CAEP光阴极RF腔注入器

【作者】 李正红；

【导师】 胡克松；

【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】 光阴极RF腔注入器是一种能产生高流强、低能散、低发射度电子束的高亮度电子束注入器，同热阴极RF腔注入器相比，这种注入器涉及到一些不同的物理和技术方面的问题。 在光阴极RF腔注入器中，电子束的时间结构由驱动激光控制、消除了热阴极RF腔注入器中的电子反轰效应，RF腔中加速场强较高，电子束受到的空间电荷效应得到很好的控制。同时，驱动激光器的参数、阴极材料的选择和制备工艺也直接影响该注入器电子束的品质。 本论文针对CAEP光阴极RF腔注入器的特点，从理论和实验上研究了驱动激光器、光阴极和光阴极RF腔涉及的有关物理问题。该注入器采用四倍频Nd：YAG锁模激光器驱动、半导体（Cs₂Te）为光阴极、L波段（2+0.5）腔为加速腔、工作频率1.3GHz、和KL-28速调管组成的微波系统作为功率源。现己获得很好的实验结果：束流70A，电子能量2MeV，发射度4mm.mrad，束团徽脉冲宽度10ps，微脉冲重复频率81.25MHz，宏脉冲宽度2.5us，宏脉冲重复频率为3.125Hz。 作为驱动激光，要求激光脉冲的时间抖动小、微脉冲宽度窄、脉冲功率高。通过研究锁模激光器的工作原理、皮秒激光脉冲的放大和信频过程，我们提出并采用了二极管泵浦的自锁模激光器改进原来灯泵浦锁模的驱动激光器，改进后驱动激光器的激光脉冲时间抖动小于2ps，微脉冲宽度由原来的60—80ps降到10ps，激光脉冲功率提高一个量级，满足了注入器的要求。 作为光阴极材料，要求量子效率高，工作稳定。在注入器实验中采用半导体（Cs₂Te）作为光阴极，该光阴极工作稳定、量子效率高、使用寿命长；实验中在注入器装置上制备Cs₂Te阴极，制备过程中用紫外灯监测阴极性能，而且在真空中将制备好的Cs₂Te阴极传递到RF腔中进行实验。除了半导体（Cs₂Te）阴极，我们还研究了金属（Sm、Cu等）和金属化合物（LaB₆）阴极的光电发射特性。论文给出了KrF准分子激光和四倍频Nd：YAG锁模激光器驱动下，金属（Sm、Cu、Y等）、金属化合物（LaB₆）和半导体（Cs₂Te）光阴极的实验结果、以及关于金属与金属化合物阴极在真空中提高量子效率的处理方法和有关实验结果的分析。 光阴极RF腔的场强较高，尤其是首腔（CAEP光阴极RF腔注入器的首腔轴向平均场强为26MV/m，阴极表面场强为37MV/m）。在首腔中电子从阴极表面发射后，在强场作用下，很快达到相对论状态，电子受到的空间电荷效应得到很好的控制。在研究RF腔中电子的运动中，论文将电子在RF腔中的运动分为阴极表面附近的非相对论运动和远离阴极表面后的相对论运动，由此根据RF 摘 要腔轴上轴向电场的强度（可用拟合解析式求得）、驱动激光的入射相位和脉冲宽度等条件，给出了电子的纵向运动表达式，包括电子能量、出口相位、电子束能散和束团长度等参数。在电子束的横向运动研究中，根据电子的横向运动特点和电子束均方根发射度的定义，研究了近轴条件下电子束的均方根包络半径和发射度增长。 在光阴极注人器实验中，电子束参数（发射度、能散、流强和束团纵向长度）的测量很重要。在实验中，束团长度（电子束微脉冲宽度）是根据驱动激光的微脉冲宽度来取值的，近似等于驱动激光的微脉冲宽度；电子束的发射度是根据包络方程，采用两屏法来测量的；在测量电子束流强和电子能量中，通过偏转磁场将电子束偏转输出，用法拉第筒测量记录偏转输出的束流脉冲电荷，实现电子能量和柬流脉冲强度的测量。 论文创新之处： l、针对不同金属及金属化合物材料，建立了一套系统的在真空中处理阴极的方法及理论分析，使得典型的金属或金属化合物光阴极材料的量子效率有量级的提高。 2、首次用分段处理方法，分析、研究光阴极RF腔注入器中电子的运动和加速过程，获得了输出束流参数（包括电子束能量、能散、束团纵向长度、电子束横向发射度和包络等参数）与驱动激光参数的关系。 3、提出并采用二极管泵浦的自锁模激光器哑100和时间同步器0X—1000改进原声光锁模振荡器和激光器，解决了激光脉冲同微波脉冲的相位同步控制和调节，激光脉冲宽度由原来的 60－100PS减小到 10PS、激光脉冲同标准信号之间的时间抖动达到了G矿＜Zps，满足光阴极 RF腔注入器对驱动激光器的要求。在光阴极RF腔注入器实验中，获得了：电子能量ZMeV、束流流强70A、发射度4mm．mr。d，亮度达到4X101。A／，的高亮度电子束流。更多还原

【Abstract】 As a promising electron injector, the photoinjector can generate electron beam of low emittance, low energy spread and high brightness. Compared with the thermionic-injector, the photoinjector has some inherent character such as low average current and high-gradient accelerating field in the RF cavity. So some physical and technological problems on photoinjector must be researched. The timing structure of electron beam is controlled by the driving-laser in the photoinjector. The space charge effect on the electron beam is greatly lessened by high-gradient accelerating field in the RF cavity. Meanwhile the driving-laser and the photocathode are also important factors, the output of the driving laser can directly affect the quality of electron beam. Based on the CAEP photoinjeetor, the driving laser, the photocathode and the relative physical problem on the photoinjector RF cavity are discussed in the thesis. The fourth harmonic Nd:YAG laser is used as a driving laser, semiconductor Cs2Te is used as the photocathode, and the RF cavity is L-band (2+0.5) cavities whose frequency is 1.3GHz. Now CAEP photoinjector can output very good electron beam whose parameters are: beam current 70A, electron energy 2MeV, emittance 4 mm.mrad , micropulse duration lOps, micropulse repetition 81.25MHz, macropulse duration 2.Sus and macropulse repetition 3.125Hz. Low timing jitter, low pulse-duration and high pulse-power is very important for the driving laser. Based on the principle of the laser mode-locking and the amplification of the ps laser pulse, we renovate the lamp-pumped mode-locked driving laser by using the diode-pumped and self mode-locked laser. The timing jitter of the renovated driving laser is reduced to less than 2ps, the pulse duration down to lOps, and the laser pulse power increased up one order of magnitude. The output of the driving laser now satisfies the requirement of the photoinjecter. As the photocathode material, it must has high quantum efficiency and work stable. The semiconductor Cs2Te was chosen in CAEP photoinjector due to its high quantum efficiency, enough long lifetime and stability. It was fabricated on the photoinjecter, the fabrication process was monitored by an arc UV lamp, and was transferred into the RF cavity in the vacuum when fabrication finished. Other photocathode such as Sm, Mg, Y, Cu, Ag, LaB6, were also investigated in the thesis. The experimental results are given in the thesis, including the process methods to increase the quantum efficiency and the relative theoretical analyses. The strong accelerating field is required at the surface of photocathode in RF cavity. In CAEP photoinjector RF cavity, 37M V/rn is reached at the surface of photocathode, and the emittance-compensating methods is taken. In the movement of electron beam, the RF effect dominates and the space charge effect is not so important as that in thermionic-injector. Because of the strong field at the surface of photocathode, the electron is accelerated to relativistic velocity quickly after emission. Based on the velocity, the movement of electron is defined as two part in the thesis: the non-relativistic movement near the surface of photocathode and the relativistic movement away from the photocathode. Then based on the accelerating field in the RF cavity, the incidence phase and the pulse-duration of the laser, the movement of electron is researched, which include the electron energy, energy spread and electron beam bunch length, the RMS emittance, envelop更多还原