[en] The limiting factors of Chirped Pulse Amplification (CPA) are discussed and experimental results of CPA in Yb:glass regenerative amplifier are given. Scaling of Yb:glass to the petawatt level is briefly discussed. (author)

[en] X-ray emission resulting from interactions of intense laser pulses with solid metal targets (Ni, Cu, Mo, Ag, and Sn) at 0.5 kHz repetition rate is measured using pulse energies of up to 12 mJ. A comparison of the conversion of laser pulse energy to total X-ray emission energy is made with respect to the previous measurements at lower energy (< 3 mJ). In the present experiments, the total bremsstrahlung conversion efficiency is found to increase by an order of magnitude for all targets as the energy in increased. The K_α line emission conversion efficiency also increases with incident pulse energy for all targets. In addition, the ratio between line and bremsstrahlung emission in the X-ray spectral region was significantly reduced at higher energies because of the large increase in bremsstrahlung. In general, the X-ray source size increases as the laser energy increased and the ellipticity of the X-ray source also increased in the laser polarization direction, with the effect becoming very pronounced at higher energies. Phase-contrast imaging of a nanospray emitter and a 3D printed plastic target was also performed using Cu and Mo targets.

[en] As lasers become progressively higher in power, optical damage thresholds will become a limiting factor. Using the non-linear optics of plasma may be a way to circumvent these limits. In this paper, we report on simulations showing an enhancement to plasma wakefield self-compression of femtosecond laser pulses due to an ionization gradient at the leading edge of the pulse. By operating in a regime where wakefield generation is driven by moderately relativistic (∼10¹⁸ W cm⁻²) laser pulses and proper choice of gas species, the ionization front of the pulse can lead to a frequency shift that enhances the ponderomotive force and therefore both the wakefield generation and subsequent pulse compression. (paper)

[en] In the interaction of a 30 fs, 40 TW Ti:sapphire Hercules laser focused to the intensity of 10¹⁹ W/cm² onto a supersonic He gas jet, we observed quasi-monoenergetic electron beams with energy up to 300 MeV and an angular divergence of 10 mrad. We found that the initial plasma density significantly affects the resultant electron beam. For plasma densities ranging between 2*10¹⁹ to 3.5*10¹⁹ cm^-3, quasi-monoenergetic electrons with energies from 80 to 160 MeV and a total charge of about 0.5 nC were produced. Lower plasma densities around 1.5*10¹⁹ cm^-3 produced quasi-monoenergetic electron beams with higher energy, up to (320 ± 50) MeV, but with a decrease of the total charge to about 5 pC. Characterization of the electron beam in terms of the electron's maximum energy, beam divergence and pointing stability is presented. The performed 2-dimensional PIC (particle-in-cell) simulations show the evolution of the laser pulse in the plasma, electron injection, and the specifics of electron acceleration. (authors)

[en] Laser plasma interaction experiments have been performed to characterize high order harmonic emission up to the 18th order using high rep rate mJ level laser pulses at relativistic intensities. The experiments were compared to two- and three-dimensional particle-in-cell simulations. The harmonic divergence was found to be less than 4° (full-width at half-maximum) at highest intensity and increased as the laser was defocused (i.e. as the intensity was reduced). The polarization dependence on the harmonic generation efficiency and divergence was also measured. Circular polarization was found to cause a deflection in the angle of emission of the harmonics—an effect which may be beneficial in the use of such harmonics for efficient isolated attosecond pulse production. (paper)

[en] The scaling of the intensity, angular and material dependence of bremsstrahlung radiation from an intense (I > 10¹⁸ W cm⁻²) laser–solid interaction has been characterized at energies between 100 keV and 1 MeV. These are the first high resolution (E/ΔE > 200) measurements of bremsstrahlung photons from a relativistic laser–plasma interaction. The measurement was performed using a high purity germanium detector at the high-repetition rate (500 Hz) λ³ laser facility. The bremsstrahlung spectra were observed to have a two effective temperature energy distribution which ranged between 80 (± 10) and 550 (± 60) keV depending on laser intensity and observation angle. The two temperatures were determined to result from separate populations of accelerated electrons. One population was isotropic and produced the lower effective bremsstrahlung temperature. The higher bremsstrahlung temperature was produced by an energetic electron beam directed out of the front of the target in the direction of the specular laser reflection, which was also the direction the bremsstrahlung effective temperature peaked. Both effective bremsstrahlung temperatures scaled consistently with a previously measured experimental electron temperature scaling on λ³. The electron populations and bremsstrahlung temperatures were modeled in the particle-in-cell code OSIRIS and the Monte Carlo code MCNPX and were in good agreement with the experimental results. The observed directionality and intensity scaling suggest a significant difference between picosecond and femtosecond duration pulse interactions. (paper)

[en] Electron bunches of attosecond duration may coherently interact with laser beams. We show how p-polarized ultraintense laser pulses interacting with sharp boundaries of overdense plasmas can produce such bunches. Particle-in-cell simulations demonstrate attosecond bunch generation during pulse propagation through a thin channel or in the course of grazing incidence on a plasma layer. In the plasma, due to the self-intersection of electron trajectories, electron concentration is abruptly peaked. A group of counterstream electrons is pushed away from the plasma through nulls in the electromagnetic field, having inherited a peaked electron density distribution and forming relativistic ultrashort bunches in vacuum

[en] While nearly all investigations of high order harmonic generation with relativistically intense laser pulses have taken place at 800 or 1053 nm, very few experimental studies have been done at other wavelengths. In this study, we investigate the scaling of relativistic high harmonic generation towards longer wavelengths at intensities of a ₀ ∼ 1. Longer driver wavelengths enable enhanced diagnostics of the harmonic emission, as multiple orders lie in the optical regime. We measure the conversion efficiency by collecting the entire harmonic emission as well as the divergence through direct imaging. We compare the emission with 2D particle-in-cell simulations to determine the experimental target conditions. This new regime of high order harmonic generation also enables relativistic scaling as well as improved discrimination of harmonic generation mechanisms. (paper)

[en] A planar Al target is excited by a 25 fs laser pulse focused to intensity up to 3x10¹⁸ W/cm² in a ∼1 μm radius spot; subsequent heat propagation along the target surface, imaged by a delayed probe pulse, appears as a roughly circular area of reduced reflectivity centered on the pump spot, that expands to as much as 12±3 μm in radius within 500 fs. We present a semiempirical model in which the pump laser pulse drives hot electrons into the target via collisionless interactions. A return current heats the target and, above a critical temperature, includes runaway electrons that return to the surface before dissipating their energy. Ultrafast radial expansion of the heated surface layer is explained by lateral diffusive motion of returning runaway electrons oscillating across the target surface layer confined by space charge. Isotropy of the observed expansion is consistent with dominance of resonance absorption over jxB heating, indicating prepulse heating is important.

[en] The electron injection process into a plasma-based laser wakefield accelerator can be influenced by modifying the parameters of the driver pulse. We present an experimental study on the combined effect of the laser pulse duration, pulse shape, and frequency chirp on the electron injection and acceleration process and the associated radiation emission for two different gas types—a 97.5% He and 2.5% N₂ mixture and pure He. In general, the shortest pulse duration with minimal frequency chirp produced the highest energy electrons and the most charge. Pulses on the positive chirp side sustained electron injection and produced higher charge, but lower peak energy electrons, compared with negatively chirped pulses. A similar trend was observed for the radiant energy. The relationship between the radiant energy and the electron charge remained linear over a threefold change in the electron density and was independent of the drive pulse characteristics. X-ray spectra showed that ionization injection of electrons into the wakefield generally produced more photons than self-injection for all pulse durations/frequency chirp and had less of a spread in the number of photons around the peak x-ray energy. (paper)