Optimization Design And Experimental Study Of The Third Harmonic Superconducting Cavity For SSRF

With the development of superconducting technology,superconducting radio frequency cavities have attracted more and more attention in the field of particle accelerators,and various types of superconducting high frequency cavities have been widely used.Superconducting elliptical cavities are used in the synchrotron radiation light source,linear accelerator and circular collider different types of particle accelerators,such as Shanghai Synchrotron Radiation Facility(SSRF)and International Linear Collider(ILC),European X-ray free electron laser device(European XFEL),phase II of the linac Coherent Light Source(LCLS-II),Shanghai high repetition rate X free electron laser and extreme light facility(SHINE),Circular Electron Positron Collider(CEPC),etc.The research of superconducting elliptical cavity has great significance to the development of accelerator.Shanghai Synchrotron Radiation Facility is a medium energy third-generation light source with energy of 3.5 GeV.It has the characteristics of low emittance and high brightness,among which beam lifetime is one of the most important parameters.The main factor affecting the beam lifetime of SSRF is the beam loss caused by touschek scattering which is electron collision in the inner of the beam.With the upgrade of the SSRF,the shorter beam lifetime will become an urgent problem to be solved.To solve this problem,some synchrotron radiation sources choose to add a harmonic radio frequency system,such as the Swiss Light source(SLS)and the Italy Light source(ELETTRA),whose beam lifetime increases by 2 to 4 times respectively without affecting the beam emittance.Based on this,in the upgrade project of SSRF,it is planned to use the passive third-order harmonic superconducting elliptical cavity to stretch the length of the beam and improve its beam quality and beam lifetime.This paper study on a passive 1.5 GHz 2-cell superconducting elliptical cavity.By using the two-dimensional electromagnetic field simulation software superfish,multipacting simulation software Multipac and three-dimensional electromagnetic field simulation software CST,the electromagnetic field optimization design and thePickup design of the superconducting cavity have been accomplished,and electromagnetic goals of the third harmonic superconducting cavity are achieved.In this paper,a series of simulation methods for the study of frequency sensitivity of superconducting cavity are obtained by using the finite element analysis software ANSYS.By comparing the experimental results of 500 MHz superconducting cavity with the results of ANSYS multi-physics field coupled simulation,the feasibility of the multi-physics field coupled simulation methods are verified.In addition,the frequency sensitivity and frequency variation of the 1.5 GHz2-cell superconducting cavity with high Q value are studied,including the frequency tuning sensitivity of the cavity,helium pressure sensitivity coefficient,Lorentz detuning sensitivity analysis,frequency change during cool-down process and modal effect analysis.Based on the simulation results of the 1.5 GHz 2-cell superconducting cavity,the analysis of the mechanical resonance mode and the structural stress analysis of the cavity,the frequency control scheme of the processing of the third harmonic cavity is presented,and the structural stability of the superconducting niobium cavity without stiffness rings is confirmed.Besides,as an alternative,the optimal stiffness rings position is also given.In this paper,the fabrication and surface post-treatment of the third harmonic superconducting niobium cavity are investigated,including the deep draw of half cell and fluted beam tube,electron beam welding,surface buffer chemical polishing,etc.It has overcome the technical difficulties in the processing and manufacturing of niobium cavity,optimized the manufacturing process of niobium cavity,enhanced the combination of production and research,and promoted the technological development of the manufacturer related to the superconducting cavity.Combined with simulation and experiment,the first third harmonic superconducting niobium cavity was successfully welded,and the resonant frequency deviation was no more than 0.3 MHz,which greatly shortened the development period of the superconducting cavity.The radio frequency test of the superconducting niobium cavity shows that the high frequency parameters of the 1.5 GHz 2-cell superconducting cavity fabricated are consistent with the designed results.In summary,using electromagnetic field simulation,multi-physical field coupling simulation and experiment,we have designed and manufactured the first one1.5 GHz 2-cell superconducting cavity with fluted beam tube used for the third harmonic cavity system of SSRF.In this paper,excellent achievements have been made in cavity design optimization,multi-physics field analysis,niobium cavity machining,electron beam welding,surface post-treatment and radio frequency testing of 1.5 GHz superconducting elliptical cavity.The completion of this paper not only has practical engineering significance to the upgrade of SSRF,but also promotes the development of the superconducting elliptical cavity.