Influence of optical thickness and hot electrons on Rydberg spectra of Ne-like and F-like copper ions

Abstract

Spectra in the 7.10 to 8.60 A range from highly charged copper ions are observed from three different laser-produced plasmas (LPPs). The LPPs are formed by a 15-ns Nd:glass laser pulse (type I: E(pulse)=1-8 J, lambda=1.064 microm), a 1-ps Nd:glass laser pulse (type II: E(pulse)=1 J, lambda=1.055 microm), and a 60-fs Ti:sapphire laser pulse (type III: E(pulse)=800 mJ, lambda=790 nm). The spectra of high-n (n<or=14) transitions in highly charged copper ions, Cu19+ to Cu21+, are recorded with a high energy resolution (lambda/deltalambda=3000-8000) spectrometer using a spherically bent mica or quartz crystal. Collisional-radiative models are computed for the emission from each plasma. The sensitivity of the model spectra to opacity effects and to populations of superthermal electrons is studied. For the type I LPPs, opacity effects, treated with escape factors, are necessary to get the correct relative intensities of high-n (n=5, 6) Ne-like Cu19+ emission features. In the case of the type II LPPs, the contrast between the laser prepulse and the main pulse has been varied from low, I(main)/I(pp)=7 x 10(4), to high, I(main)/I(pp)=3.8 x 10(7). For plasmas from low contrast shots, we find good agreement between the observed spectra and optically thin simulations with bulk electron temperatures T(bulk)=0.4 keV and a small population of superthermal electrons (T(hot)=5.0 keV) that is f(hot)<or=10(-5) of the bulk electron population. For high-contrast type II LPPs, we find higher densities and a combination of f(hot) approximately 10(-5) and escape factors best describes the data. For the type III 60-fs LPPs, a population of superthermal electrons (T(hot) approximately 5 keV) that is approximately 5 x 10(-5) of the bulk electron population (T(bulk) approximately 0.2 keV) is required to reproduce the observed spectra. The effect of both escape factors and hot electrons in the CR models is to increase the ionization balance and dramatically increase the number of strong lines for each ion considered. We have studied both opacity effects and hot-electron influence on high-n transitions of highly charged Ne-, F-, and O-like ions.

Effects of the Electron Energy Distribution Function on Line and Continuum Emission

“…Predicted values of the minimum hot electron fraction (f min ) to which modeled K-shell (solid line) and L-shell (dashed line) spectra are sensitive. Reported sensitivity limits of various collisional-radiative models are given by triangles for K-shell Al [5], Mg [6], Ar [1], and Ti and by circles for L-shell Cu [3], Kr [2], and Mo [7].…”

“…Suprathermal ("hot") electrons are of significant interest in studies of a variety of plasma sources, including laser-produced plasmas [1][2][3][4], pulsed-power plasmas [5][6][7], and astrophysical sources [8,9]. Hot electrons are typically generated in such sources by strong electric fields (including resonant laser-plasma interactions) and can have significant influence on plasma radiative properties and dynamic development.…”

“…More recently, the effects of hot electrons on L-shell line emission from pulsed power [7] and laser plasma sources [2,3] have been investigated. The models are used to generate synthetic x-ray emission spectra that can be compared with experimental data to diagnose the conditions of the experimental source.…”

“…target preheat in laser-driven inertial confinement fusion schemes [4]) or beneficial (e.g. increased high-energy radiation from relatively low-power plasma sources [1][2][3][5][6][7]). In both cases, reliable diagnostics to detect their presence and measure the properties of hot electrons are the first step towards understanding and controlling their effects.…”

Diagnostics of warm dense matter by high-resolution X-ray spectroscopy of hollow ions

“…It is well-known (Abdallah et al, 2001;Zhidkov et al, 2001;Hansen et al, 2002;Auguste et al, 2001;Fournier et al, 2003;Moribayashi et al, 2001) that in plasma produced by laser pulses with intensities higher than 10 16 W/cm 2 , hot electrons with energies about a few keV are generated very efficiently. However, the rates of electron impact ionization are proportional to n 3 (n is the principal quantum number of the ionizing electrons).…”