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Laser-plasma accelerators (LPAs) produce electric fields of the order of 100 GV m−1, more than 1000 times larger than those produced by radio-frequency accelerators. These uniquely strong fields make LPAs a promising path to generate electron beams beyond the TeV, an important goal in high-energy physics. Yet, large electric fields are of little benefit if they are not maintained over a long distance. It is therefore of the utmost importance to guide the ultra-intense laser pulse that drives the accelerator. Reaching very high energies is equally useless if the properties of the electron beam change completely from shot to shot, due to the intrinsic lack of stability of the injection process. State-of-the-art laser-plasma accelerators can already address guiding and control challenges separately by tweaking the plasma structures. However, the production of beams that are simultaneously high quality and high energy has yet to be demonstrated. This paper presents a novel experiment, coupling laser-plasma waveguides and controlled injection techniques, facilitating the reliable and efficient acceleration of high-quality electron beams up to 1.1 GeV, from a 50 TW-class laser.

International audience; Betatron x-ray sources from laser-plasma accelerators reproduce the principle of a synchrotron at the millimeter scale. They combine compactness, femtosecond pulse duration, broadband spectrum, and micron source size. However, when produced with terawatt class femtosecond lasers, their energy and flux are not sufficient to compete with synchrotron sources, thus limiting their dissemination and its possible applications. Here we present a simple method to enhance the energy and the flux of betatron sources without increasing the laser energy. The orbits of the relativistic electrons emitting the radiation were controlled using density tailored plasmas so that the energetic efficiency of the betatron source is increased by more than one order of magnitude.

The Free Electron Laser (FEL) application of Laser Plasma Acceleration (LPA) requires the handling of the energy spread and divergence. The COXINEL manipulation line, designed and built at SOLEIL for this purpose, consists of high gradient quadrupoles for divergence handling and a decompression chicane for energy sorting, enabling FEL amplification with baseline parameters. Installed at Laboratoire d’Optique Appliquee (LOA), it uses robust electrons generated and accelerated by ionization injection using a 30 TW laser. We report here on the work progress towards a FEL demonstration. The LPA measured electron beam characteristics deviates from the baseline reference case. After the installation of the equipment, the electron beam transport has first been optimized. The electron position and dispersion are independently adjusted. Then, undulator radiation has been measured. The spectral purity is controlled via the energy spread adjusted in the slit located in the chicane. FEL effect demonstration is within reach, with currently achieved performance on different LPA experiments.

International audience; Diffraction puts a fundamental limit on the distance over which a light beam can remain focused. For about 30 years, several techniques to overcome this limit have been demonstrated. Here, we propose a reflective optics, namely, the axiparabola, that allows to extend the production of "dif-fraction-free" beams to high-peak-power and broadband laser pulses. We first describe the properties of this aspheric optics. We then analyze and compare its performances in numerical simulations and in experiments. Finally, we use it to produce a plasma waveguide that can guide an intense laser pulse over 10 millimeters.

Laser-Plasma Accelerators (LPAs) produce electric fields exceeding 100 GV/m, that is 3 orders of magnitude larger than those obtained in metallic-cavity accelerators. They could thus allow for a drastic decrease of the size of accelerators for scientific, medical and industrial applications. A high field-gradient is however not sufficient for reaching high-energies; the electron beam has also to experience the accelerating field on long distances, which is challenging in a LPA because of 3 phenomenons: diffraction, pump depletion and dephasing. Diffraction and pump depletion leads to a decrease of the laser intensity during the acceleration, down to a level from which the laser can no more drive a wakefield. Dephasing corresponds to electrons reaching a decelerating phase of the electric field. It occurs because the phase velocity of the accelerating field is smaller than the velocity of the electron beam. To date, the highest beam energies have been obtained by guiding the laser in a capillary discharge, thus overcoming diffraction. Here we propose a new acceleration concept, based on the use of high-intensity quasi-Bessel beams and spatio-temporal couplings, which allows to overcome not only diffraction but also pump depletion and dephasing. The velocity of the quasi-Bessel beam is superluminal in vacuum and dephasing is suppressed by using spatio-temporal couplings to phase lock the electron beam on the accelerating field. In this scheme, the electron energy is proportional to the laser energy and inversely proportional to the laser pulse length (the shorter the laser, the higher the beam energy). We will first present Particle-In-Cell simulations demonstrating this concept. We will then show preliminary experimental results illustrating the generation of high-intensity quasi-Bessel beams as well as the generation of a 1 cm plasma-waveguide.

Betatron radiation from laser-plasma accelerators reproduces the principle of a synchrotron on a millimeter scale, but featuring femtosecond duration. Here we present the outcome of our latest developments, which now allow us to produce stable and polarized X-ray bursts. Moreover, the X-ray polarization can simply be adjusted by tuning the polarization of the laser driving the process. The excellent stability of the source is expressed in terms of pointing, flux, transverse distribution and critical energy of the spectrum. These combined features make our betatron source particularly suitable for applications in ultrafast X-ray science. In this presentation we will describe the generation process, relying on the ionization injection scheme for laser-plasma acceleration. We will show experimental measurements, numerical results and first applications in time-resolved spectroscopy.

Short-pulse x-ray/gamma-ray sources have become indispensable light sources for investigating material science, bio technology, and photo-nuclear physics. In past decades, rapid advancement of high intensity laser technology led extensive progresses in the field of radiation sources based on laser-plasma interactions - x-ray lasers, betatron radiation and Compton gamma-rays. Ever since the installation of a 100-TW laser in 2006, we have pursued the development of ultrashort x-ray/gamma-ray radiations, such as x-ray lasers, relativistic high-order harmonics, betatron radiation and all-optical Compton gamma-rays. With the construction of two PW Ti:Sapphire laser beamlines having peak powers of 1.0 PW and 1.5 PW in 2010 and 2012, respectively [1], we have investigated the generation of multi-GeV electron beams [2] and MeV betatron radiations. We plan to carry out the Compton backscattering to generate MeV gamma-rays from the interaction of a GeV electron beam and a PW laser beam. Here, we present the recent progress in the development of ultrashort x-ray/gamma-ray radiation sources based on laser plasma interactions and the plan for developing Compton gamma-ray sources driven by the PW lasers. In addition, we will present the applications of laser-plasma x-ray lasers to x-ray holography and coherent diffraction imaging. [references] 1. J. H. Sung, S. K. Lee, T. J. Yu, T. M. Jeong, and J. Lee, Opt. Lett. 35, 3021 (2010). 2. H. T. Kim, K. H. Pae, H. J. Cha, I J. Kim, T. J. Yu, J. H. Sung, S. K. Lee, T. M. Jeong, J. Lee, Phys. Rev. Lett. 111, 165002 (2013).

En transmettant, a des faisceaux d’electrons ou a des faisceaux de rayons X, les proprietes des impulsions lasers, les physiciens sont capables aujourd’hui de realiser des accelerateurs a plasma compacts et competitifs, delivrant des faisceaux de particules ou de rayonnement X tres brillants. Pour realiser cette performance, il faut, d’une part, mettre en mouvement de facon collective les electrons du plasma, et, d’autre part, controler dans un tres petit volume l’injection des particules a accelerer.

Le rayonnement X est un des outils les plus puissants pour explorer les proprietes de la matiere. Plusieurs generations de tres grands instruments, tels que les synchrotrons, puis plus recemment les lasers a electrons libres (LEL) ont ete developpes, delivrant des faisceaux de rayonnement X aux proprietes de plus en plus remarquables en termes de flux, de duree d’impulsion, d’energie ou de coherence. Les LEL sont actuellement les sources de rayonnement X les plus performantes en termes de brillance, mais tres peu d’installations seront disponibles dans le monde du fait de la taille des infrastructures necessaires et du cout de construction et de fonctionnement. Les lasers intenses ultra-brefs constituent une alternative et la physique de leur interaction avec la matiere offre des possibilites uniques pour le developpement de sources de rayonnement X qui soient a la fois ultra-breves et intenses, mais egalement compactes et a cout modere, laissant envisager l’installation de ces systemes au sein de laboratoires de recherche de taille universitaire. Aujourd'hui, grâce aux progres realises dans ce domaine, les plasmas crees par laser permettent de produire des faisceaux innovants de rayonnement X. Leur compacite, leur parfaite synchronisation avec un laser intense ou toute source lumineuse ou particulaire generee par le laser, leur brievete extreme et leur taille de source sub-micrometrique, en font des rayonnements a fort potentiel d’application et de dissemination au niveau academique et societal.

Electrons accelerated through laser-plasma interaction carry some angular momentum, which evolves during the acceleration. A source of angular momentum growth is identified through experimental and simulation results, having important consequences on transverse emittance measurements.

International audience; In 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.

International audience; 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 μm in agreement with previous studies of radiation based on coherence and far-field spatial profile.

We present the first detailed spectral measurement of 1-3 keV Betatron X-ray radiation with two high resolution crystal spectrometers. Electron trajectories in the laser produced plasma can be determined with this measurement.

International audience; Rapid progress in the development of high-intensity laser systems has extended our ability to study light–matter interactions far into the relativistic domain, in which electrons are driven to velocities close to the speed of light. As well as being of fundamental interest in their own right, these interactions enable the generation of high-energy particle beams that are short, bright and have good spatial quality. Along with steady improvements in the size, cost and repetition rate of high-intensity lasers, the unique characteristics of laser-driven particle beams are expected to be useful for a wide range of contexts, including proton therapy for the treatment of cancers, materials characterization, radiation-driven chemistry, border security through the detection of explosives, narcotics and other dangerous substances, and of course high-energy particle physics. Here, we review progress that has been made towards realizing such possibilities and the principles that underlie them.

Fresnel diffraction of X-ray beam from laser-plasma interaction shows incoherent-source diameter

We demonstrate that betatron x-ray radiation provides a direct imaging of electrons trajectories accelerated in laser wakefields. Electron excursions down to 0.7 mum plusmn 0.2 mum have been measured in our parameter regime.

We show that different schemes can be now followed to produce collimated X-ray radiation using laser systems. By focusing intense femtosecond laser light onto a gas jet, electrons of the plasma can be manipulated to generate ultrafast (femtosecond) X-ray radiation in the forward direction along the laser axis. In this chapter we discuss nonlinear Thomson scattering, betatron emission and Compton scattering. In years to come, the rapid development of laser technology will provide more intense laser systems. We can expect to see the creation of bright X-ray beams with a high degree of collimation (< 1 mrad divergence), as well as even shorter pulse durations, down to attosecond time scales. Such sources will provide multidisciplinary scientific communities with unique tools to probe and excite matter.

International audience; By focusing an ultraintense laser onto a helium gaz jet, a collimated beam of ultrafast broadband X-ray radiation can now be generated. The X-ray radiation results from the betatron oscillations of relativistic electrons in the laser created plasma channel. Thus, just as in a synchrotron, the spectral and flux properties of the X-ray beam can be linked to the electron beam through the plasma wiggler strength. The radiation has been observed within 1-10 keV with filters and presents a divergence of as low as 20 mrad. In addition, this source possesses the unique properties to be ultrafast and perfectly synchronized with the laser system, which opens the way toward new types of pump probe experiments.

International audience; 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 μm±0.5 μm in the plane of laser polarization and 0.7 μm±0.2 μm in the plane perpendicular.

Polychromatic beams of hard X-rays from ultrafast laser plasma interaction are studied. Relativistic electrons are accelerated and wiggled by a plasma along a few millimeters. Spectral, spatial characterization and electron/X-ray correlation is presented.

A practical double plasma-mirror built into a f=10 m null telescope has been added as a standard feature of a 50 fs 3J laser system for ultraintense laser-matter interaction. Contrast >10/sup 10/ is obtained, maintaining excellent focusability.

International audience; We demonstrate that a beam of x-ray radiation can be generated by simply focusing a single high-intensity laser pulse into a gas jet. A millimeter-scale laser-produced plasma creates, accelerates, and wiggles an ultrashort and relativistic electron bunch. As they propagate in the ion channel produced in the wake of the laser pulse, the accelerated electrons undergo betatron oscillations, generating a femtosecond pulse of synchrotron radiation, which has keV energy and lies within a narrow (50 mrad) cone angle.

International audience; Beams of x rays in the kiloelectronvolt energy range have been produced from laser-matter interaction. Here, energetic electrons are accelerated by a laser wakefield, and experience betatron oscillations in an ion channel formed in the wake of the intense femtosecond laser pulse. Experiments using a 50 TW laser (30 fs duration) are described, as well as comparisons with numerical simulations. These results pave the way of a new generation of radiation in the x-ray spectral range, with a high collimation and an ultrafast pulse duration, produced by the use of compact laser system.

International audience; X-ray radiation emitted by electrons during their acceleration in a laser-plasma accelerator was used to evidence two distincts self-injection mechanisms (longitudinal and transverse) and to identify one source of angular-momentum growth in laser-plasma accelerators. (c) 2014 Optical Society of America

International audience; Technology based on high-peak-power lasers has the potential to provide compact and intense radiation sources for a wide range of innovative applications. In particular, electrons that are accelerated in the wakefield of an intense laser pulse oscillate around the propagation axis and emit X-rays. This betatron source, which essentially reproduces the principle of a synchrotron at the millimeter scale, provides bright radiation with femtosecond duration and high spatial coherence. However, despite its unique features, the usability of the betatron source has been constrained by its poor control and stability. In this article, we demonstrate the reliable production of X-ray beams with tunable polarization. Using ionization-induced injection in a gas mixture, the orbits of the relativistic electrons emitting the radiation are reproducible and controlled. We observe that both the signal and beam profile fluctuations are significantly reduced and that the beam pointing varies by less than a tenth of the beam divergence. The polarization ratio reaches 80%, and the polarization axis can easily be rotated. We anticipate a broad impact of the source, as its unprecedented performance opens the way for new applications.

Laser-plasma technology has the potential to provide compact sources of bright femtosecond X-ray, which may soon serve as an alternative to their conventional counterparts. Proof-of- principle experiments have impressively demonstrated the sources’ prospects, yet the poor stability and tunability drastically limit their scope of applicability. Conventional systems have achieved their remarkable control over the source by progressive improvement of the discrete stages of injection, acceleration, beam transport and radiation generation. In this work we have adapted this approach for laser-plasma sources and made advances on all individual parts of the source.; Les progrès réalisés dans le domaine de l’interaction laser-plasma au cours des dix dernières années ont permis de produire de nouvelles sources de rayonnement X pouvant rivaliser avec les conventionnels synchrotron et tubes X. Ces nouvelles sources ont un fort potentiel mais leur domaine d’applications reste très limitées en raison d’importantes fluctuations et du peu de contrôle de leurs propriétés. Ces sources sont basées sur le même principe qu’un Synchroton. Il n’agit d’accélérer des électrons jusqu’à des vitesses relativistes et de les faire osciller de manière à ce qu’ils émettent efficacement du rayonnement X. Afin de obtenir un meilleur contrôle de la source nous avons étudié les différentes étapes conduisant à la production de rayonnement : l’injection d’électrons dans l’onde plasma créée dans le sillage du laser, l’accélération et le transport de ces électrons puis les méthodes permettant de les faire osciller. Le manuscrit présente les progrès réalisés dans ces domaines.

During the relativistic interaction between a short and intense laser pulse and an underdense plasma, electrons can be injected and accelerated up to hundreds of MeV in an accelerating structure formed in the wake of the pulse: this is the so-called laser-plasma accelerator. One of the major perspectives for laser-plasma accelerators resides in the realization of compact sources of femtosecond x-ray beams. In this thesis, two x-ray sources was studied and developed. The betatron radiation, intrinsic to laser-plasma accelerators, comes from the transverse oscillations of electrons during their acceleration. Its characterization by photon counting revealed an x-ray beam containing 10^9 photons, with energies extending above 10 keV. We also developed an all-optical Compton source producing photons with energies up to hundreds of keV, based on the collision between a photon beam and an electron beam. The potential of these x-ray sources was highlighted by the realization of single shot phase contrast imaging of a biological sample. Then, we showed that the betatron x-ray radiation can be a powerful tool to study the physics of laser-plasma acceleration. We demonstrated the possibility to map the x-ray emission region, which gives a unique insight into the interaction, permitting us for example to locate the region where electrons are injected. The x-ray angular and spectral properties allow us to gain information on the transverse dynamics of electrons during their acceleration.; Lors de l'interaction relativiste entre une impulsion laser brève et intense et un plasma sous-dense, des électrons peuvent être injectés et accélérés jusqu'à plusieurs centaines de MeV dans une structure accélératrice se formant dans le sillage de l'impulsion laser : c'est l'accélérateur laser-plasma. Une des applications majeures de ces accélérateurs réside dans le développement de sources compactes de faisceaux de rayonnement X femtoseconde. Au cours de cette thèse, deux sources de rayonnement X ont été étudiées et développées. Le rayonnement bétatron, intrinsèque à l'accélérateur laser-plasma, provient des oscillations transverses des électrons au cours de leur accélération. Sa caractérisation par comptage de photons a montré que le faisceau X contenait un total de 10^9 photons, avec des énergies pouvant être supérieures à 10 keV. Nous avons également développé une source Compton tout optique produisant des photons de quelques centaines de keV, basée sur la collision entre un faisceau de photons et un faisceau d'électrons. Le potentiel de ces sources de rayonnement a été mis en évidence en réalisant l'imagerie par contraste de phase mono-coup d'un échantillon biologique. Nous avons ensuite montré que l'émission X bétatron est un outil expérimental très puissant pour étudier la physique sous-jacente à l'accélération laser-plasma. On peut tout d'abord réaliser la cartographie de la région d'émission, ce qui donne des informations inédites, permettant par exemple de localiser l'endroit où sont injectés les électrons. Les propriétés angulaires et spectrales du rayonnement X permettent également d'avoir des informations sur la dynamique transverse des électrons au cours de leur accélération.

During the relativistic interaction between a short and intense laser pulse and an underdense plasma, electrons can be injected and accelerated up to hundreds of MeV in an accelerating structure formed in the wake of the pulse: this is the so-called laser-plasma accelerator. One of the major perspectives for laser-plasma accelerators resides in the realization of compact sources of femtosecond x-ray beams. In this thesis, two x-ray sources was studied and developed. The betatron radiation, intrinsic to laser-plasma accelerators, comes from the transverse oscillations of electrons during their acceleration. Its characterization by photon counting revealed an x-ray beam containing 10^9 photons, with energies extending above 10 keV. We also developed an all-optical Compton source producing photons with energies up to hundreds of keV, based on the collision between a photon beam and an electron beam. The potential of these x-ray sources was highlighted by the realization of single shot phase contrast imaging of a biological sample. Then, we showed that the betatron x-ray radiation can be a powerful tool to study the physics of laser-plasma acceleration. We demonstrated the possibility to map the x-ray emission region, which gives a unique insight into the interaction, permitting us for example to locate the region where electrons are injected. The x-ray angular and spectral properties allow us to gain information on the transverse dynamics of electrons during their acceleration., Lors de l'interaction relativiste entre une impulsion laser brève et intense et un plasma sous-dense, des électrons peuvent être injectés et accélérés jusqu'à plusieurs centaines de MeV dans une structure accélératrice se formant dans le sillage de l'impulsion laser : c'est l'accélérateur laser-plasma. Une des applications majeures de ces accélérateurs réside dans le développement de sources compactes de faisceaux de rayonnement X femtoseconde. Au cours de cette thèse, deux sources de rayonnement X ont été étudiées et développées. Le rayonnement bétatron, intrinsèque à l'accélérateur laser-plasma, provient des oscillations transverses des électrons au cours de leur accélération. Sa caractérisation par comptage de photons a montré que le faisceau X contenait un total de 10^9 photons, avec des énergies pouvant être supérieures à 10 keV. Nous avons également développé une source Compton tout optique produisant des photons de quelques centaines de keV, basée sur la collision entre un faisceau de photons et un faisceau d'électrons. Le potentiel de ces sources de rayonnement a été mis en évidence en réalisant l'imagerie par contraste de phase mono-coup d'un échantillon biologique. Nous avons ensuite montré que l'émission X bétatron est un outil expérimental très puissant pour étudier la physique sous-jacente à l'accélération laser-plasma. On peut tout d'abord réaliser la cartographie de la région d'émission, ce qui donne des informations inédites, permettant par exemple de localiser l'endroit où sont injectés les électrons. Les propriétés angulaires et spectrales du rayonnement X permettent également d'avoir des informations sur la dynamique transverse des électrons au cours de leur accélération.