A 250MeV superconducting cyclotron (SCC250) for proton therapy is being constructed at Huazhong university of science and technology (HUST). A well-designed magnetic field and an excellent beam extraction are critical for obtaining the desired beam in the compact cyclotron. However, the high magnetic field brings about great challenges to achieve isochronism and high extraction efficiency. Poor field isochronism will cause severe particle acceleration problems. Besides, the enormous beam loss due to the small turn separation at the extraction region may lead to the damage of the superconducting system. This paper focuses on improving the field isochronism by cutting patches at different radii of the magnetic pole, and increasing the turn separation by adjusting the amplitude and phase of the first harmonic field. The resonances tune and the acceleration process are also optimized to acquire a high-quality beam. Calculations show that the error accuracy ΔB/B is smaller than 3e-4, which meets the requirement of beam acceleration. And the turn separation can reach 7mm, which reduces the beam loss to an acceptable level.

field isochronism;,tune optimization;,precession extraction