Spatially resolved x-ray spectroscopy investigation of femtosecond laser irradiated Ar clusters

Enhanced Kα output of Ar and Kr using size optimized cluster target irradiated by high-contrast laser pulses

We observed that increasing the clusters size and laser pulse contrast can enhance the X-ray flux emitted by femtosecond-laser-driven-cluster plasma. By focusing a high contrast laser (10(-10)) on large argon clusters, high flux Kα-like X-rays (around 2.96 keV) is generated with a total flux of 2.5 × 10(11) photons/J in 4π and a conversion efficiency of 1.2 × 10-4. In the case of large Kr clusters, the best total flux for L-shell X-rays is 5.3 × 1011 photons/J with a conversion efficiency of 1.3 × 10-4 and, for the Kα X-ray (12.7 keV), it is 8 × 10(8) photons/J with a conversion efficiency of 1.6 × 10-6. Using this X-ray source, a single-shot high-performance X-ray imaging is demonstrated.

Modeling energy dependence of the inner-shell x-ray emission produced by femtosecond-pulse laser irradiation of xenon clusters

We employ the Los Alamos suite of atomic physics codes to model the inner-shell x-ray emission spectrum of xenon and compare results with those obtained via high-resolution x-ray spectroscopy of xenon clusters irradiated by 30fs Ti:Sapphire laser pulses. We find that the commonly employed configuration-average approximation breaks down and significant spin-orbit splitting necessitates a detailed level accounting. We reproduce an interesting spectral trend for a series of experimental spectra taken with varying pulse energy for fixed pulse duration. To simulate the experimental measurements at increasing beam energies, we find that spectral modeling requires an increased hot electron fraction, but decreased atomic density and bulk electron temperature. We believe these latter conditions to be a result of partial cluster destruction due to the increased energy in the laser prepulse.

Spectroscopic characterization of an ultrashort-pulse-laser-driven Ar cluster target incorporating both Boltzmann and particle-in-cell models

A model that solves simultaneously both the electron and atomic kinetics was used to generate a synthetic He alpha and satellite x-ray spectra to characterize a high intensity ultrashort laser driven Ar cluster target experiment. In particular, level populations were obtained from a detailed collisional-radiative model where collisional rates were computed from a time varying electron distribution function obtained from the solution of the zero-dimensional Boltzmann equation. In addition, a particle-in-cell simulation was used to model the laser interaction with the cluster target and provided the initial electron energy distribution function (EEDF) for the Boltzmann solver. This study suggests that a high density average, <Na>high, of 3.2 x 10(20) cm(-3) was held by the system for a time, delta tau, of 5.7 ps, and during this time the plasma was in a highly nonequilibrium state in both the EEDF and the ion level populations.

Measurement of 2l-nl' x-ray transitions from approximately 1 microm Kr clusters irradiated by high-intensity femtosecond laser pulses

X-ray line emission from 2l-nl' transitions in Ne-like Kr and nearby ions has been observed from approximately 1 microm Kr clusters irradiated by fs-scale laser pulses at the JAERI facility in Kyoto, Japan. The incident laser intensity reached 10(19) W/cm2, with pulse energies from 50 to 300 mJ and pulse durations from 30 to 500 fs. The dependence of the x-ray spectral features and intensity on the incident laser intensity is rather weak, indicating that the 1-2 ps cluster lifetimes limit the number of ions beyond Ne-like Kr that can be produced by collisional ionization. Lines from F- to Al-like Kr emitted from the cluster plasmas have been identified using data from the relativistic multiconfiguration flexible atomic code. A collisional-radiative model based on these data has been constructed and used to determine that the cluster plasma has electron densities near 10(22) cm(-3), temperatures of a few hundred eV, and hot electron fractions of a few percent.

Effects of the electron energy distribution function on modeled x-ray spectra

This paper presents the results of a broad investigation into the effects of the electron energy distribution function on the predictions of nonlocal thermodynamic equilibrium collisional-radiative atomic kinetics models. The effects of non-Maxwellian and suprathermal ("hot") electron distributions on collisional rates (including three-body recombination) are studied. It is shown that most collisional rates are fairly insensitive to the functional form and the characteristic (central or average) energy of the electron distribution function as long as the characteristic energy is larger than the threshold energy for the collisional process. Collisional excitation and ionization rates are, however, highly sensitive to the number of hot electrons. This permits the development of robust spectroscopic diagnostics that can be used to characterize the electron density, bulk electron temperature, and hot electron fraction of plasmas with nonequilibrium electron distribution functions. Hot electrons are shown to increase and spread out plasma charge state distributions, amplify the intensities of emission lines fed by direct collisional excitation and radiative cascades, and alter the structure of satellite features in both K - and L -shell spectra. The characteristic energy, functional form, and spatial properties of hot electron distributions in plasmas are open to characterization through their effects on high-energy continuum and line emission and on the polarization of spectral lines.

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

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.

Hot-electron influence on L-shell spectra of multicharged Kr ions generated in clusters irradiated by femtosecond laser pulses

Strong L-shell x-ray emission has been obtained from Kr clusters formed in gas jets and irradiated by 60-500-fs laser pulses. Spectral lines from the F-, Ne- Na-, and Mg-like charge states of Kr have been identified from highly resolved x-ray spectra. Spectral line intensities are used in conjunction with a detailed time-dependent collisional-radiative model to diagnose the electron distribution functions of plasmas formed in various gas jet nozzles with various laser pulse durations. It is shown that L-shell spectra formed by relatively long nanosecond-laser pulses can be well described by a steady-state model without hot electrons when opacity effects are included. In contrast, adequate modeling of L-shell spectra from highly transient and inhomogeneous femtosecond-laser plasmas requires including the influence of hot electrons. It is shown that femtosecond-laser interaction with gas jets from conical nozzles produces plasmas with higher ionization balances than plasmas formed by gas jets from Laval nozzles, in agreement with previous work for femtosecond laser interaction with Ar clusters.