laser-plasma acceleration of electrons

The research on laser-plasma acceleration of electrons followed two main paths, aiming at improving the beam properties, and coupling it with an undulator to generate Free-Electron-Laser (FEL) radiation, respectively.

We first worked on the injection of electrons into the accelerator. This is a critical step, since most of the final properties of the beam are determined during it.  We notably demonstrated in simulation a new injection mechanism which allows to produce electron beams gathering a high-charge and a high beam-quality, well beyond the state of the art. On the experimental side, we studied thoroughly two injection mechanisms, ‘density transition injection’ and ‘ionization injection’. We then proposed and demonstrated a new injection technique which gathers the advantages of the two previous ones, that is a good overall beam quality and a high stability. From these experiments we also gained experience in the generation of sharp density transitions. We use it, with another setup to increase the beam energy, using the rephasing technique which consists in pushing back the electron beam to an accelerating region when it reaches a decelerating one. Our experiment was the first demonstration of this technique. It led to a 60% increase of the beam energy. Finally, we introduced the principle of the laser-plasma lens and showed that this device can be used to reduce the electron beam divergence by a factor of almost three. This last result is of particular importance for applications requiring beam transport; the divergence reduction should actually be sufficient to avoid transverse emittance growth in quadrupoles triplet, for 3% energy spread electron beams (the emittance growth is due to the combination of large divergence and energy spread).

This last study was mainly driven by a groundbreaking application: the development of a FEL based on a laser-plasma accelerator. Such an achievement will allow to drastically downsize these facilities, and hence to open their access to a wider community. The transport and the shaping of the electron beam was demonstrated in 2018 and we succeeded in producing high quality synchrotron radiation.

The main perspectives on electron acceleration experiments would be driven by the starting of the Apollon multi-PW facility in 2019. For unlocking the full potential of this facility, we are developing in Salle Jaune an all-optical plasma waveguide. It will be used to guide the laser beam over several centimeters which is mandatory to get multi-GeV electron beam energies. We will also develop new injection techniques which are compatible with wave-guiding and PW system. The implementation of these devices on the Apollon facility will open the way to well-controlled QED experiments, such as the creation of pair through photon-gamma or gamma-gamma collisions, which is a major challenge for the laser-plasma community and beyond.