Development of the superconducting solenoid for the compact laser plasma accelerator for proton therapy

Abstract

Laser plasma accelerators have become a new kind of particle accelerator in recent years. Peking University has built the world’s first compact laser plasma accelerator, CLAPA I, which has an energy of 15 MeV and can adjust its energy spread to 5%. Currently, CLAPA II is being built. Its main use is for proton therapy. The beam transport system of CLAPA II can handle proton beams with energies from 40 to 230 MeV and an energy spread of less than 5% at the end. The laser interacts with the target to create an initial proton beam that has a large spread of angles and energy. This beam is first captured and focused by three superconducting solenoids. Then, the beam is sent into horizontal and vertical beamlines for treatment. This paper focuses on the three superconducting solenoids in the beam collection section. The magnetic design was completed based on beam dynamics requirements. The central magnetic fields of the three solenoids are 7.8 T, 3.2 T, and 7.8 T, respectively. A three-dimensional magnetic field measurement platform was used to measure the integral field homogeneity and magnetic center. The integral field homogeneity meets the design requirement of $10^{-2}$, confirming the magnetic design’s validity. Considering the magnetic center deviation, the alignment and installation of the three superconducting solenoids were conducted using a laser tracker, achieving an installation accuracy of less than 0.3 mm. The three superconducting solenoids are working well now, and experiments with high-power lasers impinging on the targets are about to start.