Final Report Summary - FEMTOELEC (Innovative Femtosecond laser-plasma based electron source for studying ultrafast structural dynamics) In FEMTOELEC, we explored a new path for producing extremely short electron bunches, down to femtosecond durations (1 femtosecond=10-15 s). Such ultrashort electron beams are very useful tools for studying ultrafast dynamics in matter at the atomic scale, literally enabling to see atoms in motion. These ultrafast studies are performed in pump-probe experiment and require the electron bunch to be very short but also perfectly synchronized with a laser pulse. In our project, electrons are produced in the interaction of a very intense femtosecond laser with a plasma, in a process known as laser wakefield acceleration. In this approach, the laser is focused into a plasma and drives a very large plasma wave (the wakefield), which is able to trap electrons and accelerate them close to the speed of light in micron distances. The accelerated electron beams have femtosecond durations and are perfectly synchronized to the laser pulse (jitter-free). Therefore, they have the potential to surpass existing technology by delivering the highest temporal resolution ever achieved in atomic scale imaging. Until recently, laser wakefield acceleration relied on large-scale laser systems that delivered typically one laser pulse per second. This low repetition rate made it difficult to use the electron beams for applications. In FEMTOELEC, we proposed a new approach to laser wakefield acceleration using a high-repetition rate and small energy laser system. We succeeded in producing electron beams with a laser containing 1000 times less energy and delivering 1000 pulses per second, i.e. three orders of magnitude more than before. To do this, we developed a unique laser system that delivers the shortest laser pulses ever obtained at the multi-milliJoule level. We managed to produce laser pulses with a single optical cycle of 3.5 fs, and to use them to accelerate electrons to relativistic energies. In addition, we showed experimentally that the electron beam can be used to “image” a crystal lattice via electron diffraction and to follow its dynamics on ultrafast time scales. These first proof-of-principle experiments confirm the huge potential of laser driven electron beams for ultrafast imaging. Beyond the proof-of-concept, more work is now required to turn this electron source into a user-friendly instrument, able to deliver day-to-day operation with sub-10 fs temporal resolution. In conclusion, we showed that laser wakefield acceleration could be scaled to box-size laser systems, operating at high repetition rate and opening the way to applications in ultrafast imaging.
