dee and chamber walls. The magnet is being altered to produce a more uniform increase in the radial magnetic field component with distance from the central plane. This will improve weak magnetic focusing as this component of the magnetic field creates a restoring force that pushes ions towards the central plane. Moreover, the field index value n=0.2 must not occur inside the maximum ion orbit radius to avoid coupled resonances. A computer model of the magnet and chamber was developed to test modifications of the magnet to achieve these results. A two dimensional cross-section of the magnet was modeled using Poisson Superfish, the results of which were used to track ions with the Simion 8.1 code. This model indicates a good chamber design is to replace the current aluminum chamber lids with magnetic stainless steel lids with radius 2.2 cm larger than the Dee. The steel lids direct the magnetic field lines radially outward, resulting in a more gradual change in field all the way to the outside edge of the Dee. This new design doubles the theoretical maximum kinetic energy to 900 keV for protons, but at these higher energies the ions fall out of phase before reaching the maximum radius because of the large number of orbits required. Results of the computer model will be compared with analytical results using a simplified model. Suggestions are made for future solutions.
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