K-shell Spectroscopy Research Articles

K-shell spectroscopy of plasmas created by intense laser irradiation of micron-scale pyramid and sphere targets

K-shell spectra of targets with microstructured features irradiated by an intense femtosecond laser have been studied. Examination of K α emission from laser irradiated Si targets coated with micron-scale polystyrene spheres indicates that the emission is enhanced by a factor of ∼3 over emission from planar solids. Sphere-coated targets also emit K-shell He-like Si radiation indicating the presence of a hot dense plasma beneath the microspheres. Furthermore, K α from Ti foils coupled to micro-tipped reentrant pyramid and wedge shaped targets has been studied, however, no significant enhancement of the K α yield is observed for these kinds of targets. These studies illustrate that, with correct tailoring of the target surface, field enhancements can be used to increase X-ray emission from intensely irradiated targets.

K-shell spectra from Ag, Sn, Sm, Ta, and Au generated by intense femtosecond laser pulses

K-shell X-ray spectra were recorded by the Dual Crystal Spectrometer (DCS) from Ag, Sn, Sm, Ta, and Au planar foil targets irradiated by single intense femtosecond pulses from the Titan laser at Lawrence Livermore National Laboratory. DCS implements two quartz crystals in transmission (Laué) geometry covering the X-ray energy range 10–50 keV and 20–120 keV. The spectral images were recorded on photo-stimulable phosphor image plates and on film/phosphor cassettes. The Kα and Kβ spectral lines of Ag with energies 22 keV and 25 keV, Sn at 25 keV and 28 keV, Sm at 40 keV and 46 keV, Ta at 57 keV and 66 keV, and Au at 67 keV and 79 keV were clearly resolved. The observation of these spectral lines, resulting from 1s electron vacancies created by electrons with energies up to at least 80 keV, enables the implementation of K-shell spectroscopy diagnostic techniques for understanding the laser energy deposition, energetic electron generation, ionization distribution, and X-ray conversion efficiency in plasmas produced by an intense femtosecond laser pulse. Based on the measured energies of the Au K-shell transitions, we conclude that the observed Au K-shell transitions are from ionization stages lower than Au +44, and are most likely from neutral Au atoms, that the Titan plasmas and the hard X-ray emissions are dominated by electrons with energies exceeding 80 keV, and that thermal processes play a minor role.

X-ray spectroscopy at next-generation inertial confinement fusion sources: Anticipating needs and challenges

The inertial confinement fusion community is actively preparing for the transition from kilojoule- to megajoule-class lasers. To assess laser and target performance, a sophisticated array of diagnostics that can operate in increasingly harsh environments is required. In this paper, we overview some of the needs and challenges related to performing X-ray spectroscopy with very intense and highly energetic X-ray sources. To illustrate issues that will be broadly applicable to diagnostics at high energy density facilities, we relate lessons learned during early testing of the high-energy electronic X-ray spectrometer (HENEX), a core-level diagnostic commissioned by the National Ignition Facility. A spectrometer design employing symmetric Laué diffraction has proven to be particularly well suited for the high energy density environment and will extend existing diagnostic techniques to high-Z, K-shell spectroscopy.

Development of a test bed plasma and diagnostic methods for detailed K-shell spectroscopy

The emission spectrum from a plasma is often used to extract information on the hydrodynamic, radiative and atomic processes which occur. As these processes are coupled, detailed interpretation of spectroscopic data is often complex and uncertain. We present an experiment and modelling study of a 'simple' hydrogen and helium-like plasma. The target and laser geometry were designed to minimise spatial gradients transverse to the plasma flow. The plasma was spatially and temporally diagnosed using Thomson scattering, X-ray imaging and spectroscopic measurements. The combination of the optical and X-ray results constrains the numerical modelling to provide a more reliable plasma diagnostic. This is essential for the interpretation of high-resolution measurements and of more complex, non 1-dimensional plasmas.

Line broadening analysis of implosion core conditions at Z using argon K-shell spectroscopy

We report on spectral line broadening analysis of Ar K-shell lines from argon-doped implosion cores driven by a dynamic hohlraum z-pinch. The observed Ar spectra include emissions from the resonance series in H- and He-like Ar ions, i.e., Ly α , Ly β and Ly γ , and He α , He β , He γ and He δ lines, respectively. The analysis accounts for opacity and Stark broadening to determine electron density, N e , and areal-density, N Δ R , values for the ground state populations of H- and He-like Ar ions. Furthermore, these results are combined with the ratio of H- and He-like ground state populations to extract the electron temperature, T e .

FLYCHK: an extension to the K-shell spectroscopy kinetics model FLY

The K-shell spectroscopy code FLY and its predecessors have been employed to study hot dense plasmas for decades. FLY has provided a basis for studies of K-shell ions up to Z=26, where its ease of use, PC application base, and synthetic spectral production have made it attractive to both experimentalists and modelers. However, the application has been focused on K-shell ions where less ionized species are represented as a ground state only. To attempt to make this simple model predict the ionization and recombination through the less ionized stages, we extend the model to include kinetics models for all ion stages. The emphasis of this extension is not to provide detailed spectra or population for the L- or M-shell, but to provide a K-shell model with enhanced accuracy in those cases where ionization and recombination through the L- and M-shell are important. This work is a first step in generating an application that will allow detailed analysis of sub-picosecond self-consistent velocity distribution and population kinetics solutions. We will discuss the extensions and provide examples. Finally, we will outline the progress towards the eventual goal of the self-consistent solution.

K-shell spectroscopy of an independently diagnosed uniaxially expanding laser-produced aluminum plasma.

We present detailed spectroscopic analysis of the primary K-shell emission lines from a uniaxially expanding laser-produced hydrogenic and heliumlike aluminum plasma. The spectroscopic measurements are found to be consistent with time-dependent hydrodynamic properties of the plasma, measured using Thomson scattering and shadowgraphy. The K-shell population kinetics code FLY with the measured hydrodynamic parameters is used to generate spectra that are compared to the experimental spectra. Excellent agreement is found between the measured and calculated spectra for a variety of experimental target widths employed to produce plasmas with different optical depths. The peak emission from the hydrogenic Lyman series is determined to be from a temporal and spatial region where the hydrodynamic parameters are essentially constant. This allows a single steady-state solution of FLY to be used to deduce the electron temperature and density, from the measured line ratios and linewidths, for comparison with the Thomson and shadowgraphy data. These measurements are found to agree well with time-dependent calculations, and provide further validation for the FLY calculations of the ionization and excitation balance for a K-shell aluminum plasma. We also discuss the possible application of this data as a benchmark for hydrodynamic simulations and ionization/excitation balance calculations.

Characterization of direct-drive-implosion core conditions on OMEGA with time-resolved Ar K-shell spectroscopy

Direct-drive-implosion core conditions have been characterized on the 60-beam OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] laser system with time-resolved Ar K-shell spectroscopy. Plastic shells with an Ar-doped deuterium fill gas were driven with a 23 kJ, 1 ns square laser pulse smoothed with 1 THz smoothing by spectral dispersion (SSD) and polarization smoothing (PS) using birefringent wedges. The targets are predicted to have a convergence ratio of ∼15. The emissivity-averaged core electron temperature (Te) and density (ne) were inferred from the measured time-dependent Ar K-shell spectral line shapes. As the imploding shell decelerates the observed Te and ne increase to 2.0 (±0.2) keV and 2.5 (±0.5)×1024 cm−3 at peak neutron production, which is assumed to occur at the time of the peak emissivity-averaged Te. At peak compression the ne increases to 3.1 (±0.6)×1024 cm−3 and the Te decreases to 1.7 (±0.17) keV. The observed core conditions are close to those predicted by a one-dimensional hydrodynamics code.

LTE absorption spectroscopy of an X-ray heated boron plasma

Time-resolved K-shell spectroscopy of an X-ray driven boron sample is reported. A small cavity irradiated by two frequency-doubled Nd:glass laser beams is used as an X-ray heating source, allowing to reach electron temperatures in the 10–25 eV range, in conditions close to local thermodynamic equilibrium. Synthetic spectra obtained from detailed atomic calculations are fitted to the experimental data to determine the electron temperature and the density of plasma. We compare the resulting data to radiative hydrodynamic simulations of the heating and expansion of the sample, and find lower densities and higher temperatures than predicted by the code. The reason for this discrepancy is attributed to difficulties in maintaining the boron sample homogeneity and to take into account properly the expansion of the plasma walls with such small-size cavities.

K-shell spectroscopy of radiatively heated aluminium

Thin solid tamped foils were heated by the x-rays generated in spherical gold hohlraums which were irradiated by the high power laser ASTERIX IV with an energy of up to 500 J and a pulse duration of 0.4 ns. The sample foils were probed by the radiation of a back-lighter with a variable delay to the main beam. Previous opacity studies in the extreme uv range (G. Winhart et al., PRE, 53, R1332, 1996) have now been completed by measurements in the keV range by using a crystal spectrometer. We present K-shell absorption spectra of aluminium at temperatures between 20 and 30 eV. For the analysis we used local thermodynamic equilibrium opacity codes. The resulting mean ionization of the sample is compared with hydro-code simulations which were post-processed by solving the rate equations using a collisional radiative model.

Characterization of titanium laser-produced plasmas

The development of a plasma environment that is appropriate for the study of laser-plasma processes in laser-fusion plasma is reported. A material (titanium) with medium atomic number (Z) was used to provide x-ray measurements of radial and axial plasma symmetry as well as electron temperature. The electron density evolution was measured using stimulated scattering processes and odd half-harmonic generation from probe lasers of different wavelengths. The plasmas were created by two-sided irradiation of thin foils with 24 kJ of 351 nm laser light. When the peak electron density had decayed to about 4×1020 cm−3, the density profile was estimated to have a full width at half-maximum of 2 mm and the electron temperature was measured to be about 3 keV using K-shell spectroscopy. Two-dimensional computer simulations were found to reproduce some features of both electron density and temperature evolution.

Fast and accurate line shape modeling of the H- and He-like lyman series for radiative transfer calculations in plasmas

We have modeled the Stark broadening of the H- and He-like Lyman lines for K-shell spectroscopy calculations. This approach is accurate for ionic emitters having a nuclear charge ranging between low and medium values of Z. This method is fast enough to be coupled in line with a collisional-radiative model for line transfer calculations in hot dense plasmas.

X-ray spectroscopy studies using the TRANSPEC code

TRANSPEC is a 1D planar or spherical radiation dependent collisional-radiative model for K-shell spectroscopy. The model is time dependent and acts as a postprocessor of any plasma hydrodynamics simulations relevant to K-shell emission. The coupling of the radiation with the populations is obtained self-consistently in an iterative procedure in which populations are obtained by linearization while the line transfer can be computed within the core saturation approximation. A fast and accurate in-line routine for the calculation of the H- and He-like Lyman Stark profiles has been added. The characteristics of the model are outlined. The capability of TRANSPEC is illustrated with some results of simulations for different problems relevant to laser-plasma studies like the X-ray emission of a DT-argon (or pure argon) filled ICF capsule, the X-ray emission of a short pulse laser-created plasma and the photoionization of an aluminum plasma by an X-ray source.

A model for the simulation of nonequilibrium line transfer in laboratory plasmas

A numerical model for the simulation of emission spectra of laboratory plasmas is presented. A special attention has been focused on the coupling between the line transfer and the ionic populations. This coupling is carried out by an iterative procedure, where, given the temperature, density, and velocity profiles of a plasma, the populations are obtained by linearization while the line transfer is computed within the core saturation approximation and with complete frequency redistribution. This procedure has been developed along with a collisional-radiative model for K-shell spectroscopy. As an application, the emission of a laser-created aluminum plasma was calculated, and the spectra of radiation emerging from different emission zones were accordingly computed. It is found that an accurate evaluation of line intensities involves a computation of line transfer, taking into account the macroscopic Doppler shift.

Modeling diagnostics for neon-like systems (abstract)

K-shell spectroscopy is commonly used to diagnose laboratory plasmas, such as those produced in Z-pinches or laser targets. Neon-like systems are also of special interest because they produce the next simplest spectra, and because of applications to x-ray lasers. High-resolution spectroscopy and the acquisition of complex atomic physics data make it feasible to do diagnostics directly on neon-like systems. We present a model which calculates the 3-3 and 3-2 transitions of neon-like selenium. The ground state, n=3, and n=4 excited states are included explicitly; the n=5 through n=10 states are each lumped into one generic level. The model includes collisional and radiative transitions among these levels, and solves steady-state rate equations for the populations. The code includes graphics to plot line ratios and synthetic spectra. The model is analyzed for possible diagnostic use and is compared with experimental results.

Spectra—A model for K-shell spectroscopy

X-ray spectroscopy is a powerful tool for understanding the kinetics of highly ionized plasmas. Its usefulness depends on the accuracy of the model used in analyzing spectra. We have developed a computer code for modeling and analyzing plasmas which is highly accurate as well as fast and easy to use. It produces synthetic spectra for hydrogen-like and helium-like ions at arbitrary density and temperature. Populations are calculated from rate equations including all relevant collisional and radiative process. The level populations of the hydrogen, helium and lithium-like ionization stages are calculated explicitly; those of all other ionization stages are lumped into one level. The microfield distribution and the shape of the line profiles are determined using detailed calculations. The code includes graphics to plot line ratios and synthetic spectra, and to do on-line analysis of experiments. The usefulness of this technique is demonstrated by analyzing the spectra from three different experiments: a laser irradiated aluminum disk; a neon gas puff pinch; and a laser imploded gas microballon.