Sergey Stukachev

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.

Subject of study. The paper describes the development of an optical radiation polarization analyzer for the Tachomag, a space-based solar spectromagnetograph for the International Space Station. This is the third paper in a series of articles devoted to the development of a solar magnetograph planned for deployment on the Russian segment of the International Space Station. The first two articles of the series, published earlier in this journal, described a solar telescope and an optical diffraction spectrograph, which are also components of the Tachomag, the spectromagnetograph developed for the International Space Station. Aim of study. This study developed a parallel-type polarization analyzer for the Tachomag with the necessary resolution and accuracy to measure all the components of the Stokes vector in selected spectral lines as well as the speed required to study the dynamics of fast processes in the solar photosphere. Method. This paper, for the first time, presents a polarization analyzer for a space solar magnetograph, which operates on the principle of simultaneous data acquisition under all polarizations. The characteristic features of the device include the absence of traditional polarization modulators in the composition as well as small dimensions and weight characteristics, which are particularly important for space devices. Main results. We determined that even the space version of the parallel polarization analyzer provided spectrum imaging under various polarizations with the required angular resolution of 0.35^′′ according to the Rayleigh criterion in a field of view of 5^′, along with a spectral resolution of 30 mÅ in a range of 2.52 Å, which corresponded to the characteristics of the solar optical telescope and spectrograph of the Tachomag. Practical significance. The development of the Tachomag as a spectromagnetograph for the International Space Station will aid the mitigation of several urgent issues in solar and plasma physics and will create a reserve for preparing for future complex missions related to solar research from close distances.

Subject of study. This paper is the second part of a series devoted to the development of the optical spectrograph designed for the Takhomag-International Space Station space-based spectromagnetograph planned to be deployed in the Russian segment of the International Space Station. The first paper of the series published in this journal describes a solar optical telescope in the spectromagnetograph. Aim of study. This study aimed to design an optical spectrograph for the Takhomag-International Space Station spectromagnetograph with properties required for the measurement of the Zeeman effect at selected spectral lines under magnetic fields characteristic of the solar photosphere. Method. The design of the Takhomag-International Space Station spectrograph is a classic variation of the off-axis lens scheme of the Littrow spectrograph. FeI 6301.5 Å and FeI 6302.5 Å lines were chosen as working spectral lines. The lens-based spectrograph design enabled it to satisfy the requirements of its limited size without optical elements with aspherical surface shapes. Main results. Despite the hard constraints on mass and dimensions owing to the conditions of operation, the spectrograph enables the formation of almost aberrationless images (Marechal parameter exceeding λ/20) of the solar photosphere spectrum with an angular resolution of 0.35″ by the Rayleigh criterion that corresponds to the angular resolution of the solar optical telescope of the Takhomag-International Space Station spectromagnetograph with a spectral range of 2.52 Å and spectral resolution of approximately 30 mÅ. The spectral resolution is smaller than the widths of the spectral lines used in the range of 42.4 mÅ in intense spots to 49.1 mÅ in a calm photosphere that results from the chaotic motion of atoms along the line of sight and the unresolvable structure of microturbulent velocities. Practical significance. The development of the Takhomag-International Space Station spectromagnetograph will facilitate the solution of relevant problems of solar and plasma physics and provide the groundwork for preparing more complex missions involving solar research from small distances.