A. V. Demura

Statistical model of electron impact ionization of multielectron ions

The statistical method for ab initio calculations of the electron impact ionization cross sections of multielectron ions and related ionization rates is developed. It is created based on an idea of collective excitations of atomic electrons alike in condensed medium. The Thomas–Fermi model density distribution of atomic electrons is assumed and their collective oscillations are described by the local plasma frequency model. Using a proposed statistical approach the calculations of the total single electron impact ionization cross sections of multielectron ions for several chosen chemical elements from argon up to uranium, taken in the various charge states, are performed and compared with available experimental and theoretical data, demonstrating a satisfactory agreement.

Statistical model of radiation losses for heavy ions in plasmas

The new statistical approach for calculation of radiation processes with heavy multielectron ions in plasma is developed. The method consists in consideration of atomic structure as a condensed medium, characterized by the spectrum of elementary excitations with plasma frequency, determined by local atomic electron density. The radiation losses in this model are due to excitation of plasma type oscillations in atom under its collisions with plasma electrons and could be expressed in a universal statistical form for all sorts of multielectron ions. The calculations of radiation losses on tungsten ions are performed in the wide range of plasma temperature variation, typical for physics of high temperature plasma with magnetic confinement. It is shown that the universal statistical approach results are within the data scattering of current numerical codes. The proposed statistical method for description of collective excitations in complex atoms for calculations of plasma radiation losses is of general physical interest. It allows obtaining the necessary data faster with the lesser computational resources.

Electron impact ionization of tungsten ions in a statistical model

The statistical model for calculations of the electron impact ionization cross sections of multielectron ions is developed for the first time. The model is based on the idea of collective excitations of atomic electrons with the local plasma frequency, while the Thomas-Fermi model is used for atomic electrons density distribution. The electron impact ionization cross sections and related ionization rates of tungsten ions from W+ up to W63+ are calculated and then compared with the vast collection of modern experimental and modeling results. The reasonable correspondence between experimental and theoretical data demonstrates the universal nature of statistical approach to the description of atomic processes in multielectron systems.

An analytical model for the Ly α redistribution function in conditions of tokamak edge plasmas

Radiation redistribution is investigated for applications to magnetic fusion studies. An analytical model, suitable for Monte Carlo simulations of radiative transfer, is developed for the redistribution function of the resonance line of hydrogen isotopes. The model retains Zeeman and Stark effects in the atoms rest frame and in the impact approximation. The Zeeman effect is shown to play a major role on redistribution. Discussions are given on consequences of the Doppler effect in the laboratory frame.

Interference of radiating states and ion dynamics in spectral line broadening

The influence of the plasma coupling between the populations and the coherences on the lineshape is investigated. Ion dynamics is taken into account. The present research is performed within the atomic density matrix formalism. Ion microfield dynamics is simulated by the kangaroo-Poisson stochastic process (model microfield method). Numerical calculations of both lifetimes of radiating states and lineshapes are performed for the spectral doublet (1s-2s)2 1 S-(1s-4p)4 1 P, (1s-2)2 1 S-(1s- 4d)4 1 D of helium-like multicharged ions in hot dense plasmas. It is found that the ion microfield essentially influences the difference of populations of radiating 4 1 P; 4 1 D states. Calculation of the lineshape of the doublet (1s-2p)2 3P-(1s-4d)43D, (1s-2p)23P- (1s-4f)43F of neutral helium at astrophysical plasma conditions is also performed. The contribution of nonlinear interference effects (NIEF) in both allowed and forbidden components is calculated at various plasma conditions and a comparison with binary adiabatic theory is made. The results demonstrate that it is essential to take account of NIEF in the calculation of lineshapes of multicharged ions, but not essential in the case of neutral helium. The standard theory of spectral line broadening is based on the calculation of spectral functions of radiating states, while the density matrix of these states is considered to be diagonal and the calculation of populations of radiating states is considered as a separate problem in atomic kinetics. It is well known in laser physics (1) that simultaneous consideration of the dynamics of both polarizations at optical transitions and the density matrix of radiating states is important for lineshape calculations. The effects which are due to the nondiagonal matrix elements of the density matrix (coherences) are called nonlinear interference effects (NIEF) (1). In the case of Stark-broadened lineshapes all the matrix elements of the density matrix operator (i.e. populations and coherences) are initially coupled together by the ion microfield. The optical transition takes place between these coupled states. The importance of the plasma coupling between the initial density matrix conditions and the time evolution of the optical coherences in the lineshape calculations has been demonstrated in (2) for lines of helium-like ions within the static approximation for the ion microfield, in (3) for the L -line of H-like ions within the molecular dynamics method k Permanent address: Institute of Physics and Power Engineering, 249020, Obninsk, Russia.

Density Matrix Approach to Description of Doubly Excited States in Dense Plasmas

The density matrix approach allows to consider self‐consistently the atomic kinetics and radiation dynamics in one closed system of equations. This approach is typical for laser physics, but for Stark broadening by plasmas it was introduced in the beginning of ninetieth. The present work is aimed on the specifics of such description for the located above ionization threshold doubly excited states (DES) of multiply charged ions. The Stark profiles of dielectronic satellites, originating from DES, play an important role in the diagnostic implementations. The existence of nonlinear interference effects (NIEF) in the Stark profiles of dielectronic satellites of multiply charged ions is demonstrated in the frames of the three‐level model.

Evolution of Plasma Microfield Notion

The up to date various aspects of plasma microfield notion development is outlined.The up to date various aspects of plasma microfield notion development is outlined.

On Plasma Statistics of Microfield Gradients and Line Asymmetries

These results illustrate the high sensitivity of the line asymmetry to the ion dynamics effects and to the plasma statistics of the B D function. Moreover the main contribution of field inhomogeneity to the line asymmetry, comes from the most probable values of the microfield distribution function. This explains the little difference between APEX and MC line asymmetries at high densities. Concerning the B D functions, the deviation from the Nearest Neighbor results illustrates the influence of the N-body effects. For larger plasma densities than studied in this paper, the electronic correlations are important and the simple Debye-Huckel screening, which was used here for simplicity, becomes questionable (Gilles and Peyrusse, 1995).

Asymmetry of Stark profiles in nonuniform fluctuating microfield

The Stark broadened line shapes of atomic ions are computed up to the quadrupole terms in the interaction potential of the radiator with the plasma electric microfields and their gradients, including the interaction due to the plasma polarization effects associated with the nonuniform electron distribution around ion perturbers. The relevant universal plasma functions are evaluated in a cluster expansion or by Monte Carlo simulations, and the line shape is calculated with ion dynamic effects by the Model Microfield Method. The asymmetry of the Lyman α line of hydrogenic ions is studied.

On the nature of ion dynamics

A study of physical mechanisms responsible for the ion dynamics effects is presented. It is shown that the effect is primarily caused by the fluctuations of ion microfield direction, which can naturally be represented as rotation of the electric-microfield vector. Various aspects of this problem are discussed.