Design studies on the axial injection system of the cyclone-44 cyclotron

• Main Author: Lachinov, Mikhail V.
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/8172

A comprehensive feasibility study of axial injection has been carried out for a compact cyclotron (CYCLONE-44) which the University of Louvain-la-Neuve is building as a post-accelerator/ separator for radioactive ion beams. To achieve a high overall injection efficiency it is necessary to ensure proper 6D beam matching of the injected beam to the cyclotron central region. The results presented encompass all major aspects of a working system: 4D phase-space matching, injection trajectories, spiral inflector calculations, beam bunching in spiral inflectors, choice of the bunching system and discussion of the possible approaches to resolving trade-offs in the 6D matching problem. We began by optimizing the transverse (4D) phase-space matching by reducing the emittance growth that results from the mismatch of the beam emittance with respect to the cyclotron eigenellipses, when the beam is injected by means of a spiral inflector. The matching procedure was based on the method implemented in the TRANSOPTR beam optics code. Results are presented for various sizes and tilts of the inflector and for various shapes and orientations of the cyclotron eigenellipses. Specific results for the CYCLONE-44 cyclotron are also presented. The technique used to generate inflector transfer matrices is described. Since we assumed linear optics in the inflector it was important to ensure that the derived transfer matrices were consistent with the laws of Hamiltonian mechanics and represented a real physical problem. This goal was achieved by performing 15 tests proving compliance with the symplectic conditions imposed on a (6 x 6) transfer matrix. Using semi-analytical methods, involving transfer matrix techniques, the first-order effect of a spiral inflector on the beam's longitudinal phase space was studied. The conditions for obtaining minimized bunch lengths at the inflector exit were derived. Consistent with some earlier publications, the time spread of the beam at the inflector exit for the large injected emittances required in this application was significant. It is shown that this result is a direct consequence of the inflector optics. To enhance the cyclotron acceptance of the dc beam, several bunching systems were considered with particular emphasis given to a linear buncher modulated by ideal sawtooth waveform and a double-drift bunching system consisting of two double-gap sinusoidal bunchers. Finally, since the overall goal of this work was to maximize the amount of beam that could be injected into the cyclotron, a trade-off between transverse and longitudinal beam matching was considered. As the beam sizes in the inflector increase, so do the non-linear effects, and it becomes harder to get clean transverse emittances at the entrance to the cyclotron's central region. Therefore it seems prudent to put more emphasis on obtaining good longitudinal capture into the cyclotron and limit the injected beam emittances to about 15-20 mm-mrad.