Electric field distribution in a spiral inflector for cyclotron injection applications

The results of numerical studies of the electric field distributions in a spiral inflector, designed for axial injection system of H^-, D^- and He⁺⁺ beams into the Oak Ridge Isochronous Cyclotron (ORIC), are presented in this report. Discrepancies are found between the electric fields obtained from numerical computations and those derived from theoretical field distributions within the inflector. Fringe fields at the entrance to the inflector, in combination with the field discrepancies, lead to significant deviations in particle trajectories from those derived from analytical expressions for fields within such devices. A method is described for shaping the electrode surfaces of the spiral inflector that is effective in compensating for fringe fields at the entrance to the inflector and a simple solution is introduced that eliminates the seemingly unavoidable shift in the central beam path. As a consequence of these studies, we arrive at a design for a spiral inflector that operates at a low voltage with greatly improved optical properties in which the paths of injected particles are precisely known.