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 1 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.

Ultra-intense MeV photon and neutron beams are indispensable tools in many research fields such as nuclear, atomic and material science as well as in medical and biophysical applications. For applications in laboratory nuclear astrophysics, neutron fluxes in excess of 1021 n/(cm2 s) are required. Such ultra-high fluxes are unattainable with existing conventional reactor- and accelerator-based facilities. Currently discussed concepts for generating high-flux neutron beams are based on ultra-high power multi-petawatt lasers operating around 1023 W/cm2 intensities. Here, we present an efficient concept for generating γ and neutron beams based on enhanced production of direct laser-accelerated electrons in relativistic laser interactions with a long-scale near critical density plasma at 1019 W/cm2 intensity. Experimental insights in the laser-driven generation of ultra-intense, well-directed multi-MeV beams of photons more than 1012 ph/sr and an ultra-high intense neutron source with greater than 6 × 1010 neutrons per shot are presented. More than 1.4% laser-to-gamma conversion efficiency above 10 MeV and 0.05% laser-to-neutron conversion efficiency were recorded, already at moderate relativistic laser intensities and ps pulse duration. This approach promises a strong boost of the diagnostic potential of existing kJ PW laser systems used for Inertial Confinement Fusion (ICF) research.

Thomson spectrometers (TS) are designed to detect and distinguish protons from heavier ions in experiments of intense laser-matter interaction. The combination of electric and magnetic field allows for deflecting ion species with different mass-to-charge ratio on different trajectories. However, even small distortions of the internal fields of the device can lead to a degradation of the measurement quality. Hence, TS are sensitive to both high electromagnetic pulses (EMPs) and fields due to static charge accumulation caused by the interaction. Here we report on the analysis of data obtained with a TS designed to have high sensitivity and robustness with, optimized shielding against EMPs, even when the device is placed at short distances from the interaction point, where the electromagnetic radiation is more intense. To test this, the spectrometer was thus placed ∼50 cm far from the target during an experiment at the PHELIX laser at GSI (∼180 J energy, >1020 W/cm2 intensity, sub-picosecond laser pulses on solid targets). Despite the presence of strong EMPs (beyond 100 kV/m at 1 m distance from the target), the tests were successful and the TS was able to retrieve a good-quality signal. Indeed, the close proximity to the interaction point caused a significant number of electrons, produced by the intense laser-target interaction, entering the TS and causing internal electrostatic fields up to tens of kV/m. These induced fields altered the trajectories of the detected ions, making the interpretation and characterization of the particle species not straightforward. This effect was analyzed with ad-hoc particle tracking simulations. This study is of high importance for the effective implementation of this type of high-sensitivity TSs in experiments with PW-power lasers.

High performance of laser-driven sources of radiation is in focus of research aimed at the study of high energy density matter, pair production and neutron generation using kJ PW-laser systems. In this work, we present a highly efficient approach to generate an ultra-high flux, high-energy bremsstrahlung in the interaction of direct laser-accelerated (DLA) electrons with a several-millimeters-thick high-Z converter. A directed beam of direct laser-accelerated electrons with energies up to 100 MeV was produced in the interaction of a sub-ps laser pulse of moderate relativistic intensity with long-scale plasma of near-critical density obtained by irradiation of low-density polymer foam with an ns laser pulse. In the experiment, tantalum isotopes generated via photonuclear reactions with threshold energies above 40 MeV were observed. The Geant4 Monte Carlo code, with the measured electron energy and angular distribution as input parameters, was used to characterize the bremsstrahlung spectrum responsible for the registered yields of isotopes from 180Ta to 175Ta. It is shown that when the direct laser-accelerated electrons interact with a tantalum converter, the directed bremsstrahlung with an average photon energy of 18 MeV and ∼2⋅1011 photons per laser shot in the energy range of giant dipole resonance (GDR) and beyond (≥7.5 MeV) is produced. This results in an ultra-high photon flux of ∼6 × 1022 sr−1·s−1 and a record conversion efficiency of 2% of the focused laser energy into high-energy bremsstrahlung.