Laser-plasma acceleration [1,2] is an emerging technique for accelerating electrons to high energies over very short distances. The accelerated electron bunches have femtosecond duration [3,4], making them particularly relevant for applications such as ultrafast imaging [5] or femtosecond X-ray generation [6,7]. Current laser-plasma accelerators are typically driven by Joule-class laser systems that have two main drawbacks: their relatively large scale and their low repetition-rate, with a few shots per second at best. The accelerated electron beams have energies ranging from 100 MeV [8][9][10] to multi-GeV [11,12], however a MeV electron source would be more suited to many societal and scientific applications. Here, we demonstrate a compact and reliable laserplasma accelerator producing high-quality few-MeV electron beams at kilohertz repetition rate.This breakthrough was made possible by using near-single-cycle light pulses, which lowered the required laser energy for driving the accelerator by three orders of magnitude, thus enabling high repetition-rate operation and dramatic downsizing of the laser system. The measured electron bunches are collimated, with an energy distribution that peaks at 5 MeV and contains up to 1 pC of charge. Numerical simulations reproduce all experimental features and indicate that the electron bunches are only ∼ 1 fs long. We anticipate that the advent of these kHz femtosecond relativistic electron sources will pave the way to wide-impact applications, such as ultrafast electron diffraction in materials [13,14] In a laser-plasma accelerator, a laser pulse is focused to ultra-high intensity in an underdense plasma. The laser ponderomotive force sets up a charge separation in the plasma by displacing electrons, resulting in the excitation of a large-amplitude plasma wave, also called a wakefield. The wakefield carries enormous electric fields, in excess of 100 GV/m [16], that are well adapted for accelerating electrons to relativistic energies over short distances, typically less than a millimeter. The accelerated electron beams have femtosecond duration and are intrinsically synchronized to the laser pulse, which could lift the temporal resolution bottleneck in various experimental situations. For example, in ultrafast electron diffraction, the temporal resolution is currently limited to more than 100 fs, but it could be improved to sub-10 fs using laser driven electrons [15]. Thus, laser-plasma accelerators in the MeV range could find numerous applications with unprecedented time resolution, provided they operate reliably and at high repetition-rate. Indeed, in addition to temporal resolution, ultrafast imaging and diffraction also require statistics and a high signal-to-noise ratio [5,14] that can only be reached with a reliable and high repetition-rate electron source.In this letter, we demonstrate reliable operation of a laser-plasma accelerator delivering 5 MeV electrons at kHz repetition-rate. This breakthrough was made possible by the original use of a multi-mJ lase...

International audienceWe show that electron bunches in the 50-100 keV range can be produced from a laser wake-field accelerator using 10 mJ, 35 fs laser pulses operating at 0.5 kHz. It is shown that using a solenoid magnetic lens, the electron bunch distribution can be shaped. The resulting transverse and longitudinal coherence is suitable for producing diffraction images from a polycrystalline 10 nm aluminum foil. The high repetition rate, the stability of the electron source and the fact that its uncorrelated bunch duration is below 100 fs make this approach promising for the development of sub-100 fs ultrafast electron diffraction experiments

International audienceA high-repetition rate electron source is generated by tightly focusing kHz, few-mJ laser pulses into an underdense plasma. This high-intensity laser-plasma interaction leads to stable electron beams over several hours but with strikingly complex transverse distributions even for good quality laser focal spots. We find that the electron beam distribution is sensitive to the laser wave front via the laser midfield distribution rather than the laser focal spot itself. We are able to measure the laser wave front around the focus and include it in realistic particle-in-cell simulations demonstrating the role of the laser wave front on the acceleration of electrons. Distortions of the laser wave front cause spatial inhomogeneities in the midfield laser intensity and, consequently, the laser pulse drives an inhomogeneous transverse wakefield whose focusing and defocusing properties affect the electron distribution. These findings explain the experimental results and suggest the possibility of controlling the electron spatial distribution in laser-plasma accelerators by tailoring the laser wave front

Using particle-in-cell simulations, we study the interaction of few-mJ-fewcycle laser pulses with an underdense plasma at resonant density. In this previously unexplored regime, it is found that group velocity dispersion is a key ingredient of the interaction. The concomitant effects of dispersion and plasma nonlinearities cause a deceleration of the wakefield phase velocity, which becomes sub-relativistic. Electron injection in this sub-relativistic wakefield is enhanced and leads to the production of a femtosecond electron bunch with a picocoulomb of charge in the 5-10 MeV energy range. Such an electron bunch is of great interest for application to ultrafast electron diffraction. In addition, in this dispersion dominated regime, it is shown that positively chirped laser pulses can be used as a tuning knob for compensating for plasma dispersion, increasing the laser amplitude during self-focusing and optimizing the trapped charge.

We report for the first time on the anticorrelated emission of high-order harmonics and energetic electron beams from a solid-density plasma with a sharp vacuum interface−plasma mirror−driven by an intense ultrashort laser pulse. We highlight the key role played by the nanoscale structure of the plasma surface during the interaction by measuring the spatial and spectral properties of harmonics and electron beams emitted by a plasma mirror. We show that the nanoscale behavior of the plasma mirror can be controlled by tuning the scale length of the electronic density gradient, which is measured in-situ using spatial-domain interferometry.PACS numbers: 52.38. Kd,52.38.Ph Over the past 30 years, solid-density plasmas driven by intense femtosecond (fs) pulses, so-called plasma mirrors, have been successfully tested as a source of high-order harmonics and attosecond XUV pulses in a number of experiments [1][2][3][4][5][6][7][8][9][10], where the laser intensity typically exceeds a few 10 14 W/cm 2 . Other experiments have shown it is also possible to accelerate energetic electrons from plasma mirrors for intensities above 10 16 W/cm 2 [11-13]. Attempting to understand each of these experimental observations invariably points to the key role played by the plasma-vacuum interface during the interaction both on the nanoscale spatially and on the sub-laser-cycle scale temporally [14,15].It is commonly assumed that the electronic density at the plasma mirror surface decreases exponentially from solid to vacuum over a distance L g , also called density gradient. When the laser pulse reflects on this plasma mirror, for every oscillation of the laser field, some electrons are driven towards vacuum and sent back to the plasma [16,17]. These bunches of so-called Brunel electrons [18] impulsively excite collective high-frequency plasma oscillations in the density gradient that lead to the emission of XUV radiation through linear mode conversion [19]. As illustrated in Fig 1(a), each position x of the plasma behaves as a nanoscale oscillator of frequency ω p (x) = ω 0 n e (x)/n c where ω 0 is the driving laser angular frequency, n e the local electronic density at position x and n c the critical density. This periodic mechanism, called Coherent Wake Emission (CWE), leads to efficient high harmonics generation for very short plasma scale lengths, typically L g ∼ λ/100 [19], even for subrelativistic intensities, a 0 < 1, where a 0 = eA 0 /mc is the normalized vector potential, e and m the electron charge and mass and c the speed of light. However, the efficiency significantly drops for L g >> λ/20

Recent progress in laser wakefield acceleration has led to the emergence of a new generation of electron and X-ray sources that may have enormous benefits for ultrafast science. These novel sources promise to become indispensable tools for the investigation of structural dynamics on the femtosecond time scale, with spatial resolution on the atomic scale. Here, we demonstrate the use of laser-wakefield-accelerated electron bunches for time-resolved electron diffraction measurements of the structural dynamics of single-crystal silicon nano-membranes pumped by an ultrafast laser pulse. In our proof-of-concept study, we resolve the silicon lattice dynamics on a picosecond time scale by deflecting the momentum-time correlated electrons in the diffraction peaks with a static magnetic field to obtain the time-dependent diffraction efficiency. Further improvements may lead to femtosecond temporal resolution, with negligible pump-probe jitter being possible with future laser-wakefield-accelerator ultrafast-electron-diffraction schemes.

International audienceWe propose a new concept of an electron source for ultrafast electron diffraction with sub-10-fs temporal resolution. Electrons are generated in a laser-plasma accelerator, able to deliver femtosecond electron bunches at 5 MeVenergy with a kilohertz repetition rate. The possibility of producing this electron source is demonstrated using particle-in-cell simulations. We then use particle-tracking simulations to show that this electron beam can be transported and manipulated in a realistic beam line, in order to reach parameters suitable for electron diffraction. The beam line consists of realistic static magnetic optics and introduces no temporal jitter. We demonstrate numerically that electron bunches with 5-fs duration and containing 1.5 fC per bunch can be produced, with a transverse coherence length exceeding 2 nm, as required for electron diffraction

It was recently proposed that ionization-induced self-compression could be used as an effective method to further compress femtosecond laser pulses propagating freely in a gas jet [He et al., Phys. Rev. Lett. 113, 263904 2014]. Here, we address the question of the homogeneity of the self-compression process and show experimentally that homogeneous self-compression down to 12fs can be obtained by finding the appropriate focusing geometry for the laser pulse. Simulations are used to reproduce the experimental results and give insight into the self-compression process and its limitations. Simulations suggest that the ionization process induces spatio-temporal couplings which lengthen the pulse duration at focus, possibly making this method ineffective for increasing the laser peak intensity.