The development of intense X-ray sources of femtosecond (1 fs = 10-15 s) or attosecond (1 as = 10-18 s) pulse duration is one of the main research objective in many scientific fields. In addition to the study of the sources itselves (high-energy physics, accelerators, synchrotrons and large instruments free electron laser, laser-produced plasma physics), it provides access to entirely new fields of research:
In this environment, the development of X-ray sources from laser-plasma interaction plays a leading role and offers unique radiation properties. Their potential compactness, their perfect synchronization with a laser or intense light sources or any particle generated by the laser, their extreme short pulse duration (to zeptoseconde, 1 zs = 10-21 s) and the sub-micrometer source size all provide unique opportunities for the development of innovative applications and dissemination in academic and societal areas.
The physics of these sources is very rich. It is based on the manipulation of free or bound plasma electrons with the laser and plasma electric fields: dynamics and recombination on the parent atom (harmonic generation in gases), electron ionization and population inversion in the ion energy levels atomic systems (X-ray laser), reflection of the laser on relativistic oscillating plasma electrons (harmonic generation from solids), trapping of electrons in ion cavity (laser synchrotron), relativistic oscillation of free electrons in laser electromagnetic field (Thomson and nonlinear Compton scattering), wave breaking of electron plasma waves (Ka radiation).
Principle of the betatron X-ray source (laser-based synchrotron). Electrons trapped in the ion cavity lying in the wake of the laser that propagates in a gas, perform oscillations due to electrostatic forces generated by charge separation. These oscillations generate broadband radiation in the X-ray spectral domain since the period of oscillations is about a hundred micrometers for electron energies of several hundreds of MeV.
The LOA has a strong leadership at an international level on the development of several sources. Three research groups are involved in this area: FLEX, SPL and PCO. We have recently performed pioneering works in the demonstration of new X-ray sources produced by laser-gas interaction in the relativistic regime like the micro-synchrotron (betatron radiation). The LOA also demonstrated for the first time the first seeded plasma XUV laser. "Self-compression" by filamentation in a gas of laser pulses in the infrared has been demonstrated up to 3 optical cycles to fully exploit the favorable scaling laws governing the generation of harmonics in gases (generation of isolated attosecond pulses in ultrashort short wavelength, for example).
Spectrum of the XUV laser radiation (left) used to seed a plasma XUV laser. The spectrum of the XUV line amplified with this technique is shown on the right part of the figure.