Theoretical investigation of external injection schemes for laser wakefield acceleration

This thesis reports on laser wakefield acceleration, a radically new approach for particle acceleration that builds on the huge electric fields that a plasma wave can provide. In this approach, an ultra-short laser pulse of high intensity is sent through a plasma. At sufficient intensity, the radiation pressure of the pulse expels a significant number of plasma electrons from the beam path while the ions remain at an almost fixed position due to their higher mass. This leads to a traveling charge separation wave. Associated with this wave is a traveling electric field distribution called the laser wakefield, that provides huge field strengths of hundreds of GV/m, which offer the potential to accelerate particles thousands of times more quickly than that which is currently possible with conventional accelerators.\ud \ud However, current experiments demonstrating the basic working of laser wakefield acceleration suffer from a fundamental lack of control, large shot-to-shot fluctuations and poor scalability. This is due to the fact that in all of the current schemes the injection of electrons and their subsequent acceleration is intrinsically coupled, because they are based on nonlinear dynamics such as wavebreaking in order to inject electrons from the plasma itself. This is why there is now a growing perception that electron bunches need to be injected from a separate external accelerator, so that the injection can be controlled.\ud \ud This thesis provides a theoretical investigation into the injection of electron bunches from an external accelerator. We investigate three methods in which the relatively long bunches from rf accelerators can be injected into a wakefield in such a way that the accelerated bunches attain a high quality. These schemes differ in their timing and thus in the position of the injected bunch with regard to the laser pulse. We investigated bunch injection behind the drive laser pulse, at an angle with the laser's propagation direction, and in front of the pulse. The final part of the thesis describes novel, ultrafast dynamics in laser wakefield accelerators. Previous experimental observations and theoretical investigations have shown that electron bunches can be generated with durations as short as a few femtoseconds. However, we predict that wakefield acceleration can generate significantly shorter bunches, with durations in the attosecond range.\u