Wu, Z. Ding, K.Z. Li, J. Song, Y. Wang, Z. JACoW Publishing Geneva, Switzerland 978-3-95450-205-9 10.18429/JACoW-Cyclotrons2019-MOP004 English 31-34 MOP004 extraction cyclotron proton simulation emittance Contribution to a conference proceedings 2020 2020-06 https://doi.org/10.18429/JACoW-Cyclotrons2019-MOP004 http://jacow.org/cyclotrons2019/papers/mop004.pdf In order to diversify the company’s cyclotron, a design study has been carried out on a 240 MeV superconducting cyclotron SC240 for proton therapy, which is based on our experience in design of SC200. In order to increase turn separation and extraction efficiency, resonant precessional extraction method is employed in the extraction system. A first harmonic field consistent with the Gaussian distribution is added to introduce beam precessional motion. Its effects on phase space evolution and turn separation increase is studied by a high efficiency beam dynamics simulation code. According to the study, its amplitude and phase have been optimized to meet the requirements of extraction beam dynamics. Based on beam dynamics simulation, the parameters of extraction system elements (two electrostatic deflectors and six magnetic channels) are chosen. Besides, the effects of sectors spiral direction on beam extraction are studied. Extraction efficiencies and beam parameters have been calculated.