Electromagnetic Optimization and Testing of the Vertical Beamline DCT Superconducting Magnet in CLAPA-II Proton Therapy

CLAPA-II is a laser-driven proton therapy facility designed and constructed by Peking University. Protons are generated through laser–plasma interaction, with a 2-PW laser used to accelerate the proton beam. The beam transport system is capable of transmitting protons with energies ranging from 40 to 230 MeV. The CLAPA-II beamline consists of both a horizontal transport line and a vertical beamline. To provide critical technical validation for the future engineering of superconducting rotating frames, the vertical beamline utilizes curved discrete–cosine–theta magnets, resulting in a lighter and more compact design. The vertical beamline is composed of six modular magnetic units, each containing a 2.77-T central magnetic field dipole magnet with a 900-mm bending radius, alongside two 40-T/m gradient quadrupole magnets in a quadrupole–dipole–quadrupole (Q-D-Q) configuration. This article details the electromagnetic optimization design of the vertical beamline superconducting magnet (Q-D-Q) system. In response to the beam physics requirements, magnetic fields were optimized for both pure coil and iron core models. Structural behavior under 4.2-K excitation was analyzed, and the induced magnetic field distortions due to mechanical deformation were quantitatively evaluated. In addition, excitation testing and magnetic field measurements were conducted on the first magnet in a liquid helium environment. The findings provide key technical support for the design and manufacturing of superconducting magnet systems for future laser-driven proton therapy accelerators, while also establishing the technical foundation for large-scale production.