Vol 193, No 5 (2023)

We review the current state of the kinetic theory of acceleration and propagation of energetic (thermal and nonthermal) electrons during solar flares. The classical models of a thick target, together with their extension to models that take the reverse current effect and the acceleration of electrons in collapsing magnetic traps into account, are discussed in detail. Analytic solutions of the corresponding kinetic equations are found and used to calculate the characteristics of the hard X-ray bremsstrahlung generated by energetic electrons. The results obtained are compared with modern high-precision data from satellite observations of solar flares. The radiation polarization degree is calculated and the prospects for its measurement in future space experiments are discussed.

Bound states in the continuum (BICs) are a striking example of how a solution to a simple problem of quantum mechanics, obtained about a century ago, can serve as an incentive to study a wide range of resonance phenomena in wave physics. Due to the giant radiative lifetime, BICs have found multiple applications in various fields of physics studying wave processes, in particular, in hydrodynamics, atomic phys„ics, and acoustics. In this review, we present a broad view of the physics of BICs and related effects, focusing primarily on photonic dielectric structures. We consider the history of the development of BIC studies, the main physical mechanisms of their formation, and specific examples of structures that can support such states. We also discuss possible practical applications of BICs in optics, photonics, and radiophysics.

We present results on isotopically selective lutetium photoionization using dye lasers pumped by copper vapor lasers as applied to the problem of obtaining the $^{177}$Lu radionuclide for medical applications.