| Hefei Light Source (HLS) is a dedicated synchrotron radiation research facility, spectrally strongest in Vacuum Ultra Violet and Soft X-ray. Designed and constructed in 1980's, accepted to regular service in 1991, HLS has been operational for 15 years totally, making a significant contribution to research on synchrotron radiation applications in the Mainland of China. From 1999 to 2004, the National Synchrotron Radiation Laboratory (NSRL, belonging to the University of Science and Technology of China) carried out its Phase II Project, in which quite a few sub-systems of HLS storage ring were upgraded and 8 new beam-lines were constructed. After the project, the performance of HLS is improved considerably. For example, its routine operation beam intensity, under GPLS (General Purpose Light Source) configuration, is able to be up to 300 mA, with beam lifetime longer than 8 hours and beam orbit stability better than 100μm, resulting in an enhanced capability to provide synchrotron radiation of over 600 Ampere hours per year, as well as a by far improved efficiency.As a member of NSRL machine physics group, the author takes part in the operation and upgrading of HLS and, especially in the recent years, together with many colleagues, studies the storage ring in many ways, both theoretically and experimentally. This thesis is to comprehensively and thoroughly analyze the lattice parameters and the main sub-systems of the ring, in order to better understand the phenomena observed as well as the physics behind, and contribute to the success of the project. As a summing up of the experiences and lessons we learned during the commissioning, the author hopes the thesis will help in further improving the performance of HLS to make it one of the best intermediate energy synchrotron radiation sources in the world, and will even be found useful to other light sources as well, especially for designing and commissioning of "small rings", where the curvature radii in bending magnets are comparatively small.The main contents of this thesis are as following: analysis of linear focusing parameters and stability of beam orbit, analysis of injection process, nonlinear beam dynamics, beam loading effect and Landau damping effect.In Chapter 2, a few applications of beam orbit response matrix method in HLS ring are described. Firstly, based on measured corrector to BPM response matrix, LOCO (Linear Optics from Closed Orbit) code was applied to HLS to get accurate linear focusing parameters. It was confusing for a long time that the tunes found by mathematical model of HLS did not match with measurement. The problem was solved since the strengths of quadrupole families and dipole fringe field integral, calculated as fitting variables, describe the real machine precisely and agree well with independent magnetic field measurements. Then, also using response matrix fitting technique, compensation of superconducting wiggler effect on HLS linear lattice was studied systematically. Again, the problem was not satisfactorily solved for years. A new compensation scheme was thus obtained, which almost eliminates the beta function distortion due to the wiggler and overcomes other negative effects to a large extent. Machine study and operation show that, with the new scheme, beam lifetime is increased a great deal, so as to make achievement of the Phase II Project goals possible. Finally, the effects of various factors on beam orbit stability, especially that of variation of the gap of HLS undulator, were estimated by application of error to BPM response matrix. According to analytical calculation and hardware conditions of HLS ring, a conclusion was drawn that slow orbit feedback technique has to be developed to stabilize the beam orbit. Numerical simulations and preliminary practices confirmed the conclusion.In Chapter 3, the key points of "localized bump" injection system design and the performance of the present HLS injection system are analyzed in terms of "equivalent emittance" and so on. As a result, it is mainly a mismatch between the existing beam transport line and HLS ring and a strong perturbation of the newly built kickers (due to the longitudinally tight space to accommodate them) on the stored beam, that accounts for why the performance was not satisfying. Accordingly, the thesis explains how an assistant local D.C. bump and other measures helped to fulfill the injection task, pointing out their advantages and limits; and furthermore, brings forward physical designs to upgrade the transport line and the injection system, which will, respectively, improve the phase space matching radically and reduce the perturbation largely. The two designs also meet the requirements of full energy injection. Numerical simulations demonstrated that the proposed upgrading would assure a high injection efficiency (transverse) of more than 90 % and keep the perturbation well under ordinary beam orbit stability tolerance. Suppose the Hefei Free Electron Laser Test Facility proposal should be realized and the parameters of linac beam would be as expected, there exists the potential for HLS to develop Top-Up injection mode.In Chapter 4, nonlinear properties of GPLS configuration of HLS ring are analyzed. Firstly, methods and principles in nonlinear dynamics study are briefly reviewed, including Hamiltonian mechanics, Lie Algebra and Normal Form Theory. Then, the thesis presents the author's own work in applying the above theory to study nonlinear dynamic effects of fringe fields of dipoles and quadrupoles. It was revealed that quadrupole fringe field contributes, somehow similar to an octupole, to tune shift with amplitude (detuning), second order chromaticity, third order momentum compaction and fourth order nonlinear resonances, while dipole fringe field contributes to tune shift with momentum (chromaticity), second order momentum compaction and third order nonlinear resonances, following a rule that the smaller the dipole curvature radius is, the more the fringe field contributes. Finally, a numerical tracking method in fringe field region is discussed. Since most of existing nonlinear effect calculation codes ignore some terms produced by dipoles or their fringe field and are hence not very suitable to small rings, a simple tracking code was written by the author through explicit formulae obtained via generating function method, and nonlinear properties of GPLS configuration were studied by numerical tracking. Considering fringe field contributions, dynamic aperture of the configuration is still larger than the physical aperture. The work described in Chapter 4 prepares the ground for Landau damping analysis in the next chapter, and will serve as a necessary tool in pursuing new configurations for HLS ring, in which the beam emittance is lower and the dynamic aperture usually shrinks.In Chapter 5, two of collective effects in HLS ring—beam loading effect and Landau damping effect are analyzed. Firstly, an equivalent circuit model was employed to analyze and calculate beam loading effects and high current Robinson's instability threshold in different stages, that is, in injection, energy ramping and stable operation in HLS ring. The results showed the rationality of physical parameters of the RF system, however, the cavity has to be detuned sufficiently during injection (when the cavity voltage is low and the RF power is not high) to allow the beam accumulated quickly and steadily. Also described is how the tuning loop and amplitude loop work under remote control in the whole process. Then, a transfer function model (Pederson model) was used to analyze the effects of low-level circuits on beam stability. Using Lienard-Chipart Criterion, analytical stability conditions under actions of the two loops were found, implicating that the gain of amplitude loop must not be too high. In the second part of Chapter 5, the principles of Landau damping are discussed and, on the basis of other people's work, the dispersion relation, beam transfer function and stability diagram of a "flat" double-way Gaussian beam were deduced. Such functions and diagrams were calculated quantitatively for HLS ring in various conditions. Then a physical design of an octupole system in HLS ring is introduced, together with observations of its tests and operation, and a scheme to add new octupoles. Through beam transfer functions and stability diagrams, the effects of the octupoles to stabilize the beam are evident. By successfully dealing with the two collective effects, which might make big troubles in beam injection and accumulation, the work described in this chapter is important for the Phase II Project to achieve its goals. The thesis is ended with a brief summary of the contents and an outlook of HLS storage ring. In a word, this is only a beginning to understand the physics of the ring, and a lot of questions remain yet to be discussed, studied and solved. In the future, great efforts need to be made to improve HLS performance and enhance HLS capability continuously. Making HLS an advanced VUV/Soft X Ray light source is not just a dream.
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