Research On Key Technologies Of Vacuum System In Diffraction-limited Storage Ring

The Hefei Advanced Light Facility(HALF)is a Diffraction-limited Storage Ring(DLSR)device,which has low beam emittance,high brightness and coherent photon flux.In the fields of nano-spectroscopy,coherent scattering imaging and in-situ experiment,the ultra-high spatial,temporal and energy resolution,as well as multiscale imaging capabilities,can be greatly improved by DLSR.Compared to the thirdgeneration light source,the multi-bend achromat(MBA)lattice has been applied for DLSR,so its vacuum system is more compact and the beam becomes more sensitive to coupling impedance,which brings a great challenge to the design of critical vacuum components.This dissertation mainly focuses on the study and design of critical vacuum components in the HALF storage ring,including ultra-high vacuum(UHV)flange,RF shielded bellows assembly and photon absorber.A great number of UHV flanges,capable of connecting each vacuum chamber together,are placed between magnets.So,the research and design of compact UHV flange are one of the key issues.The bellows,commonly installed on the two ends of the BPM block,can absorb vibration and thermal expansion to provide a highly stable work environment.However,the wave structure of the bellows is the source of strong coupling impedance peaks when isolating vibration,resulting in beam instability,so the RF shielding structure is needed to screen the bellows.Accordingly,the design of compact RF shielded bellows assembly,as well as the study on its coupling impedance property,are also key issues.The synchrotron radiation is generated during the orbit change of moving electronic,and only part of the radiation is sent to the beamline station for applications.So,the remanent radiation must be absorbed safely.If vacuum components and the inner wall of the vacuum chamber are directly exposed to these high-power synchrotron radiations,the temperature will rise,causing outgassing of material surface,damage to components,and even the crash of the vacuum system.Therefore,the design,thermolysis and thermal-stress analysis of photon absorbers are critical works for vacuum system construction.In this dissertation,a series of chain clamp flanges with small diameters has been designed to replace traditional CF flanges.Compared to the CF flange,the axial dimension of the chain clamp flange only depends on the width of the clamp,so the space required for installation is small.Furthermore,the chain clamp only has one bolt,which greatly improves the mounting efficiency and reduces the exposure time of operation staff under a radiation environment.For this chain clamp flange,two sealing types are designed,which are the knife-edge sealing method and the spring-energized metal C-ring(Helicoflex)sealing method.The knife edge requires high sealing force,so SUS 304,possessing high strength and hardness,is chosen to manufacture the flanges with a knife edge.With regard to the sealing method of Helicoflex,the sealing force is low.Therefore,the CuCrZr alloy(ATSM C18150),as well as stainless steel,can be used to make flanges.In HALF,Oxygen-Free with Ag(OFS)and CuCrZr copper alloy are chosen as the material for vacuum chambers,bringing the welding difficulty between the copper vacuum chamber and the stainless steel UHV flange.Choosing CuCrZr to replace stainless steel as the flange material can be one of the solutions to the welding problem.The leak rate test setup using a helium leak detector has been built.In order to evaluate the service life of the chain clamp flange,repeated mounting and unmounting tests have been carried out.Moreover,the bakeout cycle tests or liquid nitrogen immersion tests have also been performed to mimic real work conditions.The leak rates of all tests are lower than 1 × 10-11 Pa·m3·s-1,which indicates that the designed chain clamp flange has achieved the standard leakage of the UHV flange.The compact RF-shielded bellows are designed for HALF.In order to save space,two bellows are directly welded to the two ends of the BPM block.The finger-type RF shielding structure,consisting of the spring finger and contact finger,is chosen to screen the bellows.The contact force of 120 g/finger is provided to ensure good electrical contact.An impedance measurement setup based on the coaxial wire method was designed and built to investigate the longitudinal coupling impedance of RF-shielded bellows.With the help of a laser leveler,this setup was vertically placed to eliminate wire sag due to gravity.The impedances of bellows with and without RF shield have been measured.The results indicate that the finger shielding structures successfully screen the strong impedance resonances of the unshielded bellows assembly.Besides,the CST wakefield solver is used to calculate the impedance for comparison.The simulation results fit well with the measurement one,which shows the reliability of the impedance measurement setup.The photon absorbers designed in this dissertation mainly contain two types,including the crotch absorber and the photon block at the front end of RF shielded bellows.Due to the difficulty of performing experiments,the simulation method is applied to evaluate the feasibility.On the basis of lattice and vacuum system layout,Synrad is used to calculate the surface power density of synchrotron radiation on absorbers.Then the calculated results from Synrad are imported to ANSYS Workbench for further thermal-static coupling analysis.After that,the temperature and stress distribution of absorbers can be acquired.For the crotch absorber,three types of structures on the exposed part were designed,including the rectangle rack,triangle rack and plane surface.Calculation results show that all three types of absorbers can meet the material criteria,so the plane surface structure is more suitable for HALF at the low energy region.The plane surface also has a simple structure,which can reduce manufacturing difficulties and costs.The photon block is located at the end of vacuum tubes,which has a great effect on coupling impedance.In order to mitigate this negative influence,the circular-elliptic cross-section of the photon block is designed.The water cooling method was chosen to take away the heat,and two types of cooling structures were designed.The first one is an annular cooling tank around the photon block,and the second one is the cooling tube shared with the vacuum tube.The calculation results indicate that the cooling tank can benefit to reducing the temperature and stress of the photon block.All the calculation results of temperature and stress are much lower than the criterion of material,so the second cooling structure is simple and better for HALF.In this study,the research and design of key components in the storage ring vacuum system provide satisfactory results and technical reserve to HALF.Furthermore,the study of these three key components brings new technical views and design considerations to other DLSR light facilities.