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

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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.