Laser light became a robust and universal energy source with unprecedentedly high power, energy density and application accuracy. Its use for particle acceleration and generation of secondary radiation opens a broad perspective for numerous applications in fundamental plasma and high-energy density physics, nuclear physics and laboratory astrophysics, radiation biology, materials research, and more. Achievable high temporal resolution provided by lasers is crucial for investigating transient phenomena. However, one of the current challenges is the effectiveness of laser-driven schemes that enable unprecedented high power and energy density of laser-driven sources to be achieved.

The research published in High Power Laser Science and Engineering, Volume 13, Issue 1, 2025 (O.N. Rosmej, M. Gyrdymov, N.E. Andreev, P. Tavana, V.S. Popov, N.G. Borisenko, A.I. Gromov, S. Yu. Gus´kov, R.A. Yakhin, G.A. Vergunova, N. Bukharskii, P. Korneev, J. Cikhardt, S. Zähter, S. Busch, J. Jacoby, V.G. Pimenov, C. Spielmann and A. Pukhov) demonstrates a possible solution that enables a drastic increase in the conversion of laser energy into MeV particles and radiation by direct laser acceleration (DLA) of electrons in a plasma with near critical electron density.

In this scheme, the electron acceleration occurs in the presence of strong quasi-static electric and magnetic fields generated in plasma. Strongly nonlinear action of the laser beam creates a channel with a strong radial electrostatic and azimuthal magnetic field. Relativistic electrons appear to be trapped in the channel and experience transverse betatron oscillations resonantly gaining energy direct from the laser pulse when the frequency of the betatron oscillations matches the Doppler shifted laser frequency. The DLA demonstrated its high efficiency for sub-ps laser pulses like PHELIX at GSI in Darmstadt. Different from laser wake field acceleration (LWFA), the DLA does not generate electrons at very high energies, rather, it produces ample amounts of electrons with Boltzmann-like distributions carrying mega-ampere currents.

Low-density foam pre-ionized with a well-controlled nanosecond pulse is an excellent plasma target for effective DLA driven by relativistic laser pulses. Up to 20–40° of the laser energy can be transferred to MeV-energy electrons, which play a crucial role in accelerating protons, producing betatron radiation, MeV bremsstrahlung, neutrons, and more.

Experiments on the interaction of relativistic laser pulses with pre-ionized foam targets, conducted at the PHELIX facility, have demonstrated greatly improved conversion of laser energy into energy of MeV particles and radiation. The interaction of sub-ps laser pulses at ~10¹⁹ W/cm² with pre-ionized foams produces directed high-current beams of DLA electrons with energies above 100 MeV and an effective temperature and charge up to 10 to 15 times higher than in case of conventional foil targets. By using foams in combination with µm-thin foils or mm-thick high-Z converters, we successfully demonstrated generation of ultra-bright bremsstrahlung with photon energies of up to 50-60 MeV and a record-breaking conversion efficiency of up to 2% for photons > 7.5 MeV. We also demonstrated record-breaking neutron production efficiency in gamma-driven nuclear reactions, super-intense betatron radiation, and Target Normal Sheath Acceleration (TNSA) with a 1.5-fold increase in the proton energy threshold. Our results pave the way for the application of low-density foams at kJ PW laser facilities in High Energy Density (HED) and Inertial Confinement Fusion (ICF)-relevant research, as well as in laboratory nuclear astrophysics with lasers.

The idea of the pre-plasma engineering with low-density foams aims to optimize the relativistic electron flux. Polymer foams up to 0.5–1 mm thick with a volume density of 2 mg/cm³, pre-ionized by a well-controlled nanosecond pulse, are perfectly suited as plasma targets for the direct laser acceleration of electrons with high-energy, high-power sub-ps relativistic laser pulses.

This work investigates the influence of the nanosecond pulse on the DLA process. The density profile of the plasma generated after irradiating the foam with a ns pulse was simulated using a two-dimensional hydrodynamic code NUTCY-F that takes into account the properties of partly-homogenized plasma of microstructure polymer foams. Obtained plasma density profile served as input for the Virtual Laser Plasma Laboratory (VLPL) 3D PIC code to simulate the energy, angular distributions, and charge of the directed fraction of the DLA electrons. The modeling shows good agreement with experiments and, in general, a weak dependence of the electron spectra on the plasma profiles, which encompass the density up-ramp and the region of the near critical electron density. This explains the high DLA stability in pre-ionized foams, important for applications.

Figure Caption: (a) Polymer foam structure and foam within the Cu washer; (b) ns-pulse to convert foam to plasma and delayed relativistic ps-pulse to trigger DLA; (c) Electron spectra measured at different angles to the laser axis showing DLA electrons accelerated in the forward direction to energies of 90–100 MeV at 10¹⁹ W/cm² laser intensity; (d, e) 2D plasma density profile 3ns after exposure to the 10¹³ W/cm² ns pulse on 2 mg/cc 450 µm thick foam and a density profile along the ps laser beam, including a plasma density up-ramp and an NCD part. (f) Density profile along the ps laser beam 4.5 ns after exposure to the 3·10¹⁴ W/cm² ns pulse with a bell-shaped profile and 0.06 n_cr maximum electron density; (g) Electron energy distributions as result of 3D PIC simulations for different plasma profiles.

Low-density foams offer a good opportunity to generate plasma profiles for electron acceleration at multi-petawatt laser facilities by adjusting foam and ns-pulse parameters. The bell-shaped density profile with a peak density at 0.06 n_cr (Fig.1d), which occurs when the shock wave initiated by the ns pulse reaches the foam´s rear, can be used for electron acceleration with multi-PW short pulses.

In the follow-up work, we apply the concept of a low-density, pre-ionized foam plasma target in experiments at ELI (2025), ARC/NIF (early 2026), and PETAL/LMJ (2027). In preparation for the upcoming beam times at the high-energy PW-class lasers, PHELIX was used as a testbed to investigate electron acceleration in foams at large laser focal spot and near-relativistic laser intensity, characteristic of ARC. At laser intensity of 10¹⁸ W/cm², we achieved a breakthrough in generating directed electron beams with an effective temperature of up to 10 MeV, about 100 times higher than the temperature typically observed for laser shots at conventional foil targets (0.1 MeV). The combination of foam layer with high-Z converter enables the generation of gamma rays with energies up to 30 MeV and production of isotopes and neutrons in photonuclear reactions at near relativistic laser intensities and below.