高工作比、热阴极4+1/2腔微波电子枪研究

【作者】 杨兴繁；

【导师】 刘锡三；

【作者基本信息】 中国工程物理研究院北京研究生部 ， 核技术及应用， 2000， 博士

【摘要】 长期以来，探索新型短脉冲、高亮度电子源倍受关注。热阴极微波电子枪（RF-gun）是新一代高亮度电子源，它具有结构简单、运行可靠、造价低廉等优点，高工作比、热阴极RF-gun的开发和应用研究受到广泛重视。本论文针对高工作比RF-gun中的电子反轰和束流负载两个主要问题，从理论和实验两个方面进行了详细研究，论证了RF-gun高工作比运行的可行性。论文还对国际最新发展的微脉冲电子枪（MPG）工作过程进行了分析，并提出了实验方案。 论文首先分析了缩短首腔和第二腔腔长（就是加聚束段）对抑制反轰的作用，重点讨论了缩短首腔中的电子运动方程，发现对β_p＜1的聚束腔，必须对Kim表达式进行修正。用RUNGE-KUTTA法求出电子运动方程的数值解，给出了缩短首腔对增加电子俘获系数、减小电子反轰比较清晰的物理图象。利用反轰加热LaB₆阴极，建立了加热功率热力学平衡方程，设计了灯丝加热反馈控制方法。实现了高工作比、热阴极RF-gun的稳定运行。此外，叙述了新型微脉冲电子枪中的二次电子倍增工作原理，给出了共振特征关系图，从电子运动方程独立推导了电子自聚束公式，从空间电荷限制出发，讨论了饱和电流，并结合实验室的现有条件，进行了MPG初步设计。这种电子源充分利用了RF腔中电子的反向运动，无灯丝加热，不怕暴露大气，高工作比，长寿命，具有光阴极和热阴极的优点，避开了它们的缺点，很有吸引力。 论文用等效电路方法分析了高工作比RF-gun中的束流负载效应，把束流负载作为激励源来处理。讨论了束流负载对加速腔中电场、电子能量、加速腔谐振和失谐（detuning）的影响。给出了最佳耦合度公式，并将驻波加速器书中所得到的最佳耦合度公式进一步推导，得出与它相一致的结果，表述更简洁。给出了有束流负载时的最佳工作条件。论文还建立了微波源功率、束流强度、束流功率、加速腔壁耗、腔中电场强度、有效分路阻抗、无载耦合度等参数之间的用解析式表达的自治方程，并用该方程计算了4+1/2腔RF-gun的工作状态，与用PARMELA程序计算的结果基本相符，对RF-gun调试与状态分析有一定的指导意义。 论文介绍了高工作比4+1/2加速腔的物理设计与束流动力学模拟计算结果。 中 国 工程物理研究院叙述了腔的设计思想，用SUPEnyISH计算了腔的尺寸和主要特性参数，用PA删ELA程序进行了束流动力学分析，得出了不同首腔长度、实心阴极和空心阴极条件下的电子反轰情况和束斑、束径向尺寸、能谱、能量随相位分布等曲线。 论文最后是高工作比、热阴极4刊a腔y呕un的实验研究，给出了冷测和热测主要参数。调束实验中我们对实验系统进行了许多大的改进，以满足高平均功率要求，主要是：提高微波源输出功率；改进了阴极加热灯丝结构；提高出束时腔内抽真空能力；增加陶瓷窗和法拉第筒的水冷措施等，在此基础上，新型4刊Q腔RF唱tm顺利出柬，在进腔微波功率3MW时，得到：宏脉冲束流强度400InA叫，脉冲宽度印，工作比5X10＼IX10”‘，实验结果与理论计算相符。系统工作稳定性和进一步实验正在进行中。更多还原

【Abstract】 The development of high-current ,short-pulses of electron bunches has been a challenging project for many years. The thermionic cathode RF-gun is a new kind of high brightness electron source. Its advantages are simple construction, operating reliablility and low cost. Because of these merits, the thermionic RF-guns have been widely used , such as the injector of linac and FEL experiments. Especially, the thermionic RF-gun with high duty factor(DF) have potential applications in the industry. This research effort is very attractive ,although there will be some difficulties during the progress. Two key physics issues pertinent to the development of high DF RF-gun ,they are the effects of the electron back bombardment and the beam loading. They have been studied in detail from the aspects of theory and experiments in this dissertation. The purpose of these studies is to demonstrate the possibility of the RE-gun operation with high DF. Recently , a novel concept of RF- gun micropulse electron gun is being developed in the USA. A tracing study idea is also presented in the dissertation. First of all, the suppression of electron back bombardment by shortening the first and the second cells has been analysed . The essence of shortening cavity length is to add the buncher section in the starting of RF-gun. I discovered Kim equation must be modified. The study was focused on the electron motion equation in the case 3,<l .The numerical solution of the electron motion equations have been obtained using the RUNGE-KUTTA method and the results indicate clearly that the electron back bombardment can be reduced and that the electron capture coefficient can be increased. The way to use the power of electron back bombardment to heating the cathode of the electron gun and the thermodynamics balance equation of the cathode heating power is discussed. In order to operate the RE-gun steadily, the auto- controlling negative feedback method is designed. Besides, the experiment project of MPG is proposed according to the conditions of our lab. Micro-bunches are produced by resonantly amplifying current of secondary electron in RE cavity. The phase self-bunching and the saturation current are deduced individually and discussed. The multipactor operating chart is given. The MPG is one kind of excellent RE-gun. It has brilliant future. The effects of beam loading have been studied by using the equivalent circuit. In this method ,the beam loading has been regarded as another driven source. The optimum coupling coefficient with beam loading is obtained. The detuning and the optimum operation condition are analysed .A consistent equation of the relation among the operating parameters is created. The analysing results of the 4+1/2 cells RE-gun using this equation are consistent with the results of the PARMELA program. The thermionic cathode 4+1/2 cells RE-gun with high DF is analysed .The shape size and cavity operating parameters of this RE-gun and beam dynamics calculation are given out using SUPERFISH and PARMELA programs. Finally, the cold test and powered experiments of this gun are carried out .Thecommissioning experiments are conducted in the D4064 klystron microwave testing bed. In order to meet the requirement of the commissioning , we have improved the experiment system and upgraded microwave output power. The commissioning of this new high DF 4+1/2 cells RE-gun is successful , we obtain experiment parameters are: the macropulse beam current is: 400mA A, the macropulse width is 5s, the DF is 5x1OxlO3.These results are consistent with those pr更多还原