| The related issues caused by secondary electron emission(SEE)and multiplication on the surface of solid materials will have a variety of adverse effects on the performance of many devices,such as particle accelerators,spacecraft,electric vacuum devices,high-power microwave devices,and pulsed power devices.The main technology to solve these issues is to modify the SEE characteristics and reduce the secondary electron yield(SEY)of the materials used in these devices.At present,the SEE modification methods of materials which have been widely studied and applied are mainly based on the surface coating treatment of various low SEY materials,as well as various surface etching treatment based on mechanical,chemical and physical methods.New SEE modification methods of material surface have wide application demand and prospect in related fields.In this dissertation,the methods and mechanism of the SEE modification on the metal materials are studied as follows:1.Development of the test device for the SEE characteristics of materialsIn order to satisfy the requirement of SEE modification of solid materials,a set of device for measuring the SEE characteristics of materials was designed and developed based on the double grid spherical secondary electron(SE)collector.The energy of the SEs can be selected by grid bias,so that a variety of SEE parameters of the material by electron impact can be measured,including total SEY,true SEY,backscattered electron yield(BEY),and SE energy spectrum distribution.The device is equipped with an in-situ sample pretreatment system including vacuum baking degassing,Ar ion beam sputtering cleaning,and residual gas analyzer.A dielectric sample surface charge compensation system consisting of a low-energy neutralizing electron gun and a Kelvin probe,as well as an in-situ sample chemical state analysis system consisting of an X-ray source and a cylindrical mirror analyzer(CMA),by which in situ Auger electron spectroscopy(AES)and X-ray photoelectron spectroscopy(XPS)analysis of the sample can be carried out,are mounted to the device.So SEE characteristics of all kinds of materials can be tested including metal,semiconductor and dielectric.2.Study on the laws and mechanism of the influence of low-energy metal ion beam irradiation on the SEE characteristics of metal materialsBased on the principle of interaction between ion beam and matter,and the principle of SEE of solid materials by electron beam impact,this dissertation proposed that low-energy heavy ion beam irradiation with energy range of tens to hundreds keV can be used to modify the SEE characteristics of material surface.The theoretical analysis shows that there are many factors that can influence the SEE characteristics of solid materials by low energy heavy ion beam irradiation,including surface morphology variation caused by surface sputtering,chemical composition variation caused by surface sputtering and ion beam implantation,irradiation defect distribution on the surface of materials by ion beam implantation.In this dissertation,two kinds of metal materials commonly used in particle accelerators,oxygen free copper and 304 stainless steel,were irradiated by pulsed metal ion beam generated by a metal ion implanter based on metal vapor vacuum arc(MEVVA)ion source,in order to investigate the laws and mechanism of the influence of low energy heavy ion beam irradiation on the SEE characteristics of the material surface.The adjusting range of accelerating voltage of the ion implanter was 20?50 kV,and the range of ion fluence was 1×1015?1×1017 ions/cm2.The clean samples were treated by Ti ion beam irradiation,and the samples under clean storage after half a year were treated by Au ion beam irradiation.It was found that the SEY of the metal materials can be significantly reduced by the irradiation of low energy metal ion beam,and the laws of the SEY variation with ion irradiation parameters were obtained by measuring the SEY of the samples with different experimental conditions.In this dissertation,the surface morphology and chemical state of the samples before and after ion beam irradiation were analyzed by means of SEM,AFM and in situ XPS.By the comparative analysis of experimental tests and simulation results including ion range,nuclear stopping power,target atom sputtering yield,depth distribution of deposited ions and irradiation defect density,it was found that the decrease of SEY of metal materials is neither caused by the surface morphology variation nor by the surface doping after ion beam implantation.The decrease of SEY for clean metal materials was caused by the irradiation defects,and the decrease of SEY for partial oxidized metal materials was contributed by the irradiation defects and surface impurity content variation.In summary,the study of this dissertation indicated that the irradiation defects within the SEs escape depth of the material surface will increase the probability of the internal SEs being captured by the defects when they are diffused to the surface,thus reducing the SEY of the material.However,the reduction of SEY is related to the ion type,energy,fluence,and material characteristics.The research results of this dissertation will provide scientific basis and technical guidance for the application of ion beam modification technology to suppress the SEE of material surfaces. |
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