Time-resolved X-ray Spectroscopy Research Articles

Recent progress in space- and time-resolved x-ray spectroscopy of laser-produced plasmas (abstract)

New diagnostic techniques have provided measurement of relatively gradient-free x-ray spectra from high-powered laser-produced plasmas. Simultaneously space- and time-resolved x-ray spectra were measured from a variety of microdot plasmas using an array of multiframe, imaging, electronically gated x-ray crystal spectrometers with 100 ps time resolution. A multiframed, multicolored gated x-ray pinhole camera provided measurement of the plasma uniformity. A four-frame holographic interferometer was used to measure the electron density profile. Conventional x-ray streaked crystal spectrographs, spatially resolved x-ray (film) spectrometer, and pinhole cameras supplemented these new diagnostics. The instruments allow detailed studies of both the population kinetics of highly charged ions in dense plasmas and the hydrodynamics of laser-produced plasmas for the first time. The new diagnostics are described including data from ongoing experiments. Work was performed in part under the auspices of the U.S. Dept. of Energy by LLNL under Contract No. W-7405-Eng-48.

Measurements of laser plasma coronal conditions and thermal transport with time-resolved x-ray spectroscopy.

This paper presents a detailed characterization of the plasma conditions near the critical density of a laser-produced plasma. Solid spherical plastic targets with small aluminum tracer dots buried below overcoated plastic layers have been uniformly illuminated with green laser light at irradiances of ${10}^{14}$--${10}^{15}$ W/${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. Time-resolved temperature and density profiles were obtained by using x-ray emission spectroscopy. Detailed comparisons with hydrodynamic simulations show that the thermal transport is well characterized by a flux limit of 0.1.

X-ray diagnosis of high-density compression of Ar-filled polymer shell targets (abstract)

Time-resolved x-ray diagnosis is the only practical approach at present to examining the change of fuel and shell conditions during the final stages of compression of laser fusion targets. Time-resolved spectroscopy of lines emitted by the fuel or shell elements can provide information about the time history of the density, temperature, and integrity of the target. We report on time-integrated and time-resolved x-ray measurements of compressed polymer shell targets filled with Ar, or mixtures of Ar/Kr, directly irradiated by the 24-beam 2-kJ, UV (351-nm) OMEGA laser. These include x-ray photography, time-resolved and spaced-resolved x-ray spectroscopy, and x-ray diode measurements. Picosecond x-ray spectroscopy of the K shell emission from Ar has been studied, and detailed modeling and Spark Broadening analysis of the H-like and He-like line emission made. The results are compared with predictions of hydrodynamics code simulations of the implosions. From this analysis, a consistent history of the density and temperature of the compressed target is inferred. This work was supported by the U.S. DOE Office of Inertial Fusion under agreement No. DE-FC08-85DP40200 and the Sponsors of the Laser Fusion Feasibility Project at the Laboratory for Laser Energetics.

Effect of laser illumination nonuniformity on the analysis of time-resolved x-ray measurements in uv spherical transport experiments.

In this paper we present measurements of thermal transport in spherical geometry made with 24 uv (351-nm) beams from the OMEGA laser system of the Laboratory for Laser Energetics of the University of Rochester. The measurements, using time-resolved x-ray spectroscopy on solid glass targets coated with varying thicknesses of plastic, provide absolute measurements of the onset times of the resonance x-ray lines of silicon. From these measurements, the scaling of the instantaneous mass-ablation rate with absorbed intensity is obtained for times before and after the peak of the laser pulse. The observed large burnthrough depths and early onsets of the x-ray lines are explained by carrying out detailed hydrodynamic-code simulations for the range of the estimated laser-intensity distribution on target which is obtained from the superposition of the equivalent target-plane intensity distribution of a single beam. We find that neglecting the effects of laser illumination nonuniformities can lead to erroneous conclusions about the heat transport. We conclude from the analysis that the experimental results can be explained by the presence of significant energy at intensities three times the nominal intensity, contained in hot spots of size less than 20 ..mu..m. This is almost twice as much as the maximum intensitymore » obtained from the superposition of the single-beam distribution.« less

Temporal dependence of the mass-ablation rate in uv-laser-irradiated spherical targets.

In this paper we present new measurements of thermal transport in spherical geometry using time-resolved x-ray spectroscopy. We determine the time dependence of the mass-ablation rate by following the progress of the ablation surface through thin layers of material embedded at various depths below the surface of the target. These measurements, made with 6 and 12 uv (351 nm) beams from the OMEGA laser system of the Laboratory for Laser Energetics of the University of Rochester, are compared to previous thermal transport data and to detailed hydrodynamic code simulations. We find agreement with code results for the scaling of the mass-ablation rate with absorbed intensity, but measure larger absolute values than predicted. This finding is interpreted as being the result of irradiation nonuniformities on target. The sharp decrease in the mass-ablation rate after the peak of the laser pulse predicted in simulations is consistent with experimental observations.

Tokamak Fusion Test Reactor prototype x-ray pulse-height analyzer diagnostic

The x-ray pulse-height analysis (PHA) diagnostic uses a liquid-nitrogen cooled array of 5 Si(Li) and one HpGe detectors to do time-resolved (5–100 ms) x-ray spectroscopy of the central horizontal chord of the Tokamak Fusion Test Reactor (TFTR) plasmas in the 1–50-keV range. Central electron temperature Te and concentration of medium and low-Z impurities are derived from the spectra. Remotely selectable absorber-foil arrays provide selection of the energy range. Fixed and movable aperture arrays allow approximate equalization of count rates in different energy bands and extend dynamic range. Amplifier pulse shapes are approximately triangular. Main amplifier peaking time is 4 μs yielding 230-eV FWHM at 5.9 keV. Pileup inspection times are selectable at 0.13, 0.4, or 0.9 μs. Throughput is up to 42 kHz. The PHA has been used to study temperature and impurities over a wide range of TFTR operational parameters. Dramatic variations in metal impurities with density, plasma current, and major radius have been observed.

X-ray spectroscopy with an ESCA spectrometer

The use of photoelectron emission from clean solid surfaces of low- Z elements for soft X-ray spectroscopy is demonstrated. The measurement made on a clean graphite sample in an ESCA spectrometer using X-ray emission from a stainless-steel anode is promising enough to attempt the use of the technique for time-resolved X-ray spectroscopy of transient plasmas.

Toroidally curved crystal for time-resolved x-ray spectroscopy

A toroidally bent crystal for producing a focused spectrum normal to the line of sight from a laser plasma is described. Experimental results using a pentaerythritol crystal are described which show a brightness gain of 50 over a flat crystal. Improvement in the resolution of dielectronic satellites to the heliumlike 1s2-1s3p, 1s2-1s4p, and 1s2-1s5p lines of silicon is demonstrated.

Time-resolved X-ray diffraction and spectroscopy

Time-resolved X-ray diffraction and spectroscopy

Development of a flat-field grazing-incidence XUV spectrometer and its application in picosecond XUV spectroscopy

A new type of grazing-incidence spectrometer with a flat focal field is developed, and XUV spectroscopy in the extreme ultraviolet region ranging from 15 to 200 Å is carried out. Soft x-ray line spectra emitted from picosecond laser plasmas of aluminum and iron targets are measured and good resolutions are obtained in the XUV region. The spectral regions of detection are extended to shorter wavelengths (15 Å) using a finer spaced grating. Computational studies on x-ray spectra are also performed taking into account the transient characteristics of picosecond laser-produced plasmas; the importance of the transient treatment is clearly shown. This type of soft x-ray spectrometer should be useful for time-resolved picosecond soft x-ray spectroscopy.

Ratio of intercombination to resonance line intensities for He-like oxygen ions in TFR tokamak plasmas

The ratio of intercombination to resonance line intensities has been measured in TFR tokamak plasmas, using space-and time-resolved soft X-ray spectroscopy. The experimental ratio values are compared to those obtained with available theoretical atomic models.

The Secondary Electron Emission Photocathode Characteristics for Time Resolved X-Ray Spectroscopy

Recently the secondary electron energy distributions and the relative secondary electron yields for the 0.1 to 10 keV photon excitation region have been measured for gold, aluminum and for sixteen representative semiconductors and insulators (alkali halides), and a simple phenomenological model for X-ray photoemission has been developed. This work is discussed here as it might be applied to the application of streak cameras for time resolved X-ray Spectroscopy.