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

等离子体中的激光尾迹加速(LWFA)由于其极高的加速梯度和产生超短电子束和低至阿秒量级的X射线辐射的潜力而受到极大的关注。目前，极高的加速梯度已经得到了很好的证明，例如，在不到10厘米的加速距离内产生了多GeV电子束。然而，包括束流能散和发射度在内的束流质量仍然需要改进，超短电子束和X射线辐射持续到阿秒的潜力还需要进一步探索。为了产生高质量和高稳定性的电子束，电子注入到加速桶中的控制是至关重要的。这在LWFA仍然是一个巨大的挑战。人们提出了许多电子注入方案，其中一些也得到了实验验证，如电离注入、利用等离子体密度梯度的注入、碰撞激光注入等。减小束流能散的关键是将电子注入限制在短距离内，这一领域的研究表明。在这个博士项目中，学生将通过调整激光和等离子体的分布来研究基于不同注入方案的组合的新方案。例如，电离注入和密度上升注入的组合可以显著缩短注入长度。圆偏振激光和纵向直流磁场/等离子体通道的使用有助于调整激光群速度，这最终可能调整加速过程。拟议的新方案应该是稳健的，并且相对容易在实验中实施。其目标是在电荷超过30pc和/或束团持续时间小于1fs的情况下，实现电子束能量分散小于1%。新方案将建立在理论分析的基础上，并通过3DPIC模拟进行验证。

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.