D. Benredjem

Study of particle correlation effects on line profiles of Ni-like collisional XUV laser amplifier

We discuss the effects of particle correlations on the spectral broadening due to the radiator motion (Doppler broadening) for a Ni-like XUV laser line pumped in two different regimes (transient and quasi-steady state regimes) of collisional excitation. In a medium with gain, radiative transport effects modify the observed profile and these modifications depend on the homogeneous or inhomogeneous nature of the intrinsic profile (before amplification). The intrinsic line profile is usually described by a Voigt profile, which is the convolution of a Lorentzian profile due to the different homogeneous broadenings (electron collision-induced transitions and spontaneous emission) and an inhomogeneous Gaussian profile due to Doppler shifts (due to the radiator motion in the free-particle limit). In this paper, it is shown that accounting for the correlations between particles modifies noticeably the radiator-motion broadened profiles, whatever the densities and temperatures values.

Density-induced continuum resonances and quasi-bound states in the collisional-radiative equilibrium of dense plasmas

The aim of this work is to improve the treatment of density effects in non-local thermodynamic equilibrium plasmas. The density effect on atomic structure (wavefunctions and energy levels) is modelled by an ion-sphere potential. The modification of the atomic potential, continuum lowering and appearance of resonances are presented. In particular, we show that the continuum resonances are linked to the electrons in the subshells passed into the continuum. Their presence determines the existence of partially bound configurations, which must be taken into account in the collisional-radiative model. We introduce in the set of rate equations a supplementary ionization process due to the plasma environment. This process (and its inverse) enters into the balance of all the other processes. It is equivalent to tunnelling ionization where an outer electron located above the ionization threshold (and trapped by the potential barrier) crosses the barrier. As an application, we studied the influence of temperature and density on the average ionization and the ionic populations of a carbon plasma. We compared these calculations with the traditional method based on the chemical picture with continuum lowering.

RADIATION TRAPPING AND X-RAY LASING IN AL10+ IN THE RECOMBINATION SCHEME

Abstract A self-consistent model for radiation trapping in recombining plasmas is presented in view of X-ray laser gain calculations. We obtain the population density of the lasing levels in the presence of resonance photons which are emitted from the lower level of the lasing transition and which may be reabsorbed elsewhere in the plasma. The resulting gain reduction at the time and in the cell of maximum gain is about 25% and 10% for the Li-like Al transitions at 154.7 and 105.7 A, respectively. Our results for the transition at 154.7 A agree to within 10% with the most recent measured gain, in contrast with the up-to-date calculations on the escape probability which predict much smaller gains.

Opacity profiles in inertial confinement fusion plasmas

The opacity is an important issue in the knowledge of radiative properties of ICF and astrophysical plasmas. In this work we present the opacity of dopants embedded in the ablator of some ICF capsules. The silicon is used as dopant and we are interested in C+Si mixtures. We have used two methods to calculate the opacity of C+Si. The first one involves a detailed line shape calculation in which the atomic database is provided by a MCDF code. The lineshape code PPP is then adapted to the calculation of opacity profiles. Almost all spectral broadening effects, including Zeeman splitting and Stark effect, are taken into account. This method is able to provide accurate opacity spectra but becomes rapidly prohibitive when the number of lines is large. To account for many ionic stages and thousands of lines, a second method −hybrid method− is prefered. This method combines detailed-line and statistical calculations. In the spectral regions where the lines are sufficiently separated and the number of radiative transitions is moderate, the hybrid method performs detailed calculations. When the number of transitions is very large and most of them merge in broad structures due to line broadening, the hybrid method performs statistical calculations.

Collisional-Radiative modeling of the transient excitation of a carbon atoms beam crossing a tokamak plasma edge

A time-dependent collisional-radiative (CR) model is elaborated in the purpose of modeling the penetration of an atomic carbon beam in the edge plasma of a fusion machine in order to probe it. The excited states population densities of the beam are assumed to be modified by electron-induced processes only. All momentum transfer is neglected: the penetration velocity is assumed constant. In typical conditions for the electron density and temperature gradients (∂ne/∂x ≈ 1020 m−4 and ∂Te/∂x ≈ 107 K m−1), the results illustrate the electrons efficiency to ionize the beam. With a velocity equal to 1 km s−1, the beam is ionized in 20 /is in these conditions, which corresponds to a penetration depth of 2 cm. During the conversion in ions, the atomic beam is sufficiently excited to emits spectra whose measurement in coupling with the present CR model can provide information on ∂ne/∂x and ∂Te/∂x.

New modeling of germanium soft-X-ray lasers using extended atomic data

Abstract A collisional radiative model, explicitly including the interaction of the amplifying medium with the X-ray laser beam, is used to calculate the intensity of the 2–1 and 0–1 lines in neon-like germanium, as well as the population densities. The required energy levels and electron-impact collision strengths involve the configurations 1s22s22p6, 1s22s22p53l, and 1s22s2p63l (l=s,p,d). The fine-structure excitation rates coefficients are presented for temperatures in the range 100–700 eV. Well-known numerical codes are used for the atomic structure (SUPERSTRUCTURE) and electron–ion collisions (DISWAV/JAJOM). Finally, we solve the collisional radiative population equations. It is recommended to use excitation rates obtained from the extensive quantum calculations, rather than fit formulae. In this case, the calculated populations and intensities show substantial changes.

Frequency redistribution in the radiative transfer problem of amplifying media. Application to Ge22+ and C5+ x-ray lasers

This paper is devoted to the effect of frequency redistribution on x-ray laser beams. The redistribution function, calculated by extending the frequency fluctuation model to two-photon processes, shows coherent scattering and a redistributed contribution. The spectral emissivity, corrected in order to account for this effect, is calculated for lasers using neon-like germanium and hydrogen-like carbon as amplifiers. The radiative transfer equation is solved, and the intensity of the x-ray laser output is given for two 3p-3s lines in a neon-like amplifier. We also study the variation in the emissivity of the C5+ Balmer-α lasing line. The physical conditions of the calculation are derived from modelling of experiments conducted at the Rutherford Appleton Laboratory.

Calculation of atomic structures and radiative properties of fusion plasmas

The opacity is an important issue in the knowledge of the radiative properties of Inertial Confinement Fusion (ICF) and astrophysical plasmas. In this work we present the opacity of the mixture C+Si, composing the ablator of some ICF capsules. We have used Cowan’s code to calculate the atomic structure of carbon and silicon. We also have developed a collisional-radiative model in order to obtain the opacity of the mixture. Line broadening, line shift and ionization potential depression are taken into account in the opacity profile. Comparisons to other calculations are carried out. NLTE and LTE opacity calculations show discrepancies mainly in the range 1900-2000 eV for the bound-bound contribution to the total opacity and in the range 50-350 eV for the bound-free contribution. We have also accounted for photoexcitation and photoionization processes. The corresponding rates are obtained by modeling the Hohlraum radiation by a Planckian distribution at a radiative temperature of 300 eV.

Spectral Broadening of Ni-Like XUV Laser Lines

We have used the PPP lineshape code to calculate the intrinsic (i.e. before amplification) line profile of the Mo XUV laser over an extended range of plasma densities and temperatures, chosen to cover conditions for collisional excitation pumping in the transient and quasi-steady state regimes. The calculated profiles were then used to simulate the amplified line profile, using a detailed 1D-radiative transfer code, taking into account the effect of saturation. We discuss the possibility to achieve a gain bandwidth that would support pulse amplification below 1 ps.

A self‐consistent model for line trapping effect in x‐ray lasers

Line trapping appropriate for cylindrical geometry in recombining plasmas has been examined. Calculations are self‐consistent and use the results of a 1D Lagrangian simulation as inputs. Resonant photons are assumed to travel a maximum distance which is given by the inverse of the absorption coefficient.