| Particle accelerator is a kind of manual equipment which produces high energy charged particle beam.Its appearance not only promotes the progress of physics,biology,chemistry and materials and other basic subjects,but also could promotes the development of national economy.As scientists further explore the existence of elementary particles and the interactions between them,particle accelerators with higher energy and brighter brightness must be needed.However,the excellent performance requirements bring a series of serious challenges,such as ultra-strong electromagnetic fields,limited structural space and expensive construction costs.In the design and construction of the new generation of particle accelerators,researchers have proposed the use of cryogenic superconducting components to meet the physical design objectives.For example,superconducting magnets can support ultra-strong magnetic fields in limited structural space,cryogenic vacuum chamber can provide a reliable and stable operation environment for beam movement and superconducting radio frequency cavities can provide higher acceleration voltage gradient.The application of cryogenic superconducting technologies can reduce the huge construction and operation costs.Researchers have found that electron cloud effects in cryogenic vacuum chamber severely limit the performance of positively charged particle accelerators and negatively charged particle accelerators with high intensity,such as increasing beam instability,increasing dynamic pressure,and limiting beam lifetime.In addition,the multipacting phenomenon in superconducting radio frequency cavity suppresses the increase of acceleration voltage,augments the thermal load of the cryogenic system,even leads to the quench of superconducting radio frequency cavity.The secondary electron yield(SEY)which represents the ability of the material surface to emit secondary electrons,is a key influence on the occurrence of electron cloud effect and multipacting phenomenon.Over the years,the SEYs of metallic materials have been extensively studied by domestic and foreign researchers.The SEY is influenced by the intrinsic material properties,surface microstructure,surface chemical composition and temperature environment.Currently,the study on the secondary electron emission(SEE)properties of materials in cryogenic environments is still relatively scarce.It is easy to physically adsorb the residual gas layer on the material surface in cryogenic environment,and the influence of adsorption layer on the SEE properties of material surface is not negligible.Therefore,the study of the cryogenic SEE characteristics of the superconducting accelerator structure materials has become a very important topic.In this dissertation,cryogenic SEE characteristics of structural material surface have been studied based on the material types,processing technologies and application environment of cryogenic vacuum chamber and superconducting radio frequency cavity,the work contents and experimental results are as follows:1)The author independently developed a cold helium cycle cryogenic system and transformed the existing normal temperature SEY test system into a cryogenic SEY test system.The work content mainly included refrigerating capacity calculation,structural design,device selection,installation and commissioning of the cold helium cycle cryogenic system.2)The SEY of the vacuum chamber structure materials(oxygen-free copper,stainless steel and aluminum alloy)were tested at different temperatures.Based on the principle of physical adsorption,the effect of residual gas adsorption layer on the SEE characteristics was investigated.Oxygen-free copper samples were ablated by a laser etching system with different etching parameters.Combining advanced material characterization tools,the surface microstructure and chemical state of the samples with different laser etching parameters were analyzed.The laser etching process parameters were optimized to obtain an etched surface with a low cryogenic SEY maximum of 0.93 when the laser energy was 3.7 W,the scanning speed was 25 mm/s,and the etching pitch was 20 μm,and a pattern of parallel straight lines was etched on the surface of the oxygen-free copper sample by raster-type scanning.The surfaces of samples of oxygen-free copper,stainless steel and aluminum were continuously bombarded by electron gun.The experimental results showed that the cryogenic SEY decreased with increasing electron bombardment dose because electron bombardment induced dissociation and recombination of carbon-containing material on the sample surface to produce graphite-like material with low SEY.3)The influences of buffering chemical polishing and annealing treatment on the cryogenic SEY of Nb block were studied,the microscopic morphology of the Nb block was analyzed by scanning electron microscopy,the average roughness of the Nb block was quantified by atomic force microscopy,and the chemical state of the Nb block was analyzed by X-ray photoelectron spectroscopy.The experimental results showed that the surface of the Nb block treated with buffered chemical polishing and annealing has better smoothness and cleanliness,and its maximum value of cryogenic SEY 1.64,which is 0.12 lower than that of the untreated Nb block.4)Nb film and Nb3Sn film were deposited on oxygen-free copper substrates by the magnetron sputtering coating system.The dense copper-based Nb film with low cryogenic SEY were prepared by adjusting the coating process parameters such as deposition time,sputtering current and working pressure.The cryogenic SEE characteristics of Nb block,Nb film and Nb3Sn film were compared and analyzed.The experimental results showed that the surface of Nb film was covered with uniform grains and there were gaps between the grains.These microscopic structures could capture the generated secondary electrons as well as adsorb the residual gas more easily,thus they had lower cryogenic SEY.The cryogenic SEY of Nb3Sn film was larger than that of Nb film at the whole energy range of primary electron.The SEY is one of the key parameters to evaluate the performance of structural materials for cryogenic superconducting components of new generation particle accelerators.In this dissertation,the research temperature of SEE on material surface has been expanded from normal temperature to cryogenic temperature,the relationship between the surface processing process of cryogenic vacuum chamber and superconducting radio frequency cavity and the cryogenic SEE characteristics has been investigated,and the coating process of superconducting material film with low SEY has been explored.The related research results not only provide a strong theoretical basis and important reference value for the physical design and material selection of cryogenic superconducting components,but also promote the domestic development of surface technology processing process and thin-film superconducting radio frequency cavity. |
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