光阴极微波电子枪同轴型腔体物理与实验研究

The photo-cathode microwave electron gun is the critical component to develop the ultra-high brightness free electron laser in future. The side coupling hole of the traditional photo-cathode microwave electron gun input coupler destroys the rotational symmetry of the cavity, forms an asymmetric field and an asymmetric RF multipole field within the cavity, resulting in the increase of the emittance and the decrease of the beam quality. At present, there are many kinds of schemes in the world, but there are some shortcomings such as limited layout of downstream components, complex structure, difficult processing and tuning. In order to solve the problem of asymmetric field and overcome the above shortcomings, we propose a coaxial-type cavity structure scheme: in the first cavity cathode end plate to increase the coaxial cavity structure to form a new cavity as the cavity of the first cavity and move the coupling hole to the lateral end of the coaxial cavity to maintain the rotational symmetry of the cavity. In this project, we simulate and experimentally study the microwave characteristics and beam dynamics of five kinds of cavity structures (L, S, C, X, THz) to reveal the regularity of RF field symmetry in the new cavity and clarify the impact of RF cavity on the beam quality. It has important theoretical significance and engineering application value, which lays the foundation for the development of high-brightness photo-cathode microwave electron gun.

光阴极微波电子枪是未来发展超高亮度自由电子激光的关键部件。传统光阴极微波电子枪输入耦合器的边耦合孔破坏腔体旋转对称性，在腔体内部形成非对称场和激励非对称性RF多极场，导致发射度增大，造成束流品质降低。国际上有多种解决方案，但是存在下游元件布局受限制、结构复杂、不易加工和调试等缺点。为了解决非对称场的问题和克服上述缺点，我们提出同轴型腔体结构方案：在首腔光阴极端板处增加同轴腔结构，形成新的谐振腔作为腔体的首腔；将耦合孔移至同轴腔的侧边末端，保持腔体的旋转对称性。本课题通过5个波段(L、S、C、X、THz)该类型腔体结构的微波特性与束流动力学的仿真及实验研究，揭示不同波段相对应新型腔体中RF场对称性的规律，阐明新型腔体RF场对束流品质的影响规律，拟解决影响束流品质众多因素中RF场非对称性的问题，为高亮度光阴极微波电子枪的研制奠定基础，具有重要的理论意义和工程应用价值。

随着第四代先进光源、自由电子激光的发展，对电子源的要越来越高，特别是低发射度束流，成功研制高亮度电子枪是高亮度自由电子激光的关键。传统1.6cell光阴极电子枪是目前国际上主流用于自由电子激光的电子枪，但是其腔体结构的非对称性造成发射度增长，特别是对超低发射度束流影响较大。为了解决RF场非对称性问题，本项目基于传统1.6CELL光阴极电子枪，提出一种新型同轴结构，保持腔体结构的旋转对称性，即保持腔体内部RF的对称性。本项目主要研究不同波段(L、S、C等)1.6CELL光阴极微波电子枪对称性，包含优化设计不同波段腔体结构及束流动力学，优化设计不同波段腔体的耦合器，不同波段腔体的加工调谐及测试。研究结果表明，本项目提出的新型腔体结构，其非对称性由传统单馈的0.36%降低到0.01% ，基本接近于理想旋转对称性。因此，对于提高未来高品质束流源的品质至关重要，采用该结构几乎可忽略RF场非对称性对束流发射度的影响。