Superconducting solenoid design operation significantly reduces electron beam emittance, enhancing accelerator performance

Optimizing Solenoid Design to Suppress Aberrations
The team redesigned the solenoid utilized in the ELBE superconducting radio-frequency electron gun (SRF Gun-II). By increasing the radius and length of the pure iron yoke, the new solenoid significantly reduced the integral of the first derivative of the magnetic field while preserving the magnetic field integral. This optimization resulted in a reduction of the spherical aberration coefficient by approximately 47%. Simulation results demonstrate that under a 600 pC beam condition, the transverse emittance decreased from 5.458 mm·mrad to 4.768 mm·mrad.

From Simulation to Measurement: Comprehensive Validation of Magnetic Field Performance
Following the assembly of the SRF Gun-III cryomodule, the research team conducted detailed magnetic field measurements on the new solenoid. Systematic field mapping was performed along the mechanical axis and transverse planes using both one-dimensional and three-dimensional Hall sensors. The measurement results show excellent agreement between the axial magnetic field and simulations, with a slope coefficient of 35.23±0.02 mT/A and an effective magnetic length determined to be 50.990 mm. Multipole field analysis, utilizing the formalism fitting method, extracted dipole, quadrupole, and sextupole components, providing a data foundation for understanding the non-ideal field effects of the solenoid.

Impact of Multipole Fields on Emittance and Correction Strategies
The study reveals that parasitic quadrupole and sextupole fields within the solenoid can significantly degrade beam emittance. Through theoretical analysis and ASTRA simulations, the team verified that installing correction quadrupole magnets downstream of the solenoid effectively suppresses emittance growth caused by quadrupole fields. Furthermore, the sextupole field component was found to contribute approximately 20% to the emittance increase, suggesting potential for further optimization through the future addition of sextupole correctors.

Analysis of Measurement Errors and Technical Challenges
Errors in magnetic field measurement primarily arise from the mechanical alignment of Hall probes, incomplete yoke degaussing, intrinsic probe errors, and data fitting processes. Notably, fitting errors for quadrupole and sextupole components are significant, reaching 73% and 90%, respectively; these figures reflect the inherent challenges in measuring higher-order magnetic fields. The team mitigated these issues by subtracting background fields, conducting repeated measurements, and optimizing fitting procedures, thereby controlling longitudinal field errors to within 2% and transverse field errors to less than 6%.

Advancing the Prospects for High-Brightness Electron Beam Applications
This research not only enhances the performance of superconducting solenoids but also establishes key technological foundations for next-generation applications, such as high-brightness X-ray free-electron lasers (XFELs) and high-repetition-rate terahertz sources. The research team plans to further optimize sextupole field correction schemes and explore more efficient matching designs between solenoids and beam dynamics.