The energy selection system (ESS) is a key part of the cyclotron-based proton therapy facility. In addition to regulating the energy of the fixed-energy proton beam generated by the cyclotron to match the depth of the tumor, the other main role is to control the beam emittance to meet the therapeutic requirements. Existing schemes for controlling the emittance are based on switching combinations of collimators with different apertures, where the adjustable emittance is limited by the number of combinations of apertures, and the need for a small aperture collimator upstream or downstream of the ESS to suppress the beam intensity limits the possibility of increasing the high-energy beam intensity.

This study aims to propose the beam optics design of proton therapy energy selection system (ESS) on the basis of the superconducting cyclotron CYCIAE-230 under development by the China Institute of Atomic Energy (CIAE) that can flexibly control the beam emittance and transmission.

First of all, the main optical design features of this ESS was described, and the Monte Carlo method based FLUKA code was applied to calculating the beam emittance, energy, and energy spread of a proton passing through a degrader with a set of collimators. Then, beam optical software TRANSPORT and TURTLE were combined to design the linear beam optics with consideration of placing a set of width-adjustable slits in front of the achromatic section and combining the beam optical conditions at the corresponding positions to control the beam divergence. Finally, the feasibility of flexible and adjustable beam emittance and transmission efficiency for esigned ESS was verified by calculations.

The results show that the ESS meets the demand of proton therapy, and the rational use of adjustable slit can make the ESS have the characteristics of flexible and adjustable beam emittance, so as to realize the adjustable beam transmission in the energy selection section. This simple and flexible optical design proposed in this study for adjusting the state of the beam can be used for regulating the beam intensity for proton therapy and other research applications, which has a wide range of application prospects.