Jon C. Weisheit

Dense plasmas, screened interactions, and atomic ionization

Abstract There now exist many laboratory programs to study non-equilibrium plasmas in which the electron interparticle spacing ne−1/3 is no more than a few Bohr radii. Among these are short-pulse laser heating of solid targets, where n e ∼10 23 cm −3 , and inertial confinement fusion experiments, where n e >10 25 cm −3 can be achieved. Under such extreme conditions, the plasma environment is expected to have a strong influence on atomic energy levels and transitions rates. Investigations of atomic ionization in hot, dense plasmas have been motivated by the fact that the instantaneous degree of ionization is a key parameter for the modeling of these rapidly evolving physical systems. Although various theoretical treatments have been presented in the literature, here we focus on the “random field” approach, because it can readily incorporate (quasi-static) level shifts of the target ion as well as dynamic plasma effects. In this approach, the stochastic perturbation of the target by plasma density fluctuations is described in terms of the dielectric response function. Limiting cases of this description yield the familiar binary cross-sectional model, static screening collision models, and the more general dynamical screening models. Screening of the target ion is treated here with several static screening potentials, and bound state level shifts of these potentials are explored. Atomic oscillator strength densities based on these different models are compared in numerical calculations for ionization of He+ and Ar+17. Finally, we compile a list of atomic/plasma physics issues that merit future investigation.

Classical electron-ion scattering in strongly magnetized plasmas. I. A generalized Coulomb logarithm

In a strongly magnetized plasma, where the electron cyclotron radius is less than the Debye length, the Rutherford scattering formula is expected to break down. In this paper, analytic expressions are developed for classical, small-angle scattering of electrons and ions in strong magnetic fields. Numerical evaluation of these expressions shows quantitatively how strong B fields can significantly inhibit electron deflections. The influence of the field on transport phenomena is then explored—in particular, a generalized Coulomb logarithm which includes the effects of a magnetic field is formulated and computed for a wide range of trajectory pitch angles. This generalized Coulomb logarithm is used to illustrate how a strong field influences the effective electron-ion cross section, the electron velocity diffusion coefficient, and the (parallel) electrical and thermal resistivity in a variety of astrophysical and terrestrial plasmas.

The electronic structure of dense plasmas

Abstract The electronic structure of a dense plasma is considered within the context of a single electron interacting with an effective, time-independent potential. This potential consists of many screened ions which represent strong scattering sites. The strong, short range nature of the screened potentials significantly alters the plasmas electronic structure by introducing shape resonance effects. Additionally, the correlations between scattering sites can give rise to modifications analogous to band structure in crystalline matter. To lowest order these effects manifest themselves as a lowered continuum, while higher order terms generate structure in the density of states. These density-of-state modifications can alter rates of particular atomic transitions as well as change a plasmas ionization balance.

Analysis of C IV absorption lines from clouds at high redshifts

From a theoretical study of published QSO spectra, it is concluded that the Lyman-alpha forest lines and the most prevalent metal-line systems (those optically thin at the Lyman limit) arise in very different absorber populations. It is found for these systems that the observed increase with redshift of the C IV 1548, 1550A doublet ratio can be explained as being due to slowly changing metagalactic ionization conditions; there is no need to invoke any cosmic chemical evolution. No correlation is found between equivalent widths of Ly-alpha and C IV lines having the same absorption redshift, and consequently it is suggested that these C IV absorption-line systems arise in clouds strongly ionized by OB stars, presumably in young galaxies. The possibility that the clouds, which are known to exhibit clustering on a velocity scale of few hundred km/s, are protoglobular clusters is explored in some detail. 85 refs.

BLR chemistry redux

We show that at column densities higher than 1023 cm−2, the formation of H2 is enhanced significantly by the presence of dust. The radiative association of H(n=1) and H*(n=2) does not contribute much to increasing the abundance of H2 in the neutral zone. However, H2 formation via associative ionization still needs to be investigated, and several excited states of hydrogen may be important in this process.