High-current laser-driven beams of relativistic electrons for high energy density research

Abstract

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.

Transient electromagnetic fields generated in experiments at the PHELIX laser facility

“…This is the case (among these four shots) where the ratio E max T /E max RF ≈ 1.6 has the lowest value. This is compatible with what is reported by Rosmej et al [25] . The use of foam targets without a metallic converter leads to a high flux of accelerated electrons, which drive the classical RF EMP generation mechanism [8][9][10][11][12] .…”

“…As reported in Fig. 1, the area in laser forward direction, in front of the target, was devoted to diagnostics for measuring the energy, the spectra and the spatial distribution of accelerated electron beams and gamma radiation, the main purpose of the campaign [25]. At an angle of 80 ∘ degrees from the laser axis, and at a distance of 𝑑 𝐷𝑑𝑜𝑡 = 123 𝑐𝑚, we placed a D-dot differential probe (which is pictured in the photograph of Fig.…”

“…The laser light (S-polarized, λ = 1053 nm wavelength) was focused down with an off-axis parabola on a solid target (rotated by 7 • from the laser axis, made of a CHO 2 mg/cc polymer foam of 350 μm thickness stacked with a 1 mm thick Au planar converter from the rear side), yielding an intensity of I ≈ 2.5 × 10 19 W/cm 2 on a spot of approximately 15 μm FWHM. Prior to the main pulse, a secondary pulse, with a delay of 2.5 ns and an energy of 1 J, irradiated the target for pre-ionizing the foam layer and created a plasma of near critical density [25]. As reported in Figure 1, the area in the laser forward direction, in front of the target, was devoted to diagnostics for measuring the energy, the spectra and the spatial distribution of relativistic electrons generated in the foam and gamma radiation in Au-converter, the main purpose of the campaign [25] .…”

“…Prior to the main pulse, a secondary pulse, with a delay of 2.5 ns and an energy of 1 J, irradiated the target for pre-ionizing the foam layer and created a plasma of near critical density [25]. As reported in Figure 1, the area in the laser forward direction, in front of the target, was devoted to diagnostics for measuring the energy, the spectra and the spatial distribution of relativistic electrons generated in the foam and gamma radiation in Au-converter, the main purpose of the campaign [25] . A characteristic feature of laser interaction with a plasma of near critical density, is the production of high current electron and ion beams.…”

Time-of-flight methodologies with large-area diamond detectors for ion characterization in laser-driven experiments

“…A hydrodynamically stable, long scale-length near-critical density plasma was generated by irradiating low-density polymer foams with a nanosecond pulse kept at an intensity of approximately W/cm 2 . Then, with a delay of 2–3 ns, a short pulse of 750 fs having 100 J energy before compression, was used to irradiate this plasma, delivering up to approximately 20 J on target in an elliptical focal spot with FWHM diameters of 18 2 μm and 12 2 μm reaching an intensity of approximately W/cm 2[ 28 ] . The nanosecond pulse was used to trigger a super-sonic ionization wave and to prepare the near-critical density plasma for the subsequent main pulse.…”