高极化度电子源先进光阴极的研究

发布时间：2018-08-17 16:15

【摘要】：砷化镓(Ga As)类光阴极已经被广泛用于为电子枪提供自旋极化电子束。为满足现代电子加速器的苛刻需求,这些光阴极必须具备以下特征:高量子效率、高电子自旋极化度和较长的工作寿命。高质量光阴极会极大地推动新型电子加速器的成功建造,为新物理现象的探索研究提供有力的工具和手段。本论文涉及几方面有关极化光阴极的研究,旨在量化和进一步理解现有砷化镓类光阴极的局限性并提出有效方法改善其性能以满足目前和未来先进电子加速器的应用需求。基于砷化镓的应变超晶格光阴极是目前唯一能够为电子加速器提供高极化度(~90%)电子束流的阴极材料,其主要缺点是量子效率低(~1%)。虽然这种光阴极已经被成功用于大型加速器(比如美国托马斯-杰斐逊国家实验室的连续电子束加速器装置CEBAF)的运行,但是它们已经不能满足新型电子加速器对强流电子束(需求1~50mA)的紧迫需求了。大幅度提高这种光阴极的量子效率已迫在眉睫。本论文研究了一种具有分布式布拉格反射器(Distributed Bragg Reflector,简称DBR)结构的应变GaAs/GaAsP超晶格光阴极,通过严格的参数设计和反复的实验优化,在保持高极化度的同时成功地实现了量子效率的突破,远远超出现有高极化度光阴极,比现有水平提高了数倍。这一突破性成果为正在预研的若干下一代电子加速器提供了重要设计参数,也将使目前国际上急需的毫安量级强流极化电子源的研制进程显著缩短。光阴极寿命是电子枪的重要品质参数之一,长寿命是电子枪提供持续稳定束流的基础。虽然人们已经意识到限制现代直流高压电子枪中光阴极寿命的主要机制之一是离子反轰效应。然而至今为止,离子反轰效应对光阴极的影响并未得到系统性地研究,人们对其中许多物理机制尚不甚清楚。本论文首次详细研究并评估了光阴极量子效率衰减(或寿命)对离子反轰效应的敏感性,并得出电子枪使用砷化镓光阴极的最佳切面选择是(110)切面的重要结论。本研究结果对电子枪研究者理解和进一步探索离子反轰效应,以及今后进一步提高电子枪工作寿命提供了重要参考。由于电子在材料内部和表面激活层内会发生去极化过程(自旋弛豫),典型非应变砷化镓光阴极产生的极化度通常情况下小于其理论极限值50%。本论文全面阐述了光阴极材料的去极化机制,并进行了系统的实验研究,对温度、掺杂浓度、激活层和晶体切向等因素对极化度的影响进行了细致的评估。其中一个重要发现是,当低掺杂浓度的光阴极样品在冷却到77 K时,可产生与理论极限值相当的极化度(52%)。本研究结果使得人们更进一步加深了对光阴极中电子损失自旋极化度的机理的理解,进而对用光阴极产生更高极化度的电子束的工作具有指导性意义。本论文还探索了一种新颖的应变超晶格结构:GaAsSb/AlGaAs光阴极,并取得了初步结果。虽然实验结果与希望值有明显差距,但本研究成果为后续发展高极化度光阴极提供了可以直接借鉴的研究基础。本论文的最后简单介绍了利用托马斯-杰斐逊国家实验室的注入器测试装置实现高能强流极化电子束的准备工作。
[Abstract]:Ga As photocathodes have been widely used to provide spin-polarized electron beams for electron guns. To meet the harsh demands of modern electron accelerators, these photocathodes must have the following characteristics: high quantum efficiency, high electron spin polarization and long working life. High quality photocathodes will greatly promote new electron accelerators. Successful construction provides a powerful tool and means for exploring new physical phenomena. This paper deals with several aspects of polarized photocathodes, aiming to quantify and further understand the limitations of existing gallium arsenide photocathodes and propose effective ways to improve their performance to meet the current and future applications of advanced electron accelerators. GaAs-based strained superlattice photocathodes are currently the only cathode material capable of providing highly polarized (~90%) electron beams for electron accelerators. The main drawback is the low quantum efficiency (~1%). The accelerator CEBAF is running, but they can no longer meet the urgent needs of the new type electron accelerator for high-current electron beams (1-50mA). It is imminent to greatly improve the quantum efficiency of this photocathode. In this paper, a distributed Bragg Reflector (DBR) structure is studied. Strained GaAs/GaAsP superlattice photocathodes have successfully achieved a breakthrough in quantum efficiency while maintaining high polarization through rigorous parameter design and repeated experimental optimization. This breakthrough is several times higher than the existing level and far beyond the presence of high polarization photocathodes. This breakthrough is being achieved in several next-generation electron accelerators under research. The photocathode lifetime is one of the most important quality parameters of the electron gun, and the long lifetime is the basis for the electron gun to provide a sustained and stable beam. One of the main mechanisms of cathode lifetime is ion bombardment. However, the effect of ion bombardment on the photocathode has not been systematically studied so far, and many of its physical mechanisms are still unclear. The results of this study provide important reference for electron gun researchers to understand and further explore the ion bombardment effect and to further improve the gun working life in the future. The depolarization process (spin relaxation) occurs and the polarization degree of typical unstrained gallium arsenide photocathodes is usually less than 50% of the theoretical limit. In this paper, the depolarization mechanism of photocathode materials is described comprehensively, and the effects of temperature, doping concentration, activation layer and crystal tangent on the polarization degree are studied experimentally. One of the important findings is that when the photocathode sample with low doping concentration is cooled to 77 K, it produces a polarization equivalent to the theoretical limit (52%). A novel strained superlattice structure, GaAsSb/AlGaAs photocathode, has been explored and preliminary results have been obtained. Although there is a clear gap between the experimental results and the expected values, the results of this study provide a direct reference for the future development of high polarization photocathodes. In the end of this paper, the preparation of high-energy and high-current polarized electron beams by using the injector testing device of Thomas-Jefferson National Laboratory is introduced.
【学位授予单位】：中国科学院大学(中国科学院近代物理研究所)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TL503.3

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