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 MeV 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 DCT 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 Q-D-Q configuration. This paper 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. Additionally, 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.