Dimitri V. Fisher

A scaling of multiple ionization cross sections

On the basis of average experimental data we demonstrate scaling laws of electron-impact multiple ionization cross sections and propose expressions for the cross sections for arbitrary atoms and ions.

Intraband and interband absorption of femtosecond laser pulses in copper

We investigated the optical properties of pure copper irradiated by a femtosecond laser pulse. Self-absorption of 50-fs laser pulses at 800 nm and 400 nm wavelengths (below and above the interband absorption threshold, respectively) is studied for peak laser intensities up to 10 15 W/cm 2 . Theoretical description of laser interaction with copper target is developed, solving numerically the energy balance equations for electron and ion subsystems together with Maxwell equations for laser radiation field inside the target. The theory accounts for both intraband and interband absorption mechanisms. We treated in detail the changes in electron structure and distribution function with an increase in electron temperature, as well as the ensuing changes in thermodynamic properties, collision frequencies, optical and transport coefficients. Experimental work on self-absorption of femtosecond laser pulses in copper targets at 800 nm and 400 nm wavelengths is ongoing. Results for 800 nm wavelength are reported. Theory and experiment are in good agreement.

Progress in line-shape modeling of K-shell transitions in warm dense titanium plasmas

Modeling of x-ray spectra emitted from a solid-density strongly coupled plasma formed in short-duration, high-power laser–matter interactions represents a highly challenging task due to extreme conditions found in these experiments. In this paper we present recent progress in the modeling and analysis of Kα emission from solid-density laser-produced titanium plasmas. The self-consistent modeling is based on collisional-radiative calculations that comprise many different processes and effects, such as satellite formation and blending, plasma polarization, Stark broadening, solid-density quantum effects and self-absorption. A rather strong dependence of the Kα shape on the bulk electron temperature is observed. Preliminary analysis of recently obtained experimental data shows a great utility of the calculations, allowing for inferring a temperature distribution of the bulk electrons from a single-shot measurement.

Charge state of Zn projectile ions in partially ionized plasma: Simulations

This study deals with the simulation of the experimental study of Roth et al. ~2000! on the interaction of energetic Zn projectiles in partially ionized laser produced carbon targets, and with similar type experiments. Particular attention is paid to the specific contributions of the K and Lshell target electrons to electron recombination in the energetic Zn ionic projectile. The classical Bohr–Lindhard model was used for describing recombination, while quantum mechanical modelswerealsointroducedforscalingtheLtoKcross-sectionratios.Itwasfoundthatevenforahydrogen-likecarbon target, the effect of the missing five bound electrons brings about an increase of only 0.6 charge units in the equilibrium charge state as compared to the cold target value of 23. A collisional radiative calculation was employed for analyzing the type of plasma produced in the experimental study. It was found that for the plasma conditions characteristic of this experiment, some fully ionized target plasma atoms should be present. However in order to explain the experimentally observed large increase in the projectile charge state a very dominant component of the fully ionized plasma must comprise the target plasma. A procedure for calculating the dynamic evolvement of the projectile charge state within partially ionized plasma is also presented and applied to the type of plasma encountered in the experiment of Roth et al. ~2000!.The low temperature and density tail on the back of the target brings about a decrease in the exiting charge state, while the value of the average charge state within the target is dependent on the absolute value of the cross-sections.

Core holes, charge disorder, and transition from metallic to plasma properties in ultrashort pulse irradiation of metals

We study the details of a gradual change in electron properties from those of a nearly-free-electron (NFE) metal to those of a strongly-coupled plasma, in ultrashort pulse energy deposition in solid metal targets. Time scales shorter than those of a target surface layer expansion are considered. Both the case of an optical laser (visible or near infrared wavelengths range) and of a free electron laser (vacuum ultraviolet or X-ray) are treated. The mechanisms responsible for the change in electron behavior are isochoric melting, lattice charge disordering, and electron mean free path reduction. We find that the transition from metal to plasma usually occurs via an intermediate stage of a charge-disordered solid (solid plasma), in which ions are at their lattice sites but the ionization stages of individual ions differ due to ionization from localized bound states. Charge disordered state formation is very rapid (typically, few femtoseconds or few tens of femtoseconds). Pathway to charge-disordered state differs in simple metals and in noble metals. Probabilities are derived for electron impact ionization and 3-body recombination of a bound ionic state in solid-density medium, applicable both in metal and in plasma regime. An evolution of energy coupling between electron and ion subsystems, from metallic electron-phonon (e-ph) to plasma electron-ion (e-i) coupling, is considered. Substantial increase in coupling parameter is expected as a result of charge disorder.

Characterization of electron states in dense plasmas and its use in atomic kinetics modeling

Abstract We describe a self-consistent statistical approach to account for plasma density effects in collisional-radiative kinetics. The approach is based on the characterization of three distinct types of electron states, namely, bound, collectivized, and free, and on the formalism of the effective statistical weights (ESW) of the bound states. The present approach accounts for individual and collective effects of the surrounding electrons and ions on atomic (ionic) electron states. High-accuracy expressions for the ESWs of bound states have been derived. The notions of ionization stage population, free electron density, and rate coefficient are redefined in accordance with the present characterization scheme. The modified expressions for the probabilities of electron-impact induced transitions as well as spontaneous and induced radiative transitions are then obtained. The influence of collectivized states on a dense plasma ionization composition is demonstrated to be strong. Examples of calculated ESWs and populations of ionic quantum states for steady state and transient plasmas are given.

Measurement of magnetic field produced by the interaction of ultra‐short, ultra‐intense fs laser pulse with matter

We report on the preliminary results of a first experiment where the Zeeman effect in the X‐ray range was exploited to monitor the quasi static magnetic field, created by the interaction of an ultra‐short (80 fs) ultra‐intense fs laser pulse with solid matter.The involved laser intensities ranged between 1018 and 5⋅1019 W/cm2. The target was a 25 μm thick Ti foil. We monitored the X‐ray emissions of the Kα1 and Kα2 lines by means of a spectrometer with 0.3 eV energy resolution and 20 μm spatial resolution. The spectrometer was composed by a toroidally bent GaAs crystal and a Si plane crystal in dispersionless setup at 45°, working as polarizer, to distinguish contribution to the line braodening coming from other mechanisms.We observe an increasing broadening of the linewidth in dependence of the intensity but no differences between polarized and non‐polarized spectra, probably because of a too low spatial resolution. A linewidth increase was measured to be 30%. We are working to determine the role of mech...