S. Alexiou

ASPECTS OF PLASMA SPECTROSCOPY : RECENT ADVANCES

Abstract Two important recent developments in Stark broadening are presented in this work: The first describes the Frequency Separation Technique (FST), which is motivated by the standard electron-ion separation and has important applications in that, coupled to any of a variety of methods capable of treating the intermediate ion-dynamical regime, (but not capable of treating ion impact), presents a unified and practical solution to the ion-dynamical problem. That is, this technique allows the relaxation of the quasistatic approximation. The second describes recent improvements that allow the accurate calculation of electron impact widths.

FREQUENCY SEPARATION METHOD FOR RELAXATION PROBLEMS

The modern era in spectral line broadening began with the understanding that the slow(quasistatic) ion and fast(impact) electron perturbers could be treated separately. The problem remained of unifying these two theoretical limits. A scheme for this unification is presented here that has at its foundation a fundamental observation that is supported by analytical theory and is further demonstrated by computer simulation. The fundamental observation is that the ions and electrons can be separated most of the time, and that a frequency separation within each perturber subsystem can be used for unification. That is, the rigorous inclusion of slow, but not necessarily static ions together with the correct impact ion perturbations, will produce valid ionic line shapes. We show that a frequency separation may be effected to exactly include the fast modulation limit in a variety of modern methods that can deal with the intermediate regime between the fast and slow frequency limits of the perturbation.

On the pure dipole shift for hydrogen lines in a plasma

In the light of recent discussion on the shift of hydrogen lines in dense plasmas we give an analytic proof that within the standard assumptions of no quenching, no fine structure and density matrix factorization the dipole shift is identically zero, regardless of the details of the interaction, as long as it is dipole. This refutes some theoretical explanations that match observed shifts by invoking various effects based on a dipole interaction.

Hyperbolic trajectory parametrization for spectral line broadening calculations

Abstract In this work we express the parameter u employed for the parametrization of the hyperbolic trajectory in a Coulomb potential in terms of time. These expressions are useful for ionic lineshape calculations by the independent particle simulation method, permitting benchmark calculations to test simplified methods.

Problems with the use of line shifts in plasmas

In this work a nonperturbative semiclassical treatment with proper account of penetrating collisions and monopoles is used to illustrate the relative importance of penetration, close collisions and monopoles to the line shift. The main result of this work is that shifts are very sensitive, not linear in the density and even the sign of shifts may be affected by fairly fine effects, such as small deviations from thermal behavior.

Fast analytic formulas for the modified Bessel functions of imaginary order for spectral line broadening calculations

Abstract In this work we review the status of the standard line broadening theory for plasmas and fill in the existing gap, i.e., the partially overlapping case for ions lines, by deriving expressions as well as fast and accurate numerical approximations for the relevant functions, namely the modified Bessel function of imaginary order and its derivative with respect to argument. These functions also arise in the context of the theory of Coulomb excitation.

Width functions for the perturbative dipole impact line-broadening theory with correct unitarity cutoff

Abstract In this work accurate analytic formulas are given that enable the correct determination of the minimum impact parameter ϱ min ( v ) for isolated ion lines in the dipole approximation.

IMPROVED CLOSED-FORM FORMULAS FOR THE COLLISION OPERATOR FOR ISOLATED ION LINES IN THE STANDARD STARK-BROADENING THEORY

Abstract In a recent paper, Alexiou and Maron gave a closed-form formula for the collision operator for Stark broadening of isolated ion lines. In this work we improve on the above-mentioned work by giving simpler and more accurate expressions. As in the work of Alexiou and Maron, the new expressions involve no integrations and assume a pure dipole interaction, and a Maxwellian electron velocity distribution. As in the original work by Alexiou and Maron, these expressions are exact within the stated approximations, and apply equally to hot and cold plasmas as long as the above approximations are satisfied. Further, they also give an estimate of the theoretical error. The present formulas should be useful for fast data analysis and for the calculation of a large number of lines, as, for example, in opacity calculations.

Recent advances in spectroscopy of strongly correlated plasmas

The Quasimolecular Model using a Two Centre basis to describe the electronic emitting structure gives an alternative treatment of line broadening in dense and hot plasmas. Two codes are developed: IDEFIX for the radiative properties, QMSPECTRA (postprocessed to the first one) for the spectral line shapes. The observability of dense plasma effects (PPS, asymmetries and satellite features) in spectroscopic measurements is analysed within the proposed model and taking care of the eventual integrations over density gradients.