Studies On Low-energy Highly-charged Ion Beam Transport From An ECR Ion Source

As one of the most powerful devices to produce intense highly-charged heavy ion beams, ECR ion sources are widely adopted by many large accelerator facilities.With the increasing of the beam current request by the accelerators, the total beam current of the multi-component ion beam from an ECR ion source will exceed 20 em A, the serious space charge effect will produce profound influences on the beam quality and transport in the low energy beam section. Besides, due to the complicated magnetic field configuration of the ECR ion source, the ion beam extracted from which has a special and complicated structure, which will bring a big challenge to the subsequent transmission and matching with the downstream accelerators. In this thesis, beam property and transport from high charge state ECR ion sources are studied both by experiments and simulations. The research mainly focuses on three aspects: 1. Transverse coupling property of beam from ECR ion sources, 2. Effects of the sextupole field in the ECR and the sextupole component in the analyzing dipole on the ion beam, as well as the corresponding compensation, 3.Effects of pre-focusing solenoid on the beam quality and transport. The results of the research are then verified and applied in the existing beam line and new beam line design.Transverse coupling is one of the most important properties of ECR source beam.The extraction apertures of ECR ion sources are mostly located near the center of the extraction solenoid coils, particles are extracted and accelerated in a strong descending axial magnetic field, which adds a rotational component to the beam and couples the transverse phase space of the beam. This property of ECR source beam is firstly proven by experiment in this thesis. Based on the measurements, a particle distribution in the 4D phase space is obtained by beam back-tracking simulation,which has been proven to include the real coupling configuration between the transverse phase spaces and can be used as the initial condition in the beam line study and design. Beam simulation also demonstrates the periodic property of the beam coupling, a decouping scheme to make the projection emittance product get close to the minimum value by changing the strength and polarity of the pre-focusing solenoid is proposed.Both the sextupole field in the ECR and the sextupole component in the analyzing dipole can lead to beam distortion. The effects of both sextupole fields on the beam are firstly taken into account simultaneously, and a compensation scheme by using a rotatable hexapole magnet is proposed. A hexapole magent is designed and fabricated and installed in the SECRAL line. Measurement results have verified the feasibility of the sextupole compensation scheme. In addition, based on the experimental results, the complexity of the sextupole compensation is analyzed,which indicates that different compensation scheme should be used for different ion species.The pre-focusing solenoid has a fundamental impact on the beam quality and transport. Effects of the solenoid on the location of the beam waist formation after the analyzing magnet both for low-current and high-current ion beams are studied both in experiment and simulation, which shows that for high-intensity ion beams, increasingthe initial focusing strength of the solenoid is beneficial to avoid the large-radius aberration in the analyzing magnet and improve the beam quality, but with the sacrifice of the momentum resolution of the system due to the space charge effect.The results of the research are verified and applied in the existing beam line study and new beam line design. Firstly, by taking the beam transverse coupling property into the SFC injector study, an improved operation scheme for the SECRAL associated line has been proposed, what is to adjust the solenoid lenses to achieve a transversely decoupled beam so that the projection transverse emittances reaches the minimum value. Corresponding experiment indicates that by using this operation scheme, the beam transmission efficiency of the injector line of the SFC can be increased by 24%, while the overall efficiency of SFC is almost doubled. Secondly,based on the study towards the function of the pre-focusing solenoid in the SECRAL line, a new Q/A selector system is designed for SECRAL-2, in which a quadrupole triplet is introduced to handle the conflict between the beam quality and the resolution of the system. Finally, general requirements of the high charge state ion beam transport and matching from ECR to the downstream accelerator are discussed and summarized, on this basis, a LEBT is designed for LEAF program, and the corresponding simulations are performed.