Christian Brabetz

This work presents the results of a nuclear fission experiment with depleted uranium, driven by ultra-intense multi-MeV bremsstrahlung, generated by laser-accelerated electrons in a high-Z converter, which enables the production of radioisotopes on the picosecond timescale. The advanced plasma target made of pre-ionized low-density foam enabled effective electron acceleration to energies of up to 100 MeV at a laser intensity of 1019 W cm−2 and the generation of ultra-bright bremsstrahlung with an effective temperature of 11.8 MeV in the giant dipole resonance region, with a previously unattained high conversion efficiency of 1%–2%. A major hazard in high-power laser experiments is the presence of harmful radiation and its impact on sensitive detectors. A gas flow system was used to transport volatile fission products from the laser-target interaction point to remote detectors in under ten seconds. This spatial separation of the detectors from the point of laser-matter interaction significantly reduces background noise, the impact of harmful radiation, and enables the detection of signals from short-lived radioisotopes—the products of induced nuclear fission—with high sensitivity.

We have recently proposed a new technique of plasma tailoring by laser-driven hydrodynamic shockwaves generated on both sides of a gas jet [Marquès et al., Phys. Plasmas 28, 023103 (2021)]. In a continuation of this numerical work, we study experimentally the influence of the tailoring on proton acceleration driven by a high-intensity picosecond laser in three cases: without tailoring, by tailoring only the entrance side of the picosecond laser, and by tailoring both sides of the gas jet. Without tailoring, the acceleration is transverse to the laser axis, with a low-energy exponential spectrum, produced by Coulomb explosion. When the front side of the gas jet is tailored, a forward acceleration appears, which is significantly enhanced when both the front and back sides of the plasma are tailored. This forward acceleration produces higher-energy protons, with a peaked spectrum, and is in good agreement with the mechanism of collisionless shock acceleration (CSA). The spatiotemporal evolution of the plasma profile is characterized by optical shadowgraphy of a probe beam. The refraction and absorption of this beam are simulated by post-processing 3D hydrodynamic simulations of the plasma tailoring. Comparison with the experimental results allows estimation of the thickness and near-critical density of the plasma slab produced by tailoring both sides of the gas jet. These parameters are in good agreement with those required for CSA.

Strong electromagnetic pulses (EMPs) are generated from intense laser interactions with solid-density targets and can be guided by the target geometry, specifically through conductive connections to the ground. We present an experimental characterization by time- and spatial-resolved proton deflectometry of guided electromagnetic discharge pulses along wires including a coil, driven by 0.5 ps, 50 J, 10¹⁹ W/cm² laser pulses. Proton-deflectometry allows us to time-resolve first the EMP due to the laser-driven target charging and then the return EMP from the ground through the conductive target stalk. Both EMPs have a typical duration of tens of ps and correspond to currents in the kA-range with electric-field amplitudes of multiple GV/m. The sub-mm coil in the target rod creates lensing effects on probing protons due to both magnetic- and electric-field contributions. This way, protons of the 10 MeV-energy range are focused over cm-scale distances. Experimental results are supported by analytical modeling and high-resolution numerical particle-in-cell simulations, unraveling the likely presence of a surface plasma, in which parameters define the discharge pulse dispersion in the non-linear propagation regime.

In this work, we propose and verify experimentally a model that describes the concomitant influence of the beam size and optical roughness on the temporal contrast of optical pulses passing through a pulse stretcher in chirped-pulse amplification laser systems. We develop an analytical model that is capable of predicting the rising edge caused by the reflection from an optical element in a pulse stretcher, based on the power spectral density of the surface and the spatial beam profile on the surface. In an experimental campaign, we characterize the temporal contrast of a laser pulse that passed through either a folded or an unfolded stretcher design and compare these results with the analytical model. By varying the beam size for both setups, we verify that optical elements in the near- and the far-field act opposed to each with respect to the temporal contrast and that the rising edge caused by a surface benefits from a larger spatial beam size on that surface.

We demonstrate here, that for clever stretcher designs the dependency between beam size and the slow rising edge in the ps-scale, that is present in most CPA laser systems, can be used for contrast improvements.

A new ultra-high contrast uOPA module is being developed for the PHELIX- and PENELOPE frontend-chains. This module will enhance the ASE-contrast and prevent the formation of pre-pulses through a bypass of high-gain amplifiers.