Zhang, H.J. Fan, K. Rao, Y.-N. Yan, Y. Zhang, L.G. JACoW Publishing Geneva, Switzerland 978-3-95450-205-9 10.18429/JACoW-Cyclotrons2019-MOP035 English 113-115 MOP035 extraction cyclotron MMI resonance proton Contribution to a conference proceedings 2020 2020-06 https://doi.org/10.18429/JACoW-Cyclotrons2019-MOP035 http://jacow.org/cyclotrons2019/papers/mop035.pdf A superconducting cyclotron based on proton therapy facility is being developed at Huazhong university of science and technology (HUST). Due to the compact size of the main magnet, the beam orbits at the extraction region are distributed densely, which creates difficulties for beam extraction leading to severe beam loss. In order to deal with these challenges, the orbit precession method has been employed in the extraction system design. In this paper, we introduce a method of employing a first harmonic field near the nu_{r}=1 resonance where the beam energy is about 248 MeV to adjust the amplitude of beam orbit oscillation. The optimum amplitude and phase of the first harmonic field are designed to obtain a large turn separation in the extraction region. Three different ways of generating the first harmonic field are compared for optimization.