合肥光源储存环同时运行长短束团方案的概念设计研究

Short electron bunch has interesting application in synchrotron radiation light source, such as production of powerful coherent THz radiation, Time Resolving spectrum analysis, etc. Except for linac-based SR light source, various methods were developed for production of short electron bunch or short radiation pulse, for example, laser-slicing method, isochronous storage ring, crab cavity pair technique, etc. Although the results are very exciting, the defects are also remarkable. In the isochronous storage ring, other usual beamlines can't work well due to very low beam intensity. The photon flux of short radiation pulse is very weak in laser slicing method due to tiny electrons and low repetition frequency. In crab cavity pair scheme, short radiation pulse is only available to single or several beamlines, it is not enough for multi-user research platform. Our aim is to study production of short electron bunch globally in Hefei Light Source storage ring. First, based on available conditions of HLS storage ring, except for lowering momentum compaction factor of the ring, passive 4th harmonic cavity can be used to reduce bunch length futher in quasi-isochronous operation mode. The dependance of bunch length on momentum compaction factor, parameters of basic and harmonic RF cavities, beam intensity will be studied experimentally. Then, considering the empty long straight section of HLS storage ring, conceptual design study of simulataneous long and short bunch operation mode would be made with two additional active harmonic cavities. Compromising and combining bunch length, beam intensity and beam lifetime, main parameters of RF system will be optimized. The final beam performance can be estimated by numerical emulation software of electron storage ring, where the impedance-driven effects, non-uniform filling effects, coherent synchrotron radiation effects, etc, will be taken into consideration. According to physical requirements of conceptual design, we will design two harmonic cavities with HOM damper using electro-magnetic simulation code CST-MWS. Through theoretical analysis of longitudinal beam dynamics, numerical emulation of electron storage ring, technical design of key components-harmonic cavities, the feasibility of new simultaneous long and short bunch operation mode will be approved.

短束团在同步辐射光源中有非常重要的意义，不仅用于产生时间分辨光谱研究的短同步光脉冲，还可以产生高功率相干THz辐射。目前电子储存环中发展了多种技术用于产生短电子束团，如等时性储存环、激光切片技术、偏转腔技术等，获得了良好的效果，也暴露出明显不足。如等时性储存环中束流强度弱，与其它用户研究不兼容；激光切片技术中，短脉冲辐射光通量弱，重复频率低；偏转腔技术中，短束团只针对个别束线存在，无法满足多用户的需求。本项目将研究在合肥光源储存环上如何产生短束团。实验上，研究储存环准等时性模式下，应用现有高次谐波腔压缩电子束团长度。理论上研究储存环增加2套主动式谐波腔，实现储存环中长-短束团同时运行模式。通过数值仿真研究，确定最佳谐波腔参数，评估束流耦合阻抗、束团非均匀填充、相干同步辐射等对束流动力学的影响，准确预测储存环中长和短束团的表现。最后根据物理要求完成高次模阻尼的高次谐波腔的参数设计和数值模拟。

近些年来，关于短束团的研究得到了越来越多的关注，而得到短束团需要在电子储存环中实现束团的压缩。传统的束团压缩方法是通过减小动量压缩因子（Low-Alpha)来得到短束团，但是这种方法会使得储存环平均流强减小到微安量级，远无法满足用户需求。近几年来，一个新的压缩束团的方法在国际上得到了广泛的关注，我们可以通过加入两个超导高频腔来实现束团的压缩，并且在这种情况下，可以同时得到多种不同长度的束团，同时大幅度提高了电子储存环平均流强。然而对于“Bucket”个数不同的电子储存环，在利用新型压缩束团方法时会略有不同。本论文讨论了如何在合肥光源中实现利用两个超导高频腔来压缩束团，并且将这个计划推及到适用与所有储存环的情况。同时，我们发现，由于合肥储存环自身的加速腔低腔压低频率的特性，只用 single-cell 的结构就可以实现超导腔的设计，而其他大部分电子储存环需要 multi-cell 的结构来实现超导腔。因此合肥光源储存环可以很大程度上减小在建造超导腔过程中会遇到的高次模（HOM）问题，同时 single-cell 的加速结构可以节省很多超导腔的经费。因此合肥光源很有可能成为世界上第一个利用两个超导腔实现束团压缩的储存环。