Lossless generation of pulse trains to drive high quality LWFA

High energy and high quality electron bunches are needed for advanced all-optical X/gamma-ray secondary sources. In this context, schemes relying on the decoupling of the ionization process from the plasma wave excitation are highly desirable and actively pursued. Efficient plasma waves can be resonantly excited using trains of ultrashort pulses instead of a single pulse. This is exploited, for instance, in the recently proposed ReMPI scheme, which holds the promise for producing remarkably high quality bunches, while keeping at the same time a reasonable complexity. The generation of ultrashort pulse trains is not so straightforward; over the past few years, not so many schemes have been proposed and, possibly, experimentally studied. Most often, these schemes lead to the loss of a sizeable fraction of the available laser energy; this is of a particular concern for the design of high rep rate (tentatively accessing the 100Hz level) systems aimed to drive secondary sources, such as, for instance, the lasers currently under design for the envisioned EuPRAXIA facility. Here we report on the study of novel optical schemes leading to the generation of ultrashort pulse trains with negligible energy losses. Two different schemes will be presented, based on the splitting of the original pulse at different levels of the laser transport and focusing chain. Results from both numerical simulations and experimental tests will be shown, and the perspectives for scaling to the pulse requirements of large scale facilities discussed.