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FEMTOELEC Sintesi della relazione

Project ID: 306708
Finanziato nell'ambito di: FP7-IDEAS-ERC
Paese: France

Mid-Term Report Summary - FEMTOELEC (Innovative Femtosecond laser-plasma based electron source for studying ultrafast structural dynamics)

The field of ultrafast science has recently undergone impressive progress with the emergence of new instruments such as X-ray free electron lasers which allow scientists to probe matter on femtosecond time scale and unravel its dynamical properties. Ultrashort electron bunches are also a powerful way to probe structural dynamics via time resolved electron diffraction. In FEMTOELEC, we are developing a new technology for generating ultrashort electron bunches for probing dynamics with sub-10 fs temporal resolution. This new emerging technology is based on the use of ultraintense lasers interacting with plasmas. These laser-plasma accelerators are compact devices that can accelerate electron bunches to relativistic energies with femtosecond duration, making them a unique tool to probe the dynamics of matter on atomic time scales. However, their use for scientific and industrial applications requires improvement in the repetition rate, stability and quality of the electron beams. Hence, one of our endeavors has been to develop a compact laser-plasma accelerator operating at kHz and using modest energy (<10 mJ), kHz lasers, rather than the larger and more costly laser systems that are usually used for accelerating electrons.
In the first part of this project, we were able to develop the first high-repetition rate laser-plasma accelerator. Our experiment provides kHz electron bunches at the 100 keV level. We showed that the operation at the kHz enhances considerably the stability of the electron source. The high repetition rate and stable source allowed us to perform static transmission electron diffraction on ultrathin (<100 nm) solid samples, therefore confirming the potential of such an electron source for probing matter. Time resolved studies are currently under way. We have also shown theoretically that using shorter laser pulses (5 fs) composed of less than two optical cycles should enable the acceleration of 5 MeV electron bunches with a duration of only a few fs. We are currently working on shortening the pulse durations of our laser system in order to reach this regime and we expect to increase the electron energy from 100 keV to a few MeV.
In these experiments, we also explored some fundamental aspects of these relativistic laser-plasma interactions. We studied in details the role of the laser wavefront on the acceleration of electrons and found that it has a major impact on the angular distribution of the electrons. Our work underlines the importance of measuring/controlling the laser wavefront and provides a path for active control and optimization of the electron beam quality in laser-plasma accelerators. In addition, we explored the complex physics of nonlinear effects occurring during the interaction and showed that the laser pulses can be shortened during their interaction with the ionizing gas, providing a path for the post-compression of high intensity laser pulses. The understanding of these fundamental aspects is very important for developing and optimizing this new plasma-based technology.
We also investigated the interaction of ultra-intense laser pulses with solid surfaces and gained new insights on the fundamental physics of these processes. We showed that under the right conditions, the plasma surface can be used to inject electrons into the optical waveforms. Electrons are then accelerated to high energies (10 MeV) by surfing along the optical cycles in a process known as “Vacuum Laser Acceleration”. In this process, the energy flows directly from the photons to the electrons in a very efficient manner and provides a train of sub-femtosecond electron bunches. This research is opening fascinating perspectives for the direct acceleration of electrons by laser fields as engineering and tailoring of the laser field should provide a direct control over the electron beam, providing a new path for the generation of ultrashort electron bunches.

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