Design and optimization of beam optics for a superconducting gantry applied to proton therapy.

PURPOSE: Proton therapy (PT) is a precise and effective radiotherapy method for tumors. To reduce the weight and footprint of normal conducting gantries applied to PT, a lightweight superconducting (SC) gantry with large momentum acceptance is studied at Huazhong University of Science and Technology. METHODS: To limit the frequency of field changing in SC magnets, a local-achromaticity beamline is designed with large momentum acceptance. Based on the analysis of high order aberrations, the lattice of the gantry beamline is composed of two symmetric bending sections to limit high-order aberrations, and sextupole fields are superimposed to further eliminate dispersion up to second order. RESULTS: We presented a second order beam optics design of a SC gantry. The optics fitting is completed with COSY Infinity and the result of particle tracking shows a momentum acceptance of ±8%. Alternating gradient canted-cosine-theta (CCT) magnets are applied to implement combined functions of beam bending and focusing. Some methods are proposed to minimize the field distortion in curved CCT magnets. CONCLUSIONS: The beam optics with high order considerations of a lightweight SC gantry with large momentum acceptance is presented.

Study of beam transverse properties of a thermionic electron gun for application to a compact THz free electron laser.

A novel thermionic electron gun adopted for use in a high power THz free electron laser (FEL) is proposed in this paper. By optimization of the structural and radiofrequency (RF) parameters, the physical design of the gun is performed using dynamic calculations. Velocity bunching is used to minimize the bunch's energy spread, and the dynamic calculation results indicate that high quality beams can be provided. The transverse properties of the beams generated by the gun are also analyzed. The novel RF focusing effects of the resonance cavity are investigated precisely and are used to establish emittance compensation, which enables the injector length to be reduced. In addition, the causes of the extrema of the beam radius and the normalized transverse emittance are analyzed and interpreted, respectively, and slice simulations are performed to illustrate how the RF focusing varies along the bunch length and to determine the effects of that variation on the emittance compensation. Finally, by observation of the variations of the beam properties in the drift tube behind the electron gun, prospective assembly scenarios for the complete THz-FEL injector are discussed, and a joint-debugging process for the injector is implemented.