Sandrine Ferri

Temporal Coherence and Spectral Linewidth of Neon-Like XUV Lasers Pumped in the Quasi-steady State Regime

We report recent experimental progress in the characterization of the temporal coherence and related spectral linewidth of plasma-based soft X-ray lasers (SXRL). New measurements were carried out with two types of SXRLs pumped in the quasi-steady state (QSS) regime, in a capillary-discharge plasma and in a laser-produced plasma. We describe the main results obtained from both experiments and compare them to dedicated numerical simulations. We discuss the results in the context of the possibility to achieve XUV lasers with pulse duration below 1 picosecond.

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.

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.

Spectral width of seeded and ASE XUV lasers: experiment and numerical simulations

We describe our recent progress in the investigation of the spectral properties of collisional XUV lasers, including both experimental measurements and numerical calculations. Using a wavefront-division, variable path-difference interferometer, we have characterized the temporal coherence and the spectral width of an injection-seeded transient XUV laser emitted at 18.9 nm from a Ni-like Mo plasma. Our results show that the temporal coherence of the beam is significantly increased by the injection-seeded operation, compared to the standard ASE mode, in agreement with detailed numerical simulations. Using the PPP code we have calculated the intrinsic linewidth of the same lasing line over a range of electron density and at temperatures that are relevant to transient collisional pumping. We discuss the relative contributions of homogeneous and inhomogeneous broadening to the overall profile.

Ionization-potential depression and other dense plasma statistical property studies -Application to spectroscopic diagnostics

The radiative properties of an emitter surrounded by a plasma, are modified through various mechanisms. For instance the line shapes emitted by bound-bound transitions are broadened and carry useful information for plasma diagnostics. Depending on plasma conditions the electrons occupying the upper quantum levels of radiators no longer exist as they belong to the plasma free electron population. All the charges present in the radiator environment contribute to the lowering of the energy required to free an electron in the fundamental state. This mechanism is known as ionization potential depression (IPD). The knowledge of IPD is useful as it affects both the radiative properties of the various ionic states and their populations. Its evaluation deals with highly complex n-body coupled systems, involving particles with different dynamics and attractive ion-electron forces. A classical molecular dynamics (MD) code, the BinGo-TCP code, has been recently developed to simulate neutral multi-component (various charge state ions and electrons) plasma accounting for all the charge correlations. In the present work, results on IPD and other dense plasma statistical properties obtained using the BinGo-TCP code are presented. The study focuses on aluminum plasmas for different densities and several temperatures in order to explore different plasma coupling conditions.

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.