Plasma and Thermal Physics

Symmetrically designed fusion and heat concepts, space propulsion, and energy conversion issues with a particular interest in kinetic analysis, plasma power balance, advanced fuels, and alternative systems as new trends in experiments and theory, in physics, power engineering, and in very specific related areas such as space processes, cosmology, and turbulence are very important in fundamental and applied science from an engineering physics perspective.

Plasma and thermal physics have collaborated to increase our understanding of several phenomena. In this Special Issue, some recent advances in the modeling of plasma flows, fusion systems, thermal physical properties and particle simulations in strong magnetic and electric fields are reviewed.

This Special Issue consists of eleven papers, including four reviews, recently published in MDPI’s journal Symmetry under the general thematic title “Plasma and Thermal Physics” (see [1,2,3,4,5,6,7,8,9,10,11]).

In Ref. [1], heat transfer and entropy of steady Williamson nanofluid flow based on fundamental symmetry is studied. The fluid is positioned over a stretched flat surface moving non-uniformly. The nanofluid is analyzed for its flow and thermal transport properties by consigning it to a convectively heated slippery surface. Thermal conductivity is assumed to vary with temperature impacted by thermal radiation along with axisymmetric magnetohydrodynamics (MHD). The resulting ODEs are solved via a finite difference-based Keller box scheme. Various interesting physical parameters that affect fluid movement, difference in temperature, system entropy, skin friction and Nusselt numbers around the boundary are graphically presented and numerically discussed.

Ref. [2] is devoted to the calculation and theoretical analysis of physical processes in the powerful electric discharge sources of UV radiation and shock waves with required and controlled technical and physical characteristics. Based on the calculations, the processes of converting the initially stored electromagnetic energy into internal, kinetic, magnetic and radiation energy formed in the electro-discharge plasma sources of plasma formation were studied, and the interactions of discharged plasma and its radiation with matter in different aggregate states were also studied. All the main magneto-plasma dynamic and radiative parameters of plasma formation in the electric discharge sources of UV radiation and shock waves were obtained.

Paper [3] deals with the theoretical estimation of the strong-field-modified vibrational parameters of three-atomic ABA molecules with D∞h symmetry in laser-induced plasma. The linear CO22 and CS22 molecules in the X1Σg state are considered as examples. We show that double degeneracy of the Πu mode is removed due to a reduction in the ABA molecule symmetry by the laser field. The linear form of the ABA molecule is preserved, with the bond length being elongated.

In Ref. [4], the presence of the double-layer (DL) was confirmed by a drastic change in the plasma potential along the axis, and a specific impulse of 1100 s was indirectly measured. The measurement results show the potential of the WPS based on the open-ended chamber for efficient operations.

In manuscript [5], the dynamic electrical conductivity of dense Zr plasma near melting is calculated using ab initio molecular dynamics and the Kubo–Greenwood formula. The antisymmetrization of the electronic wave function is considered with the determinant of one-electron wave functions; exchange and correlation effects are treated via an exchange–correlation functional. Optical properties are restored using the Kramers–Kronig transformation. The influence of computational parameters and inner shell electrons on the results is thoroughly investigated. We demonstrate the convergence of our computations and carry out comparisons with the experimental data.

In research paper [6], the polarization dependence of the cross sections of two-photon transitions including X-ray scattering was analyzed. We developed the regular approach to the derivation of the polarization parameters of photoprocesses. Our approach is based on the tensor representation of the photon density matrix, which is written in terms of the unit vectors directed along the major axis of the polarization ellipse and the photon propagation. Explicit expressions for the product of two photon density matrices were derived. As an example, the parametrization of the polarization dependence of the X-ray scattering by closed-shell atoms is given both in terms of (i) scalar products of photon vectors and (ii) X-ray Stokes parameters. We discuss the applicability of atomic scattering for the measurement of the polarization of X-rays.

A detailed review [7] of computational–theoretical and experimental works devoted to the study of the flow of bodies by two-phase (dispersed) flows was carried out. The features of particle motion in the vicinity of bodies of various shapes, as well as the effect of the dispersed phase on resistance and heat transfer, are considered. Some consequences of the interaction of particles and droplets with the surface of streamlined bodies (erosive destruction, gas-dynamic spraying, icing, glowing) are analyzed.

The review [8] is intended to some extent to fill in the currently existing gap associated with the absence of dimensionless criteria (or complexes of physical parameters) responsible for the direction (attenuation or enhancement) of turbulence modification, and the value of this change. Possible directions for further research are given in the conclusion of the review.

The objective of the review [9] is to describe the specific structure of refractory metals after high-temperature plasma irradiation and the potential application of plasma processing of materials in order to create heat exchange surfaces that provide a significant intensification of two-phase heat transfer. Refractory metals with such a highly porous, rough surface can be used as plasma-facing components for operation under extreme heat and plasma loads in thermonuclear and nuclear reactors, as catalysts for hydrogen production, as well as in biotechnology and biomedical applications.

A review [10] of calculation–theoretical and experimental works devoted to the study of the motion of particles (or droplets) in various concentrated vortex structures, as well as their inverse effect on vortex characteristics, was conducted. The main characteristics (inertia, concentration) as well as dimensionless parameters (Reynolds, Stokes, Froude, Tachikawa numbers) determining the interaction between the dispersed phase and vortices are described.

Article [11] addressed the problem of the influence of the properties of the profile of a radial static electric field E(r) on the evolution of an unstable ion temperature-gradient (ITG) drift wave in a non-uniformly rotating plasma column in a magnetic field. The effect of symmetry on the decrease in the level of turbulent fluctuations, which are associated with the limiting state of the ITG-wave during its destruction, is discussed. The level of turbulence is estimated in the framework of approximation of finite amplitudes, depending on the electric field structure. It is shown that the maximum decrease in the level of fluctuations occurs with a symmetrical configuration of the radial electric field.

Finally, I would like to thank all the contributors to the Special Issue for sharing their research, and the anonymous reviewers for donating their time to improve the quality of the submitted papers.