激光尾波场加速中横向光离化注入对电子的控制

Electron injection has been a bottle-net to increase the final accelerated beam's quality. Initial injection number, transversal and longtitudinal momentum spreads are critically important to the final accelerated beam's energy spectrum and transverse emittance, which are also the critial index for their potential applications. We propose to use a three-pulse scheme to perform ionization injection in a laser plasma wakefield accelerator to reduce the transverse emittance of the beam by one order compared wtih current injection methods, and we want to actively control the transverse injection position for the future studies of X-ray source based on laser plasmaaccelerator. In our scheme, the driver pulse will partly ionize the high Z neutral gas medium and excite a wakefield. Two transvese propagating ultra short laser pulses collide in the following wakefield and form a beat wave. The electric field of the beat wave ionizes the high-order electrons of the ions inside the wake, and some of these electrons will be trapped and accelereted by the wakefield. Since the direction of the electric field inside the beat wave is along the longitudinal direction of the wake, the residual ioniation momentum will no longer in the transverse direction, which could reduce the beams's final transverse emittance by one order. Through control the position and length of the beat wave, electron's transverse injection position and radius can be actively controlled as well. These would benefit the accelerated beam's potential applications, such as X-ray radiation source, injection source for electron-position collider and free electron laser.

激光尾波场加速中的电子注入控制已成为制约电子束质量的一个关键问题。初始注入电量、能散和横向动量分布直接关系到最终束团能谱和横向发射度。而这些又是将来电子束在后续应用中的关键性指标。我们拟利用三光束方案进行尾波场中的电子离化注入控制，以期得到比现有注入方式小一个量级的横向发射度；主动控制电子束团在尾波场中的横向位置，为将来的基于激光尾波场的辐射源做准备。方案中驱动激光部分电离具有高Z的气体，并形成尾波场；两超短脉冲在后续尾波场中进行横向碰撞，形成拍频场离化尾波中离子的内壳层电子，并使之被尾波捕获，进而得到加速。由于拍频场的电场方向与驱动激光的传输方向相同，所以由于离化而产生的残余动量不再在尾波场的横向，可以大大减小注入电子的横向发射度；通过调节拍频场的位置和长度可以对电子束的横向注入位置和大小进行控制。研究对电子束的应用（如X射线辐射源、自由电子激光、正负电子对撞机注入源等）具有重要意义。

本项目以激光尾波场加速中的电子注入这一关键问题作为研究对象，通过多束激光脉冲与离化注入机制，分离尾场激发及电子注入过程；通过横向注入光脉冲改变残余离化动量的方向，实现了超低发射度（归一化发射度比立项前水平小一个量级，结果小于0.1mm mrad）且能散控制在1%左右的电子束加速；利用该机制还实现了横向注入位置的调节。项目执行期间在原定计划的基础之上，进一步开展了光离化注入的二维效应研究，发现了激光自演化导致的离化注入自截止现象，为实现高品质的离化注入提供了新的解决方案；研究了通过激光尾场加速电子束与激光汤姆逊散射过程，提出了利用尾场加速不同距离处的具有能量啁啾的电子束与高斯强度分布的激光束进行匹配，从而实现窄谱、高亮度的汤姆逊散射X射线辐射方案，使得辐射强度达到利用非匹配电子束时的5倍。通过该项目的研究，对激光尾场加速中的离化注入过程，及电子束的操控有了更深入的理解，为实现高品质、可调谐电子束及其次级X光源提供了若干新的方法，为相关实验研究提供了新的思路和解决方案。