自由电子激光装置直线加速器中微束团不稳定性的理论及实验研究

The microbunching instability is an important factor that has strong effects on the beam quality in the LINAC of an X-ray FEL facility, i.e.，the 4th generator light source.This issue must be solved in the LINAC before it reaches the serious level. In the case that the instability gain is out of control, the beam quality will be damaged， in such a way that the the whole FEL faclity would not work properly.As of today,there is almost no systematic theoretical and experimental studies on this topic all around the world. On the other hand,the theoretical study only focuses on the Linear theory of the longitudinal space charge (LSC) and long beam approximation, and it is not enough.Therefore in this proposal, we plan to study the microbunching instability in the aspects of both theory and experiment thoroughly and systematically. ..In this study, the density modulation of the beam bunch will be produced in the way of laser pulse stacking, which is an innovative method; and the idea of plasma oscillation will also be introduced in the development of the instability model.The project will be based on the Linac of Shanghai Deep Ultra-Voilet (SDUV) facility at Shanghai Institute of Appiled Physics (SINAP), Chinese Academy of Sciences.The purpose of the study is to deeply investigate the mechanism and the measurement methods of the instability,to furtherly discover the essential physics behind it and the main factors to produce it; and to build the solid basis for the success and the optimization of the future X-ray FEL facility.

微束团不稳定性是X射线自由电子激光装置即第四代光源中影响束流品质的一个重要因素，是主加速器中必须解决的难点。如果其增益不能得到有效控制，将对束流品质有很大的危害，会导致后续辐射段激光的质量变差甚至无法出光。迄今为止世界上还几乎未曾为此做过系统与深入的理论与实验研究；对于作为主要因素的纵向空间电荷，也仅限于通常的线性理论与长束团假设。此项申请计划通过理论探索和实验测试，在实验上采用激光脉冲堆积的办法产生束团的密度调制，以及在理论上引入等离子体震荡的概念，基于中科院上海应用物理研究所已有的深紫外自由电子激光（SDUV）直线加速器装置，深入探讨微束团不稳定性的机理与实测方法，进一步揭示其物理本质及主要影响因素。为将来X射线自由电子激光装置的顺利实现和优化打下基础。

微束团（微聚束）不稳定性是X射线自由电子激光装置即第四代光源中影响束流品质的一个重要因素，是直线加速器中必须解决的难点。如果其增益不能得到有效控制，将会对束流品质带来很大的危害，导致后续辐射段激光的质量变差甚至无法出光。 本项目基于中国科学院上海应用物理研究所深紫外自由电子激光（SDUV）装置，通过理论分析和实验测试，采用激光脉冲堆积的办法产生束团的密度调制并通过偏转腔来观察微聚束不稳定性的形成与增长，探索微束团不稳定性的成因与实测方法，进一步揭示其物理规律并研究控制方案。通过本课题的研究，建立了基于横向梯度波荡器的微聚束不稳定性控制方案，得到了国内外同行的高度认可；并且在实验上观测到了微聚束增长。此外，在本项目的实施过程中，对激光的远距离传输与紫外脉冲堆积以及偏转腔的控制与使用方面学习获得了大量的经验与技术，为未来我国更先进的新一代X射线自由电子激光装置的顺利实现和优化打下基础。