Table of contents

A review is given of the current status of theoretical and experimental investigations of laser thermonuclear fusion. A wide range of physical problems encountered in fusion research and ways of solving them are discussed. Attention is concentrated on current problems such as fusion energetics, adiabatic target compression, conditions for development of hydrodynamic instabilities, absorption of laser radiation in plasma, and plasma heating. Detailed analysis is made of the results of numerical fusion simulations that have been carried out subject to various assumptions relating to the type of target, laser pulse parameters, and predominant processes in the laser plasma. Some results are given of recent

The physical principles are given of laser jet engines, in which the source of energy ("fuel") is a laser source of electromagnetic energy located outside the flight vehicle being accelerated. An analysis is made of two mechanisms of producing a thrust: one is the evaporation mechanism involving the evaporation of the propulsive mass of the flight vehicle by the incident laser radiation and the other is the "explosive" mechanism due to the laser-induced breakdown of air ("explosion") and excitation of a shock wave which can propel the flight vehicle. The first mechanism can be used in the atmosphere and in space, whereas the second can be used only in the atmosphere (laser air-breathing jet engine). The main characteristics of laser jet engines (the ratio of the laser power to the thrust obtained, specific impulse, and power conversion efficiency) are obtained and some of them are confirmed experimentally. The promising nature of the use of lasers in jet engines of this kind is stressed.

This review treats the little-studied magnetic phase transitions of the order-order, spin-reorientation type, in which a rotation of the magnetization vector (or of die antiferromagnetism vector) with respect to the crystallographic axes occurs upon change of temperature, magnetic field, or external pressure. It presents theoretical ideas about magnetic phase transitions of the spin-reorientation type; it considers the effect of domain structure on spin-reorientation phase transitions, aad critical fluctuations during such transitions. Taking as an example rare-earth magnets (orthoferrites, iron-garnets, and rare-earth metals and intermetallic compounds), it considers the experimental data on various types of spin-reorientation transitions and on the anomalies of physical properties at such transitions.

We present the theoretical foundations of the method of using multipass Fabry-Perot interferometers and an iodine light filter to get high-contrast spectra. Plane Fabry-Perot interferometers are considered with two, three, or five successive passes of the light through them, as well as one- and two-pass spherical Fabry- Perot interferometers, and also their tandem combinations. Examples are given of experimental studies of molecular light-scattering spectra in phase transitions in imperfect crystals and liquid crystals, pressed powders, opaque crystals, and also acoustic magnon spectra. The resultant contrast of the spectra was as large as 10¹² when multipass and tandem interferometers were used, and 10⁵ when an iodine light filter was used.

A review is given of the first investigations formulating the idea of direct conversion of thermal energy into coherent electromagnetic radiation in quantum oscillators. It is shown that the operation of a gasdynamic CO₂ laser can be described, from the energy point of view, by the second law of thermodynamics.

The main ideas and methods currently used to obtain upper limits on the photon rest mass from astrophysical data are briefly reviewed. One method is based on the fact that if the photon has nonzero rest mass m magnetoacoustic waves cannot have frequencies lower than a certain critical frequency, which depends on m. If wisps in the Crab Nebula are interpreted as magnetoacoustic waves of extremely low frequency, then the data of many-year observations of the wisps put an upper limit on m which is much better than the one established under terrestrial conditions. An error is pointed out in a different method which has been proposed in the literature in which the contribution of the energy of the galactic magnetic field (or rather, the vector potential) to the mass of Galaxy is estimated on the basis of gravitational effects. The point is that in the general theory of relativity not only energy but also pressure has a weight. In the case under consideration, these two contributions cancel each other and the galactic magnetic field cannot produce anomalously strong gravitational fields. The most stringent upper limit on the photon rest mass, m≲ 3·10^–60 g, is obtained from the analysis of the mechanical stability of magnetized gas in the galaxies with allowance for the specific pressure forces of the vector potential. This upper limit is 12 orders of magnitude better than the best upper limits obtained under terrestrial conditions. This result clearly demonstrates the effectiveness of astrophysical methods.

The present state of theoretical and experimental research on forbidden rotational transitions in polar and nonpolar molecules, initiated mainly in 1971, is reviewed. Three mechanisms whereby forbidden rotational transitions appear are considered: 1) centrifugal distortion, 2) anharmonicity of the potential energy, 3) electron-vibrational interaction (the Jahn-Teller effect). A method is described for calculating the effective dipole moment and the strength of a transition, based on sucessive contact transformations of the Hamiltonian and of the dipole moment operator. Formulas are given for the line strengths of the most important classes of molecules. Results are presented of experimental investigations of centrifugal transitions of tetrahedral hydride molecules by the double resonance and microwave and infrared spectroscopy methods. Particular attention is paid to Jahn-Teller transitions, which have not yet been observed. An approximate estimate is made, for the first time, of the dipole moment of such transitions in triply degenerate electronic states of molecules (such as the ground state of methane) and it is shown that the Jahn-Teller transition strengths can be of the same order as of the anharmonic and centrifugal transitions.