[en] The current status of the problem of electron–ion collisions in strong electromagnetic fields is presented. The collision operator is expressed in terms of an integral over test particle trajectories for an arbitrary alternating field. The equation for test particles is analyzed. It is shown that none of the energy processes involved (Joule heating, bremsstrahlung and fast electron generation) diminishes as the electromagnetic field amplitude increases. The collision frequency, the momentum distribution of fast electrons, and the electron–ion collision operator are calculated in the classical framework. (reviews of topical problems)

[en] Electron-ion collisions in relativistically strong electromagnetic fields are considered. Analytical and numerical analyses both show that all qualitative effects characteristic of collisions in nonrelativistic strong fields [1-3] occur at relativistic intensities of an electromagnetic wave as well. Expressions for Joule plasma heating and for the energy distributions of fast particles are derived from simple analytic considerations and are confirmed by numerical simulations. It is found, in particular, that, due to the relativistic increase in the mass of a scattered electron, Joule heating in ultrarelativistic fields becomes more intense as the field amplitude grows

[en] Collisions of electrons with ions in the presence of an alternating electromagnetic field are considered. Based on the first principles (the Liouville equations for N particles), a general expression for the collisional operator in the approximation of pair collisions at an arbitrary scattering potential, including that depending periodically on time, is derived. The problem of collisions in plasma in the presence of an electromagnetic field can be reduced to this case by introducing drift coordinates. It is shown that the method of test particles can be applied to the problem of particle collisions in an alternating electromagnetic field.

[en] Electron-ion collisions in a high-density plasma in strong electromagnetic fields are considered. The applicability condition for the approximate model of pair collisions in strong fields are determined. It is shown that this condition is identical to the condition for the plasma to be transparent. Investigations were carried out by the test particle method generalized to the case of several scattering centers. An accurate calculation of short-range collisions is provided by a 'jump' method that is based on the exact solution to the problem of the motion of a particle in a Coulomb potential. This method can also be applied in other approaches to simulating a collisional plasma (such as particle-in-cell and molecular dynamics methods).

[en] A numerical analysis of the electron cyclotron (EC) wave beam propagation in the presence of edge density fluctuations by means of a quasi-optical code [Balakin A. A. et al, Nucl. Fusion 48 (2008) 065003] is presented. The effects of the density fluctuations on the wave beam propagation are estimated in a vacuum beam propagation between the edge density layer and the EC resonance absorption layer. Consequences on the EC beam propagation are investigated by using a simplified model in which the density fluctuations are described by a single harmonic oscillation. In addition, quasi-optical calculations are shown by using edge density fluctuations as calculated by two-dimensional interchange turbulence simulations and validated with the experimental data [O. E. Garcia et al, Nucl. Fusion 47 (2007) 667].

[en] The propagation and absorption of microwave radiation at the periphery of a tokamak plasma under ECR conditions are considered. For microwaves propagating in a quasi-transverse direction, the range of plasma parameters is determined in which the effective plasma permittivity can be approximated by a piecewise linear function. With this approximation, it is possible to obtain analytic solutions to the wave equation and to use them to estimate the width of the power deposition region for different modes of microwave launching. A detailed analysis is given of tangential launching-the propagation of microwaves along a tangent to the resonance magnetic surface at which they begin to be absorbed under ECR conditions. Using the ITER tokamak as an example, it is shown that this launching method is most efficient in providing the narrowest power deposition profile at the plasma periphery. The results obtained are of interest for the problems of suppressing tearing-mode instabilities by localized ECR heating

[en] The formal procedure for the formulation of the relation between displacement and electric fields in operator form is suggested for a smoothly inhomogeneous media with spatial dispersion. This procedure allows one to estimate and consequently reduce errors caused by a lack of information about microscopic processes. Expressions for power flux, power density and power deposition are derived for these media. (paper)

[en] A heuristic procedure is proposed for deriving quasi-optical equations for wave beams in anisotropic gyrotropic media with allowance for aberrations, spatial dispersion, and absorption. To solve such equations numerically, a method is developed that generalizes the operator exponent method. The applicability limits of the aberration-free approximation for simulating the propagation of beams in absorbing media are determined. Numerical examples of the propagation of beams in the vicinity of the electron cyclotron resonance in plasmas in actual devices are presented

[en] Electron-ion collisions in plasma in a strong electromagnetic field are considered in the ultrarelativistic limit (in which the vector potential A is such that a = eA/mc² >> 1). Expressions relating the electron drift coordinates and momentum to those in the laboratory frame are obtained using exact canonical transformations with allowance for adiabatic effects. The appearance of ultrafast particles with a maximum energy proportional to the third power of the laser pulse vector potential is predicted. Expressions for the energy (and number) distribution function of such high-energy (hot) electrons appearing as a result of electron-ion collisions are obtained. These distribution functions obey a power law, which agrees with the results recently obtained by Mangles et al. in experiments with a petawatt laser

[en] The quasi-optical description of wave beams is extended to smoothly inhomogeneous anisotropic and/or gyrotropic media. A method of deriving quasi-optical equations for a slow-varied scalar beam amplitude from Maxwellian equations is proposed. Transversal localization of the beam near the central ray allows one to separate and sequentially derive the refractional, focusing (or defocusing) and aberrational terms. The correct relationship between the scalar beam amplitude and the vector electric field is conserved for every order. For simplicity, a cold collisionless magnetized plasma is considered. However, all basic results are valid, with minor amendments, for media with spatial dispersion. The aberration-free parabolic-like equation for a complex beam envelope is considered in detail. Its physical properties are investigated and a general solution is found