Theoretical And Experimental Research On The Realization Of Ultralow Beam Emittance For Storage Ring Light Source

Ultralow beam emittance is the main development goal of the storage ring light source.This dissertation mainly focuses the realization of ultralow emittance for the storage ring light source,and it includes two parts:ultralow emittance lattice design and round beam mode.Fully understanding of the optimal distributions of dipole and quadrupole fields that achieve the minimum emittance inside a lattice cell will help ultralow emittance lattice design and improve the cost performance of a real machine.And the round beam mode will be necessary for not only the SR users but also the machine operation when the natural emittance of storage ring approaches the diffraction-limitation.Using this mode the photon brightness can be further enhanced,and some effects such as IBS and Touschek-loss in the storage ring can be alleviated.The main works and results are given in the following.*Ultralow emittance lattice designThe beam emittance in the storage ring is mainly determined by the magnetic lattice composed of magnets with dipole and quadrupole field.Therefore,a question arises directly:what are the optimal distributions of the bending and focusing fields inside a lattice cell to achieve the minimum beam emittance?To response to this question,we numerically optimized the dipole and quadrupole distributions of the normalized cell using the particle swarm optimization algorithm.Optimization is first done using the photon brightness of an insertion device as the objective while both the quadrupole and dipole field distributions are varied as optimized variables.The maximum strength of the quadrupole gradient Kmaxis the main constraint in the optimization.When Kmax is relatively low,the longitudinal gradient bending magnet(LGBM)and anti-bending magnet(ABM)appear simultaneously in the optimized cell,and the quadrupole field naturally groups into one focusing magnet and one defosuing magnet.As the Kmax is increased,it shows that the cell tends to split into two identical cells with similar features.We further studied the FODO cell optimization using the natural emittance as the objective.The optimized cell st.ructure again shows longitudinal field variation with negative bending.The LGBM and ABM have shown their abilities of reducing the beam emittance to a ultralow level(diffraction-limitation)in storage ring design.However,a low emittance lattice is usually having a poor nonlinear performance.So we analyzed the ultralow emittance lattice composed of both of the LGBM and ABM in terms of the natural chromaticity,momentum compaction factor,and the phase advance.The results show that a well-designed linear lattice is first step to have a good nonlinear dynamic performance.In addition,a 9BA lattice composed of both of LGBM and ABM is present here.And for this lattice,an innovative phase cancellation scheme is proposed.The preliminary nonlinear optimization of this lattice shows a good nonlinear dynamic performance.*Round beam modeRound beam mode(with full coupling)means that the natural beam emittance of storage ring is divided equally into the horizontal and vertical planes.This mode is usually assumed in the diffraction-limited storage ring light source designs.Among all the methods of generat.ing round beam,operating on the coupling resonance is most popular one for its simpleness and no special requirements on accelerator hard-ware.To comprehensive understanding of this method,we did the experiments to generate the round beam on SPEAR3 machine.The results show that the round beam can be generated by careful control of the working point of the stored beam and coupling coefficient of the storage ring.We also did the measurements of the injection efficiency and the dynamic aperture when the round beam experiment is conducted.An interesting phenomenon is founded when the coupling coefficient is corrected to a low level.It is that the stored beam with working point sat on the coupling resonance has a good injection efficiency and dynamic aperture,while the stored beam with working point shifted away the coupling resonance has a poor in-jection efficiency and dynamic aperture.A simulation is implemented to account for this phenomenon.The results indicate that the tune of oscillating beam with large amplitude will be shifted from the nominal working point due to the tune shfit with amplitude.