Study Of The High Power Coupler For The Superconducting Cavity Of Hefei Advanced Light Facility

Hefei Advanced Light Facility(HALF)is a fourth-generation synchrotron radiation light source based on diffraction-limited storage ring.Superconducting cavity will be used to provide energy for the beam.The superconducting cavity module is mainly composed of superconducting cavity,high power coupler,high order mode damper and cryostat,etc.The high power coupler plays important roles in the superconducting cavity module,such as feeding power,isolating vacuum and temperature transition.In this dissertation,the design researching and manufacturing of the high power coupler of HALF 499.8 MHz superconducting cavity was presented.Based on the design of the high power coupler of the KEK-B 509 MHz superconducting cavity and BEPCII 500 MHz superconducting cavity,the high power coupler of HALF 499.8 MHz superconducting cavity is optimized.The choke structure of the ceramic window is optimized to get good microwave transmission characteristics,and to reduce the maximum electric field and the asymmetric distribution of the electric field in the ceramic.In this thesis,the microwave transmission performance deterioration caused by machining error is analyzed and the error compensation method is proposed.Multipacting(MP)is a phenomenon of resonant secondary emission multiplication which can cause breakdown in high RF power components,thus affecting the stable operation of superconducting RF system.The accurate and efficient analysis of multipacting effect is a major challenge in the design process of the high power coupler.In this dissertation,the characteristics of multipacting effect on the high power coupler has been analyzed.The simulation results indicate that MP occurs near the ceramic when the power level is greater than 150 kW.The MP suppression effect of different DC biasing voltages is investigated,the simulation results shows that MP can be well suppressed in the power range of 1 kW to 250 kW when the DC biasing voltage is 3 kV.Specially,the delay phenomenon of the MP process is studied and the physical mechanism of this phenomenon is explained.Moreover,a new MP identification method has been proposed,which can be used to identify MP efficiently and accurately.Thermal analysis of the high power coupler has been presented in this thesis.The computational fluid dynamics(CFD)method and the correlation equations method for dealing with the problem of convective heat transfer between fluid and coupler are studied.The CFD method directly solves the flow state of the fluid,while the correlation equations method regards the heat transfer surface between fluid and coupler as an ideal boundary surface with a specific convective heat transfer coefficient,which can be calculated by the correlation equations.A set of correlation equations which can be applied for the convective heat transfer in annular pipes in the high power coupler are found.These correlation equations are the theoretical model of convective heat transfer in the annular pipe based on the experimental data.The calculation time for the thermal analysis of the coupler can be greatly reduced by using this model.The radiation heat transfer between the cold part inner and outer conductors is considered.According to the thermal analysis results,the cooling scheme of the coupler and the heat load to the cryogenic system are optimized.In addition,the process scheme and the manufacturing of the high power coupler is introduced in this dissertation.The problems encountered in the manufacturing process and the solutions of the problems are also described.