非常规电子分布及相关束流不稳定性和非线性动力学的研究

The distribution of the electrons in the phase space determines the emittance of the electron beam in a storage ring，while the beam emittance impacts the brightness of the synchrotron radiation light source based upon the storage ring. Trapped by the focusing magnetic field in the transverse plane and the RF field in the longitudinal plane, and impacted by radiation damping and excitation, the electrons in the storage ring usually have a solid distribution in the phase space. However, impacted by beam instabilities and nonlinear effects of the magnetic field, electrons might have a non-solid distribution. For example, an anomalous hollow electron beam was found in the Duke FEL storage ring in 2003. The primary studies performed in 2003-2005 indicates that the solid beam breaks up due to some beam instability and recaptured in a new potential well at a large transverse amplitude to form a ring beam. Under this distribution, the electrons show strong nonlinear dynamics effects in their movement. The results were reported in PRL94-134802(2005). The hollow beam is an unique model system for studying the transverse instabilities, in particular, the interplay of the wakefield and the lattice nonlinearity. In addition, the hollow beam can be used as a powerful tool to study the linear and nonlinear particle dynamics in the storage ring. So, based upon these studies, we will perform more detailed and insight researches on this phenomenon. We will study the mechanism which forms and sustains the ring beam in the phase space with a new physics model. This physics model will be proved by simulations and experiments. At the same time, we will also perform studies on the single-bunch beam instabilities, and linear and non-linear dynamics of the electrons using this anomalous hollow electron beam.

电子在相空间内的分布决定着储存环的发射度，进而影响着光源的亮度。在聚焦磁场和加速场的约束及辐射阻尼和激发效应的作用下，电子在相空间内一般呈实心分布。由于束流不稳定性和磁场非线性效应的影响，电子会呈现非实心分布，如在2003-2005年间，一种新的环形分布现象在Duke FEL储存环上被发现。研究表明，该现象的成因是由于某种不稳定性的影响而造成实心分布电子束的爆发(blow up)，逃逸电子随后又被非线性势阱所捕获而成。该成果发表在PRL94-134802(2005)上。环形分布的电子束是研究束流不稳定性的一个理想模型系统，也是研究电子线性及非线性动力学的一个强有力的工具。本项目以该研究为基础，通过建立新的物理模型和理论分析来解释该现象形成和维持的物理机制，并通过模拟计算和实验研究等来检验物理模型和理论结果。在此基础上，利用非常规分布的电子束进行束流不稳定性和电子线性及非线性动力学的研究。

在本基金的支持下，我们极大地提升了合肥光源储存环的性能，包括提高了合肥光源储存环的束流轨道稳定性；优化了高次谐波腔性能，提高束流纵向运动稳定性和束流寿命；通过优化储存环磁铁电源的静态和动态特性，提高束流横向运动稳定性；补偿储存环插入件对束流动力学的影响，提高束流运动稳定性等，这些措施不仅为本课题的其他研究提供了保障，同时也保障了合肥光源的高质量运行。此外，我们研究了束流在六维相空间的特种分布，以及纵向分布与横向分布之间的耦合关系，使得对束流的分布进行操控成为可能。而且，通过研究束团内部电子分布（束团尺寸）在不同能量、不同流强和不同共振耦合下的变化，让我们清楚地认识到影响束团分布的各种效应的贡献。