Relativistic interaction of short-pulse lasers with underdense plasmas has recently led to the emergence of a novel generation of femtosecond x-ray sources. Based on radiation from electrons accelerated in plasma, these sources have the common properties to be compact and to deliver collimated, incoherent and femtosecond radiation. In this article we review, within a unified formalism, the betatron radiation of trapped and accelerated electrons in the so-called bubble regime, the synchrotron radiation of laser-accelerated electrons in usual meter-scale undulators, the nonlinear Thomson scattering from relativistic electrons oscillating in an intense laser field, and the Thomson backscattered radiation of a laser beam by laser-accelerated electrons. The underlying physics is presented using ideal models, the relevant parameters are defined, and analytical expressions providing the features of the sources are given. Numerical simulations and a summary of recent experimental results on the different mechanisms are also presented. Each section ends with the foreseen development of each scheme. Finally, one of the most promising applications of laser-plasma accelerators is discussed: the realization of a compact free-electron laser in the x-ray range of the spectrum. In the conclusion, the relevant parameters characterizing each sources are summarized. Considering typical laser-plasma interaction parameters obtained with currently available lasers, examples of the source features are given. The sources are then compared to each other in order to define their field of applications.Comment: 58 pages, 41 figure

To take full advantage of a laser-plasma accelerator, stability and control of the electron beam parameters have to be achieved. The external injection scheme with two colliding laser pulses is a way to stabilize the injection of electrons into the plasma wave, and to easily tune the energy of the output beam by changing the longitudinal position of the injection. In this Letter, it is shown that by tuning the optical injection parameters, one is able to control the phase-space volume of the injected particles, and thus the charge and the energy spread of the beam. With this method, the production of a laser accelerated electron beam of 10 pC at the 200 MeV level with a 1% relative energy spread at full width half maximum (3.1% rms) is demonstrated. This unique tunability extends the capability of laser-plasma accelerators and their applications.

We demonstrate that betatron x-ray radiation accurately provides direct imaging of electrons trajectories accelerated in laser wakefields. Experimental far field x-ray beam profiles reveal that electrons can follow similar transverse trajectories with typical excursions of 1.5 microm+/-0.5 microm in the plane of laser polarization and 0.7 microm+/-0.2 microm in the plane perpendicular.

Relativistic electrons accelerated by laser wakefields can produce x-ray beams from their motion in plasma termed betatron oscillations. Detailed spectral characterization is presented in which the amplitude of the betatron oscillations r is studied by numerical analysis of electron and x-ray spectra measured simultaneously. We find that r reaches as low as 1 mum in agreement with previous studies of radiation based on coherence and far-field spatial profile.

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The features of Betatron x-ray emission produced in a laser-plasma accelerator are closely linked to the properties of the relativistic electrons which are at the origin of the radiation. While in interaction regimes explored previously the source was by nature unstable, following the fluctuations of the electron beam, we demonstrate in this Letter the possibility to generate x-ray Betatron radiation with controlled and reproducible features, allowing fine studies of its properties. To do so, Betatron radiation is produced using monoenergetic electrons with tunable energies from a laser-plasma accelerator with colliding pulse injection [J. Faure et al., Nature (London) 444, 737 (2006)]. The presented study provides evidence of the correlations between electrons and x-rays, and the obtained results open significant perspectives toward the production of a stable and controlled femtosecond Betatron x-ray source in the keV range.The continuous progress made over the past decade in the production of ultrashort x-ray radiation has opened novel research horizons with countless applications. The most advanced short pulse x-ray source to date is the free electron laser (FEL), producing x-ray pulse orders of magnitude brighter than any other source [1]. However, at such large facilities, beam time access is inevitably limited and there is therefore an interest in producing compact sources delivering x-ray pulses, even if less intense than in a FEL, sufficiently bright to satisfy the need of many applications. For this reason, the research on complementary femtosecond x-ray sources remains dynamic and novel sources are developed in both synchrotron and laser-plasma interaction communities. Produced in relativistic laser-plasma interaction, the Betatron radiation presents promising features to become a bright and compact femtosecond x-ray source. Demonstrated in 2004, this scheme allowed us to produce for the first time broadband low divergence femtosecond x-ray beams in the keV energy range from laser-plasma interaction [2]. Since then the source has been developed and widely characterized. It can now generate radiation with divergence down to below 10 mrad and in the 10 keV range [3]. However, since its first demonstration, the Betatron radiation has always been produced by self-injected electrons in the bubble or blowout regime [2][3][4][5][6][7]. In that case, with present laser technologies, the laser pulse has to be shrunk in time and space to generate an appropriate bubble structure. These effects, that result from the relativistic self-focusing and self-shortening, are strongly nonlinear and by nature unstable; small fluctuations of any of the experimental parameters will likely lead to important fluctuations of the Betatron radiation properties. Self-injected laser-plasma accelerators do not allow us to easily control the electron and radiation properties. In addition, correlations between the electron beam energy and x-ray properties are difficult to observe because the relevant parameters can not be controlled i...

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International audienceIn relativistic laser plasma interaction, electrons can be simultaneously accelerated and wiggled in an ion cavity created in the wake of an intense short pulse laser propagating in an underdense plasma. As a consequence of their motion, the accelerated electrons emit an intense x-ray beam called laser produced betatron radiation. Being an emission from charged particles, the features of the betatron source are directly linked to the electrons trajectories. In particular, the radiation is emitted in the direction of the electrons velocity. In this article we show how an image of electrons orbits in the wakefield cavity can be deduced from the structure of x-ray spatial profiles