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Directed x-rays produced in the interaction of sub-picosecond laser pulses of moderate relativistic intensity with plasma of near-critical density are investigated. Synchrotron-like (betatron) radiation occurs in the process of direct laser acceleration (DLA) of electrons in a relativistic laser channel when the electrons undergo transverse betatron oscillations in self-generated quasi-static electric and magnetic fields. In an experiment at the PHELIX laser system, high-current directed beams of DLA electrons with a mean energy ten times higher than the ponderomotive potential and maximum energy up to 100 MeV were measured at 1019 W/cm2 laser intensity. The spectrum of directed x-rays in the range of 5–60 keV was evaluated using two sets of Ross filters placed at 0° and 10° to the laser pulse propagation axis. The differential x-ray absorption method allowed for absolute measurements of the angular-dependent photon fluence. We report 1013 photons/sr with energies >5 keV measured at 0° to the laser axis and a brilliance of 1021 photons s−1 mm−2 mrad−2 (0.1%BW)−1. The angular distribution of the emission has an FWHM of 14°–16°. Thanks to the ultra-high photon fluence, point-like radiation source, and ultra-short emission time, DLA-based keV backlighters are promising for various applications in high-energy-density research with kilojoule petawatt-class laser facilities. [ABSTRACT FROM AUTHOR]

We report here on the results of comparative experimental measurements of laser energy absorption in a bulk and different morphology nanowire arrays interacting with relativistically intense, ultra-high temporal contrast femtosecond laser pulses. We compare polished, flat bulk samples with vertically and randomly oriented nanowires made of ZnO semiconductor material. The optical absorption of the 45° incident laser pulses of ∼40 fs duration with a central wavelength of 400 nm at intensities above 10 19 W cm 2 was determined using an integrating Ulbricht sphere. We demonstrate an almost twofold enhancement of absorption in both nanowire morphologies with an average of (79.6 ± 1.9)% in comparison to the flat bulk sample of (45.8 ± 1.9)%. The observed substantially enhanced absorption in nanowire arrays is also confirmed by high-resolution x-ray emission spectroscopy. The spectral analysis of the K-shell x-ray emission lines revealed that the He-like resonance line emission from highly ionized Zn (Zn28+) is only present in the case of nanowire arrays, whereas, for the flat bulk samples, only neutral and low charge states were observed. Our numerical simulations, based on radiative-collisional kinetic code FLYCHK, well reproduce the measured He-like emission spectrum and suggest that high charge state observed in nanowire arrays is due to substantially higher plasma temperature. Our results, which were measured for the first time with femtosecond laser pulses, can be used to benchmark theoretical models and numerical codes for the relativistic interaction of ultrashort laser pulses with nanowires. [ABSTRACT FROM AUTHOR]

Our previous experimental and three-dimensional (3D) particle-in-cell (PIC) simulation results demonstrated that a well-directed electron beam with space charge of about μC and maximum energy of 100 MeV can be generated via a sub-petawatt, picosecond laser pulse interacting with a long-scale near-critical-density plasma. Effective laser energy coupling into hot electrons occurs in the presence of strong self-generated quasistatic electric and magnetic fields that confine fast electrons in relativistic ion channels. Here, we report results of 3D PIC simulations, which show that this direct laser accelerated electron beam can work as a compact high-brightness source of x rays. The relativistic electrons make betatron oscillations in the transverse fields of the ion channel and emit a bright broadband x-ray radiation with the critical energy of about 5 keV. Due to the huge number of accelerated electrons, our simulation shows that with a picosecond, 20 J laser pulse, an x-ray spectrum with a photon number of 7 × 10 11 (>1 keV) can be generated, resulting into a peak flux of 2 × 1 0 8 photons/eV and a brilliance of 3.3 × 10 20 photons/s/mm2/mrad2/0.1%BW. [ABSTRACT FROM AUTHOR]

We report on enhanced laser driven electron beam generation in the multi MeV energy range that promises a tremendous increase of the diagnostic potential of high energy sub-PW and PW-class laser systems. In the experiment, an intense sub-picosecond laser pulse of ∼1019 Wcm−2 intensity propagates through a plasma of near critical electron density (NCD) and drives the direct laser acceleration (DLA) of plasma electrons. Low-density polymer foams were used for the production of hydrodynamically stable long-scale NCD-plasmas. Measurements show that relativistic electrons generated in the DLA-process propagate within a half angle of 2 ± 1° to the laser axis. Inside this divergence cone, an effective electron temperature of 10–13 MeV and a maximum of the electron energy of 100 MeV were reached. The high laser energy conversion efficiency into electrons with energies above 2 MeV achieved 23% with a total charge approaching 1 μC. For application purposes, we used the nuclear activation method to characterize the MeV bremsstrahlung spectrum produced in the interaction of the high-current relativistic electrons with high-Z samples and measured top yields of gamma-driven nuclear reactions. The optimization of the high-Z target geometry predicts an ultra-high MeV photon number of ∼1012 per shot at moderate relativistic laser intensity of 1019 Wcm−2. A good agreement between the experimental data and the results of the 3D-PIC and GEANT4-simulations was demonstrated. [ABSTRACT FROM AUTHOR]