Controlled injection for Multi-pulse laser wakefield accelerators

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Context, aims and objectivesLaser-driven plasma accelerators can already accelerate electrons (and, in principle, positrons) to multi-GeV energies in accelerator stages only a few centimetres long. However, several issues must be overcome before these systems can realize their enormous promise: (i) the shot-to-shot jitter of the bunch parameters is too high for many applications; (ii) the lasers used to drive the accelerator operate at only a few pulses per second, which is too low for applications such as driving compact light sources; and (iii) the driving lasers are very inefficient (below 0.1%), which will be an issue for long-term applications such as driving particle colliders. In an effort to address the low repetition rate and efficiency of the driving lasers used at present, our group has proposed that a train of low-energy laser pulses could be used to drive the plasma wave, rather than a single high energy pulse. If the pulses in the train are spaced by the plasma period then the wakefields add coherently to produce a plasma wave with an amplitude which grows towards the back of the pulse train. This approach, known, as multi-pulse laser wakefield acceleration (MP-LWFA), could enable novel lasers to be employed which cannot easily generate high energy pulses, but which can generate few-milijoule laser pulses at mulit-kilohertz pulse repetition rates, with high efficiency. The MP-LWFA approach therefore offers a route to achieving compact accelerators able to generate GeV-scale electron bunches at high repetition rates, and in turn a route to a new generation of compact light sources. In order to address the issues of high shot-to-jot jitter of the bunch parameters it will be necessary to develop strategies for controlling the injection and trapping of electrons into the plasma wakefield. This is especially challenging for the linear, or quasi-linear, plasma wakefields driven by a MP-LWFA. The objectives of this work are to develop methods for controlling electron injection into the quasi-linear wakefields produced by MP-LWFAs. Methodology: Two methods for controlling injection will be explored: (i) ionization injection using additional laser pulses; and (ii) injection at plasma density ramps. These possible methods will be explored both experimentally and through numerical simulations. The experimental work will be undertaken in our labs in Oxford and at national laser facilities such as the Central Laser Facility at Rutherford Appleton Laboratory. Numerical simulations will be undertaken using particle-in-cell (PIC) codes run on national and local computer cluster facilities.