J. Nilsen

NEW COMPREHENSIVE THEORETICAL ANALYSIS OF THE DOUBLY EXCITED 3LNL' STATES OF SODIUM-LIKE IRON

Abstract Energy levels, radiative transition probabilities and autoionization rates for Fe XVI including the 1s 2 2s 2 2p 5 3 l n l ′ (n=3–8, l ′≤(n−1) ) and 1s 2 2s2p 6 3 l n l ′ (n=3–4, l ′≤(n−1)) doubly excited states were calculated by using the multi-configurational Hartree–Fock (Cowan code) method. The contribution of relativistic effects is studied in comparison with the data obtained using the multi-configurational Dirac–Fock (YODA code) method. The comparison of the theoretical data with experimental spectra obtained by double-electron capture in slow ion-atom collisions is discussed.

A picosecond 14.7 nm x‐ray laser for probing matter undergoing rapid changes

With laser‐driven tabletop x‐ray lasers now operating in the efficient saturation regime, the source characteristics of high photon flux, high monochromaticity, picosecond pulse duration, and coherence are well‐matched to many applications involving the probing of matter undergoing rapid changes. We give an overview of recent experiments at the Lawrence Livermore National Laboratory (LLNL) Compact Multipulse Terawatt (COMET) laser using the picosecond 14.7 nm x‐ray laser as a compact, ultrafast probe for surface analysis and for interferometry of laser‐produced plasmas. The plasma density measurements for known laser conditions allow us to reliably and precisely benchmark hydrodynamics codes. In the former case, the x‐ray laser ejects photo‐electrons, from the valence band or shallow core‐levels of the material, and are measured in a time‐of‐flight analyzer. Therefore, the electronic structure can be studied directly to determine the physical properties of materials undergoing rapid phase changes.

Overview of Tabletop X-Ray Laser Development at the Lawrence Livermore National Laboratory

It is almost a decade since the first tabletop x-ray laser experiments were implemented at the Lawrence Livermore National Laboratory (LLNL). The decision to pursue the picosecond-driven schemes at LLNL was largely based around the early demonstration of the tabletop Ne-like Ti x-ray laser at the Max Born Institute (MBI) as well as the established robustness of collisional excitation schemes. These picosecond x-ray lasers have been a strong growth area for x-ray laser research. Rapid progress in source development and characterization has achieved ultrahigh peak brightness rivaling the previous activities on the larger facilities. Various picosecond soft-x-ray based applications have benefited from the increased repetition rates. We will describe the activities at LLNL in this area.

Soft x-ray laser interferometry of colliding plasmas

We have used soft x-ray laser interferometry to study dense colliding plasmas produced by laser irradiation of semi-cylindrical targets. Results are reported on the evolution of 1 mm long plasmas created by heating 500 μm diameter half holhraum copper targets with an intensity of ~1.6 1012 W.cm-2 from 120 ps duration laser pulses of 800 nm wavelength. The setup combines a robust high throughput amplitude division interferometer based on diffraction gratings with a 46.9 nm table-top capillary discharge laser. Series of high contrast interferograms were obtained depicting the evolution of the copper plasmas into a localized plasma that reaches densities above 1×1020 cm-3 when the plasmas collide near the center of the cavity. The technique allows the generation of high resolution density maps of colliding plasma with various degree of collisionality for comparison with code simulations.

Hydrodynamic Simulations and Soft X-Ray Laser Interferometric Studies of Energy Transport in Tightly Focused Laser-Heated Aluminum Plasmas

A plasma generated by a tightly focused ~ 14 μm (FWHM), 600 ps duration laser beam at an irradiance of 1013 W cm-2 on a flat aluminum target is investigated. We report new findings that give a better understanding of the energy transport mechanisms in the measured plasma. The formation of a small, on-axis dip or concave electron density profile is observed. Detailed modeling of the spatial and temporal profile of a laser-produced plasma with the 2-D LASNEX hydrodynamics code gives good agreement with the observed features. The observed cold plasma formation along the target surface hundreds of microns away from the small focal spot is generated by heated material outside of the laser focus. Plasma generated by low intensity wings in the laser spatial profile is augmented by soft x-ray radiation from the hot coronal plasma heated by the laser. The simulations show that the x-ray heating will produce a plasma outside the focal spot without the low intensity wings. Strong thermal electron conductive heating due to large thermal gradients continues to generate ablation outside the laser spot and formation of lobes on each side of the focal spot.

Soft x-ray laser interferometry unveils plasmas with index of refraction greater than one

We report clear evidence of the existence of multiply ionized plasmas with index of refraction greater than one at soft x-ray wavelengths. Moreover, it is shown to be a general phenomenon affecting broad spectral regions in numerous highly ionized plasmas. The experimental evidence consists of the observation of anomalous fringe shifts in soft x-ray laser interferograms of laser-created Al plasmas probed at 14.7 nm and of Ag and Sn laser-created plasmas probed at 46.9 nm. The comparison of measured and simulated interferograms shows that these anomalous fringe shifts result from the dominant contribution of low charge ions to the index of refraction. This usually neglected bound electron contribution can affect the propagation of soft x-ray radiation in plasmas and the interferometric diagnostics of plasmas for many elements and at different wavelengths.

Picosecond x-ray laser interferometry for probing dense laser-produced plasmas

Summary form only given, as follows. We describe precision interferometric characterization experiments using the picosecond, 14.7 nm x-ray laser source generated on the Compact Multipulse Terawatt (COMET) laser facility together with the Mach-Zehnder type diffraction grating interferometer. A review of the results from dense, mm-scale line focus plasma experiments will be described with detailed comparisons to 1-, 1.5- and 2-D hydrodynamic simulations. Ongoing experiments on smaller spot focus high intensity plasmas will be discussed.

Investigations on soft x‐ray lasers with a picosecond‐laser‐irradiated gas puff target

We present results of experimental studies on transient gain soft x‐ray lasers with a picosecond‐laser‐irradiated gas puff target. The target in a form of an elongated gas sheet is formed by pulsed injection of gas through a slit nozzle using a high‐pressure electromagnetic valve developed and characterized at the Institute of Optoelectronics. The x‐ray laser experiments were performed at the Lawrence Livermore National Laboratory using the tabletop Compact Multipulse Terawatt (COMET) laser to irradiate argon, krypton or xenon gas puff targets. Soft x‐ray lasing in neon‐like argon on the 3p‐3s transition at 46.9 nm and the 3d‐3p transition at 45.1 nm have been demonstrated, however, no amplification for nickel‐like krypton or xenon was observed. Results of the experiments are presented and discussed.

Modeling of transient X-ray lasers: comparison with experiments

We discuss progress in numerical and experimental research of table-top X-ray lasers based on several schemes that have emerged successfully in recent years. We discuss the progress, trends and major differences of modern approaches in X-ray laser design to previous efforts. The efficiency issues and main properties of X-ray lasers based on laser-produced and electrical discharge-created plasmas that have made this improvement possible will be outlined.

X-ray spectroscopic diagnostics of high-temperature dense plasmas created in different gaseous media

Summary form only given, as follows. The investigations of emission X-ray spectra of multicharged ions of some chemical elements (S, F, Ar, Fr, O) have been carried out. These atoms are contained in a gases and consequently can be used as diagnostic elements in a dense plasma focus experiments. Our investigations were done in the dense high-temperature plasma (N/sub e//spl sime/10/sup 21/ cm/sup -3/, T/sub e//spl ap/500 eV) created by laser heating of high-pressure gas puff targets, and X-ray spectrographs with a spherically bent mica crystals were used for spectra observations. Some new spectroscopic results (line identifications, high-precision wavelength measurements) have been obtained and have been applied to determine a spatial distribution of plasma parameters. It is shown that spectroscopic techniques used is a very suitable tool for studies of a plasma with complicated spatial structure.