Context. Numerical studies as well as scaled laboratory experiments suggest that bipolar outflows arising from young stellar objects (YSOs) could be collimated into narrow and stable jets as a result of their interaction with a poloidal magnetic field. However, this magnetic collimation mechanism was demonstrated only for the simplified topology of the uniform poloidal magnetic field. Aims. We have extended the experimental studies to the case of a plasma outflow expanding in a region of strong poloidal magnetic field and then propagating through divergent magnetic field lines. In this case the magnetic field distribution is closer to the hourglass magnetic field distribution expected near YSOs. Our aim was to find out whether (and under what conditions) magnetic collimation is possible in such a strongly nonuniform B-field configuration. Methods. The experiments were carried out on the PEARL high-power laser facility. The laser produced plasma outflow was embedded in a strong (~10T) magnetic field generated by our unique magnetic system. The morphology and dynamics of the plasma were diagnosed with a Mach-Zehnder interferometer. Results. Laboratory experiments and 3D numerical modeling allow us to reveal the various stages of plasma jet formation in a divergent poloidal magnetic field. The results show (i) that there is a fundamental possibility for magnetic collimation of a plasma outflow in a divergent magnetic field; (ii) that there is good scalability of astrophysical and laboratory flows; (iii) that the conditions for the formation of a magnetic nozzle, hence collimation by poloidal magnetic field, have been met; and (iv) that the propagation of the jet proceeds unimpeded through the region of weak and strongly divergent magnetic fields, maintaining a high aspect ratio. Conclusions. Since we have verified that the laboratory plasma scales favorably to YSO jets and outflows, our laboratory modeling hints at the possibility of the YSO jet collimation in a divergent poloidal magnetic field.

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.

Experiments with interacting high-velocity flows of laser plasma can help answer the fundamental questions in plasma physics and improve the understanding of the mechanisms behind astrophysical phenomena, such as formation of collisionless shock waves, deceleration of accretion flows, and evolution of solar (stellar) flares. This work presents the first direct experimental observations of stagnation and redirection of counterstreaming flows (jets) of laser plasma induced by ultra-intense laser pulses with intensity $I \sim$ 2 $\times$ $10^{18}$ $W/cm^2$. Hybrid (PIC - 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 the 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.