E. D'HUmieres

We report the generation and transport of laser-driven discharge currents in the sub-100 ps, kiloampere regime. A target and transport architecture was developed to mitigate dispersion, distortion, reflections, and the risk of electrical breakdown, enabling efficient extraction and guided propagation of the laser-induced discharge current. Experiments performed with a petawatt-class laser demonstrate measured peak currents up to 1.6 kA into 50 Ω (∼80 kV) with a measured pulse duration of 70 ps, limited by the 6 GHz diagnostic bandwidth. Bandwidth-corrected extrapolation suggests intrinsic pulses of a few tens of picoseconds with multi-kiloampere amplitudes, requiring confirmation by higher-bandwidth measurements. These results establish laser-driven targets as a viable source of ultrafast, high-amplitude electrical pulses and open new opportunities for applications in pulsed power and strong-field science.

The expansion of two circular rarefaction waves in vacuum or in a thin ambient plasma is examined with particle-in-cell simulations that resolve two spatial dimensions. In the simulation with no ambient plasma, the rarefaction waves interpenetrate near the symmetry line between both rarefaction wave centers. The exponential density decrease in rarefaction waves with distance implies that the sum of their density does not lead to a density maximum near the symmetry line. The absence of a density maximum, which would yield a repelling electric potential for the inflowing rarefaction wave ions near the symmetry line, and the high interpenetration speed of the ion beams lead to ion–ion instabilities rather than shocks in the overlap layer. The simulations with ambient plasma show that the rarefaction waves pileup the ions of the ambient plasma near the symmetry line. A localized piston of hot ambient ions forms. If its density is large enough, its thermoelectric field allows reverse shocks to grow in the rarefaction waves. These reverse shocks move slowly in the simulation frame and enclose a slab of downstream plasma. A decrease in the speed of the rarefaction wave ions upstream of the shocks with time leads to their collapse.

Stimulated Brillouin and Raman scattering (SBS and SRS) are instabilities that affect the propagation of high-power lasers in plasmas. The latter is further affected by cross-talk (CT) effects when multiple laser beams are simultaneously propagated in the plasma, as found in the schemes proposed for inertial confinement fusion (ICF). Here, we develop a new theoretical model that allows us to evaluate the impact of CT on SBS and SRS in low-density plasmas. As supported by experiments, we demonstrate that CT can lead to a reduction of both SBS and SRS, due to the destabilization of the individually triggered instabilities. We further demonstrate that this destabilization effect is accelerated by applying an external magnetic field to the plasma, which is also beneficial for the hydrodynamics or fuel heating of ICF targets. By shedding new light on the promising scheme of magnetized ICF, our findings thus offer beneficial prospects for ICF.

This paper reports a numerical study based on previously published experiments with high-power laser systems, in which high-energy bunches were observed in proton spectra. In these experiments, thin targets were irradiated with hundreds of joules and pulse durations of several hundred femtoseconds. We carried out a simulation campaign combining hydrodynamic and particle-in-cell codes to analyze the origin of the proton energy bunch. The study demonstrates how laser parameters, target thickness, and ionization dynamics influence proton bunch formation. Possible acceleration mechanisms were explored, and the role of a Coulomb piston of highly ionized carbon ions in shaping the proton spectrum within the target normal sheath acceleration regime is demonstrated.

This paper investigates the impact of self-consistency in modeling the laser-driven post-acceleration of proton beams using a helical coil target (HC). We adapt and apply the 1D Hamiltonian multi-particle traveling-wave tube model, DIMOHA, to simulate the HC post-acceleration. While DIMOHA does not incorporate the effects of the radial component of the coil and space charge fields, it effectively captures the axial interaction between accelerated protons and the coil's electric field. A key advantage of DIMOHA is that it is a self-consistent code significantly faster than traditional Particle-In-Cell codes, offering an efficient balance between computational time and modeling accuracy. Using DIMOHA, we present a method to assess the significance of self-consistent effects in the post-acceleration process. Through extensive simulations, we explore various beam densities and RF pulse amplitudes to identify the conditions under which self-consistency becomes critical. Our findings show that in setups with high beam densities, self-induced fields modify proton bunch energies within the range of shot-to-shot experimental fluctuations. Furthermore, under typical TNSA conditions, where the bunch energy remains significantly below the cutoff energy, these effects appear negligible for the high-energy population. Thus, simplified models with precomputed fields can be reliably used to satisfactorily evaluate proton spectra.

The expansion of a dense plasma into an ambient plasma with a step-like distribution is studied with a series of one-dimensional particle-in-cell (PIC) simulations. Several electron-to-proton temperature ratios Te/Tp in the ambient plasma and density ratios nr between the dense and ambient plasmas are examined. The collisionless plasma is unmagnetized. If nr≥10, hybrid structures form, which are composed of an electrostatic shock for the ambient plasma and a double layer for the expanding dense plasma. If nr=5, the electrostatic shocks become separated from the rarefaction waves by a contact discontinuity. If Te/Tp=5, the steepening of the electrostatic shock is halted by dissipation of wave energy by Landau damping. Its transition layer is wide, it propagates at the ion acoustic speed in the downstream frame of reference, and it hardly compresses the ambient plasma that crosses its transition layer. The contact discontinuity is wide, and the total density varies across its layer, creating a thermoelectric field. With a temperature ratio of Te/Tp=100, shock steepening is halted by wave dispersion. The shock transition layer is thin; it doubles the density of the ambient plasma that crosses it as expected, and the contact discontinuity is thin and has a uniform density across its layer. PIC simulations with nr=5 and several ratios Te/Tp between 5 and 100 show how the electrostatic shock changes from being dissipative to dispersive. Non-linear Landau damping allows the dispersive shock collapse, changing it into an ensemble of waves resembling solitary ion acoustic waves.

Laser-driven fusion reactions for the development of bright particle sources or the production of short-lived isotopes remain an active research theme. We propose a comprehensive analysis and a fast modeling approach to highlight optimization strategies on the fusion yields stemming from advanced target designs. Particle-in-cell simulations were supplemented with an inline Monte Carlo approach to evaluate the proton–boron fusion events under direct laser illumination for planar and spherical target geometries. The time evolution and the spatial localization of the nuclear reactions are systematically compared to reveal a multi-phase mechanism in the folded geometries. For both architectures, a shared initial phase of fusion occurs in the laser spot vicinity, following from ions accelerated by the laser radiation pressure forward into the target. Meanwhile, the circulation of hot electrons issued from the laser-plasma interaction leads to the development of sheath fields along the target walls. In a closed geometry, the corresponding accelerated ions may be directed toward dense parts of the target, significantly increasing the number of created α-particles. We analyze the energy distribution of the ion populations issued from the different acceleration mechanisms and assess the corresponding efficiencies in the fusion processes. The use of a double cavity allows for further exploitation of the sheath field acceleration, raising the α yield almost an order of magnitude higher than for the planar case.

We present a comprehensive study unveiling advancements in α particle spectra manipulation that can be achieved through the implementation of a helical coil target with tube (HCT). Leveraging particle-in-cell simulations, we demonstrate the ability to control the energy distribution of α particle bunches within a narrow range, down to a few MeV. This development marks the first instance of successful ion energy manipulation facilitated by the HCT configuration. Importantly, our investigations reveal a significant enhancement in radioisotope production, with yields ranging from 10 to 3000 times greater with an HCT than without. These findings underscore the transformative potential of the HCT approach in improving α particle production in the context of laser-plasma acceleration and its consequential impact on radioisotope production for diverse applications. We also investigate the production of radioisotopes using proton acceleration with the HCT configuration and demonstrate that the yield can be increased by a factor of 30.

We present the results of the second commissioning phase of the short-focal-length area of the Apollon laser facility, located in Saclay, France. This phase was conducted using the main laser beam (F1), scaled to a peak power of 2 PW. Under the tested conditions, the F1 beam delivered on-target pulses with a maximum energy of up to 45 J and a duration of 22 fs. Several diagnostics were deployed to assess the facility's performance. Key measurements included the on-target focal spot and its spatial stability, along with characterizations of secondary sources generated by irradiating solid targets. These evaluations aim at assisting users in designing future experiments. The laser-target interactions were thoroughly characterized, with emissions of energetic ions, x rays, and neutrons recorded, demonstrating good laser-to-target coupling efficiency. Additionally, we successfully demonstrated the simultaneous operation of the F1 beam with the auxiliary 0.5 PW F2 beam of Apollon, enabling dual-beam operation. This commissioning phase paves the way for the next stage in 2025, which will involve scaling the F1 beam to 8 PW, progressing toward the ultimate goal of achieving 10 PW power.

Laser-driven ion acceleration, as produced by interaction of a high-intensity laser with a target, is a growing field of interest. One of the current challenges is to enhance the acceleration process, i.e., to increase the produced ion energy and the ion number and to shape the energy distribution for future applications. In this paper, we investigate the effect of helical coil (HC) targets on the laser–matter interaction process using a 150 TW laser. We demonstrate that HC targets significantly enhance proton acceleration, improving energy bunching and beam focusing and increasing the cutoff energy. For the first time, we extend this analysis to carbon ions, revealing a marked reduction in the number of low-energy carbon ions and the potential for energy bunching and post-acceleration through an optimized HC design. Simulations using the particle-in-cell code SOPHIE confirm the experimental results, providing insights into the current propagation and ion synchronization mechanisms in HCs. Our findings suggest that HC targets can be optimized for multispecies ion acceleration.