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...

CEP-stable 4 fs near-IR pulses with 3 mJ energy were generated by spectral broadening of circularly polarized 8 mJ pulses in a differentially pumped 2-m-long composite stretched flexible hollow fiber. The pulses were characterized using both SHG-FROG and SHG d-scan methods.

We report on recent progress on laser-plasma acceleration using a low energy and high-repetition rate laser system. Using only few milliJoule laser energy, in conjunction with extremely short pulses composed of a single optical cycle, we demonstrate that the laser-plasma accelerator (LPA) can be operated close to the resonant blowout regime. This results in the production of high charge electron beams (>10 pC) with peaked energy distributions in the few MeV range and relatively narrow divergence angles. We highlight the importance of the plasma density profile and gas jet design for the performance of the LPA. In this extreme regime of relativistic laser-plasma interaction with near-single-cycle laser pulses, we find that the effect of group velocity dispersion and carrier envelope phase can no longer be neglected. These advances bring LPAs closer to real scientific applications in ultrafast probing.

The development of ultra-intense and ultra-short light sources is currently a subject of intense research driven by the discovery of novel phenomena in the realm of relativistic optics, such as the production of ultrafast energetic particle and radiation beams for applications. It has been a long-standing challenge to unite two hitherto distinct classes of light sources: those achieving relativistic intensity and those with pulse durations approaching a single light cycle. While the former class traditionally involves large-scale amplification chains, the latter class places high demand on the spatiotemporal control of the electromagnetic laser field. Here, we present a light source producing waveformcontrolled 1.5-cycle pulses with a 719 nm central wavelength that can be focused to relativistic intensity at a 1 kHz repetition rate based on nonlinear post-compression in a long hollow-core fiber. The unique capabilities of this source allow us to observe the first experimental indications of light waveform effects in laser wakefield acceleration of relativistic energy electrons.

We report on electron wakefield acceleration in the resonant bubble regime with few-millijoule near-single-cycle laser pulses at a kilohertz repetition rate. Using very tight focusing of the laser pulse in conjunction with microscale supersonic gas jets, we demonstrate a stable relativistic electron source with a high charge per pulse up to 24 pC/shot. The corresponding average current is 24 nA, making this kilohertz electron source useful for various applications.

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

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

We present a practical spatial-domain interferometer for characterizing the electronic density gradient of laser-induced plasma mirrors with sub-30-femtosecond temporal resolution. Time-resolved spatial imaging of an intensity-shaped pulse reflecting off an expanding plasma mirror induced by a time-delayed pre-pulse allows us to measure characteristic plasma gradients of 10-100 nm with an expansion velocity of 10.8 nm/ps. Spatial-domain interferometry (SDI) can be generalized to the ultrafast imaging of nm to μm size laser-induced phenomena at surfaces.