Zhang, H. Baumgarten, C. Frey, P. Hartmann, M. Kan, R. Kostezer, M. Mülhaupt, A. Schippers, J.M. Schmidt, A. Snuverink, J. JACoW Publishing Geneva, Switzerland 2673-5482 978-3-95450-212-7 10.18429/JACoW-CYCLOTRONS2022-WEAO01 English 148-151 cyclotron simulation proton ion-source experiment Contribution to a conference proceedings 2023 2023-10 https://doi.org/10.18429/JACoW-CYCLOTRONS2022-WEAO01 https://jacow.org/cyclotrons2022/papers/weao01.pdf The use of the medical cyclotron COMET for FLASH proton therapy requires a high beam transmission from the ion source through the central region apertures. This paper first presents a model of the COMET cyclotron featuring a rotatable ion source, a movable puller, and an adjustable first fixed slit (FFS), implemented with the OPAL framework. The electromagnetic field is individual-ly created to match each specific configuration. The beam optics parameters, especially beam position and beam size upon approaching and after passing FFS, have been studied in detail. The OPAL simulations demon-strate that an optimal configuration of the ion source, the puller and the FFS is key to achieve a high beam trans-mission. An experimental test gave a 2.8 times higher intensity within COMET cyclotron with the modifications derived on the basis of the simulations: a 0.57 mm shift of puller and a 5.6° rotation of ion source. The simula-tions indicate that, with these modifications, the beam can still be centered and accelerated to the extraction energy of 250 MeV. Next step is to investigate the influ-ence of such modifications upon the acceleration and the extraction, again with an iterative approach combining simulations and experiments.