Laser wakefield acceleration of electrons is studied experimentally in the strongly mismatched regime. Up to 20 J, 60 fs laser pulses are focussed at the input of a gas cell with the f /46 focussing system. The resulting 44 µ m focal spot at the cell entrance is about two times greater than the matched spot size. Experimental results are supported by 3D particle-in-cell (PIC) simulations of the laser–plasma dynamics as well as by numerical simulations of the gas distribution inside the gas cell. Special attention is paid to the accuracy of spectra reconstruction affected by low pointing stability of the accelerated electron beam. It is demonstrated experimentally that the electron energy in the mismatched regime can be higher than that in the matched one and can reach GeV level for the used laser parameters.

The design of ellipsoidal plasma mirrors (EPMs) for the PEARL laser facility is presented. The EPMs achieve a magnification of 0.32 in focal spot size, and the corresponding increase in focused intensity is expected to be about 8. Designing and implementing such focusing optics for short-pulse (<100 fs) systems paves the way for their use in future high-power facilities, where they can be used to achieve intensities beyond 10²³ W/cm². A retro-imaging-based target alignment system is also described, which is used to align solid targets at the output of the ellispoidal mirrors (with a numerical aperture of 0.75 in this case).

A pioneering detailed comparative study of the dynamics of plasma flows generated by high-power nanosecond and high-intensity femtosecond laser pulses with similar fluences of up to $3\times {10}^4$ J/cm² is presented. The experiments were conducted on the petawatt laser facility PEARL using two types of high-power laser radiation: femtosecond pulses with energy exceeding 10 J and a duration less than 60 fs, and nanosecond pulses with energy exceeding 10 J and a duration on the order of 1 ns. In the experiments, high-velocity (>100 km/s) flows of «femtosecond» (created by femtosecond laser pulses) and «nanosecond» plasmas propagated in a vacuum across a uniform magnetic field with a strength over 14 T. A significant difference in the dynamics of «femtosecond» and «nanosecond» plasma flows was observed: (i) The «femtosecond» plasma initially propagated in a vacuum (no B-field) as a collimated flow, while the «nanosecond» flow diverged. (ii) The «nanosecond» plasma interacting with external magnetic field formed a quasi-spherical cavity with Rayleigh–Taylor instability flutes. In the case of «femtosecond» plasma, such flutes were not observed, and the flow was immediately redirected into a narrow plasma sheet (or «tongue») propagating across the magnetic field at an approximately constant velocity. (iii) Elongated «nanosecond» and «femtosecond» plasma slabs interacting with a transverse magnetic field broke up into Rayleigh–Taylor «tongues». (iv) The ends of these «tongues» in the femtosecond case twisted into vortex structures aligned with the ion motion in the external magnetic field, whereas the «tongues» in the nanosecond case were randomly oriented. It was suggested that the twisting of femtosecond «tongues» is related to Hall effects. The experimental results are complemented by and consistent with numerical 3D magnetohydrodynamic simulations. The potential applications of these findings for astrophysical objects, such as short bursts in active galactic nuclei, are discussed.

Experiments with interacting high-velocity flows in a laser plasma can help answer fundamental questions in plasma physics and improve understanding of the mechanisms behind some astrophysical phenomena, such as the formation of collisionless shock waves, deceleration of accretion flows, and evolution of solar and stellar flares. This work presents the first direct experimental observations of stagnation and redirection of counterstreaming flows (jets) of laser plasma induced by intense laser pulses with intensity I ∼ 2 × 1018 W/cm2. Hybrid particle-in-cell–fluid modeling, which takes into account the kinetic effects of ion motion and the evolution of the pressure tensor for electrons, demonstrates the compression of counterdirected toroidal self-generated magnetic fields embedded in counterstreaming plasma flows. The enhancement of the toroidal magnetic field in the interaction region results in plasma flow stagnation and redirection of the jets across the line of their initial propagation.

A pioneer detailed comparative study of the dynamics of plasma flows generated by high-power nanosecond and high-intensity femtosecond laser pulses with similar fluences of up to 3 × 10^4 J/cm^2 is presented. The experiments were conducted on the petawatt laser facility PEARL using two types of high-power laser radiation: femtosecond pulses with energy exceeding 10 J, duration less than 60 fs, and nanosecond pulses with energy exceeding 10 J and duration on the order of 1 ns. In the experiments, high-velocity (>100 km/s) flows of femtosecond (created by femtosecond laser pulses) and nanosecond plasmas propagated in vacuum across a uniform magnetic field with a strength over 14 T. A significant difference in the dynamics of femtosecond and nanosecond plasma flows was observed: (i) The femtosecond plasma initially propagated in vacuum (no B-field) as a collimated flow, while the nanosecond flow diverged; (ii) The nanosecond plasma interacting with external magnetic field formed a quasi-spherical cavity with Rayleigh–Taylor instability flutes. In the case of femtosecond plasma, such flutes were not observed, and the flow was immediately redirected into a narrow plasma sheet (or tongue) propagating across the magnetic field at an approximately constant velocity; (iii) Elongated nanosecond and femtosecond plasma slabs interacting with a transverse magnetic field broke up into Rayleigh–Taylor tongues; (iv) The ends of these tongues in the femtosecond case twisted into vortex structures aligned with the ion motion in the external magnetic field, whereas the tongues in the nanosecond case were randomly oriented. It was suggested that the twisting of femtosecond tongues is related to Hall effects. The experimental results are complemented by and consistent with numerical 3D magnetohydrodynamic simulations. The potential applications of these findings for astrophysical objects, such as short bursts in active galactic nuclei, are discussed.