Control of electron injection in laser wakefield acceleration for high quality electron beams

等离子体驱动的激光韦克菲尔德加速（LWFA）具有极高的加速梯度和产生超短电子束团和低至阿秒的X射线辐射的潜力，是一种非常有吸引力的加速技术。目前，极高的加速梯度已经得到了很好的证明，例如，通过在小于10厘米的加速距离内产生多GeV的电子束。然而，包括束流能量扩展和发射度在内的束流质量仍需要改进，并且需要进一步探索超短电子聚束和X射线辐射的潜力，直到阿秒持续时间。为了产生高质量和高稳定性的电子束，电子注入加速桶的控制是关键。这仍然是LWFA的一个巨大挑战。许多计划的电子注入已被提出，其中一些也进行了实验测试，如电离注入，注入等离子体密度梯度，碰撞激光注入等的关键，以减少光束能量扩散是限制电子注入到一个短的距离长度所建议的以前的调查在这方面。在这个博士项目中，学生将通过定制激光和等离子体分布来研究基于不同注入方案组合的新方案。例如，电离注入和密度上升注入的组合可以显著地减小注入长度。使用圆偏振激光和纵向DC磁场/等离子体通道有助于调谐激光群速度，这最终可以调谐加速过程。所提出的新方案应该是鲁棒的，并且相对容易在实验上实现。目标是实现电子束能量扩散小于1%，电荷超过30 pC和/或聚束持续时间小于1fs。新的方案将基于理论分析和三维PIC模拟。

The laser wakefield acceleration (LWFA) driven in plasma is highly attractive for its extremely high acceleration gradients and the potential to produce ultrashort electron bunches and X-ray radiation down to attosecond duration. Presently, the extremely high acceleration gradients have been well-demonstrated, for example, by producing multi-GeV electron beams in an accelerating distance of less than 10cm. However, the beam quality including the beam energy spread and emittance still needs to be improved and the potential to ultrashort electron bunches and X-ray radiation down to attosecond duration needs to be further explored. To produce electron beams with high quality and high stability, the control of electron injection into the acceleration buckets is critical. This remains a great challenge in LWFA. Many schemes of electron injection have been proposed and some of them have also been tested experimentally, such as ionisation injection, injection by use of plasma density gradients, and colliding laser injection etc. The key to reduce the beam energy spread is to confine the electron injection to a short distance length as suggested by previous investigations in this area. In this PhD project, the student will study new schemes based upon the combination of different injection schemes by tailoring both the laser and plasma distributions. For example, the combination of the ionisation injection and density upramp injection may considerably reduce the injection length. The use of circularly-polarised laser and a longitudinal DC magnetic field/plasma channel helps to tune the laser group velocity, which may eventually tune the acceleration process. The proposed new schemes should be robust and relatively easy to implement experimentally. The aim is to achieve electron beam energy spread less than 1% with charge over 30pC and/or bunch duration less than 1fs. The new schemes will be based upon theoretical analysis and demonstrated by 3D PIC simulations.