Showing papers by "Siegfried Glenzer published in 1997"

Thomson scattering from inertial-confinement-fusion hohlraum plasmas

Abstract: We present the first Thomson scattering measurements of local plasma conditions in ignition-relevant, gas-filled, inertial-confinement-fusion hohlraums. The experimental data provide a benchmark for two-dimensional hydrodynamic simulations using LASNEX, which is presently in use to predict the performance of future megajoule laser-driven hohlraums of the National Ignition Facility. The data are consistent with modeling using significantly inhibited heat transport at the peak of the drive. Further, we find that stagnating plasma regions on the hohlraum axis are well described by the calculations. {copyright} {ital 1997} {ital The American Physical Society}

Observation of multiple mechanisms for stimulating ion waves in ignition scale plasmas

Abstract: The laser and plasma conditions expected in ignition experiments using indirect drive inertial confinement have been studied experimentally. It has been found that there are at least three ways in which ion waves can be stimulated in these plasmas and have significant effect on the energy balance and distribution in the target. First ion waves can be stimulated by a single laser beam by the process of Stimulated Brillouin Scattering (SBS) in which an ion acoustic and a scattered electromagnetic wave grow from noise. Second, in a plasma where more than one beam intersect, ion waves can be excited at the `beat` frequency and wave number of the intersecting beams, causing the side scatter instability to be seeded, and substantial energy to be transferred between the beams [R. K. Kirkwood et. al. Phys. Re0319v. Lett. 76, 2065 (1996)]. And third, ion waves may be stimulated by the decay of electron plasma waves produced by Stimulated Raman Scattering (SRS), thereby inhibiting the SRS process [R. K. Kirkwood et. al. Phys. Rev. Lett. 77, 2706 (1996)].

Electron temperature and density measurements in laser-produced large-scale-length gas-bag plasmas by x-ray spectroscopy

Abstract: We present temporally and spatially resolved measurements of the K-shell emission from argon and chlorine dopants in laser-produced mm-size gas-bag plasmas. Particularly useful for the diagnostics of these plasmas are the line intensity ratios of the He- and H-like resonance lines to their respective Li- and He-like dielectronic satellite transitions. By Abel inverting the experimental spectra and applying time-dependent collisional-radiative modeling, local electron temperatures, and densities are deduced. About 0.4 ns after the beginning of the laser heating, we observe a homogeneous plasma center which heats steadily until the end of the heating pulse. Although the heating is slower than predicted by hydrodynamic simulations, the measured peak electron temperature of T{sub e}=3 keV for neopentane-filled gas bags is in good agreement with the hydrodynamic simulations. In addition, the electron densities inferred from the line intensity ratio of the intercombination to the resonance line of heliumlike argon are consistent with 10{sup 21} cm{sup {minus}3} as expected from the initial gas fill density. Further gas fill densities have been investigated, and in agreement with simulations we find lower temperatures for lower gas densities. {copyright} {ital 1997} {ital The American Physical Society}

Imaging backscattered and near to backscattered light in ignition scale plasmas (invited)

Abstract: Diagnostics have been developed and fielded at the Nova laser facility that, for the first time, image nearly all the light scattered within 20° of the laser axis, including the light collected by the laser focusing lens as well as that just outside the lens. Absolute calibration of optical components exposed to the target debris have been achieved by a combination of routine in situ calibration and maintenance. Measurements from plasmas relevant to ignition experiments indicate that scattering is peaked in the direction of backscatter with significant energy scattered both into the lens and just outside the lens. The scattering outside the lens is found to be dominant when the f number is large (f/8).

X-ray diagnostics of hohlraum plasma flow

Abstract: In this study we use spectroscopy and x-ray imaging to investigate the macroscopic plasma flow in mm-sized laser-produced hohlraum plasmas. By using multiple diagnostics to triangulate the emission on a single experiment, we can pinpoint the position of dopants placed inside the hohlraum. X-ray emission from the foil has been used in the past to measure electron temperature. Here we analyze the spatial movement of dopant plasmas for comparison to hydrodynamic calculations.

Thomson scattering in the corona of laser-produced gold plasmas

Abstract: Thomson scattering measurements of the electron temperature in laser-produced gold plasmas are presented. We irradiated a flat gold disk target with one laser beam of the Nova laser facility at an angle of 64° from the normal. A second laser beam probed the plasma at a distance of 500 μm with temporally resolved Thomson scattering. The electron temperature measurements are compared with hydrodynamic simulations using the code LASNEX for experiments applying smoothed and unsmoothed heater beams. In case of an unsmoothed heater beam the simulations predict temperatures which are about 40% higher than our measured data. Although the agreement is improved for a smoothed heater beam, discrepancies exist in the decay phase of the plasma. We discuss possible explanations for these observations.

Thomson scattering from two-species laser-produced plasmas (invited)

Abstract: We report on recent Thomson scattering measurements from two ion species laser-produced plasmas. A flat disk coated with multiple alternate thin layers of gold and beryllium was irradiated with one laser beam of the Nova laser facility and the resulting two ion species plasma was probed with a second laser beam at a distance of 500 μm with time-resolved spectroscopy of the Thomson scattered light. A controlled variation of the Au fraction of the plasma was achieved by changing the relative thicknesses of the individual multilayers of the target. Two ion acoustic waves belonging to the Au and Be species were clearly observed. Besides their usual application to deduce electron temperatures, the relative damping of both waves provides an accurate measurement of the ion temperature of the plasma. In addition, the relative ion densities can be measured with high accuracy from the relative phase velocities of both waves if the ion charge state is known independently or vice versa.

Experiments on hot and dense laser-produced plasmas

Abstract: Plasmas generated by irradiating targets with ∼20 kJ of laser energy are routinely created in inertial confinement fusion research. X-ray spectroscopy provides one of the few methods for diagnosing the electron temperature and electron density. For example, electron densities approaching 1024 cm−3 have been diagnosed by spectral linewidths. However, the accuracy of the spectroscopic diagnostics depends the population kinetics, the radiative transfer, and the line shape calculations. Analysis for the complex line transitions has recently been improved and accelerated by the use of a database where detailed calculations can be accessed rapidly and interactively. Examples of data from Xe and Ar doped targets demonstrate the current analytic methods. First we will illustrate complications that arise from the presence of a multitude of underlying spectral lines. Then, we will consider the Ar He-like 1s2(1S0)−1s3p(1P0) transition where ion dynamic effects may affect the profile. Here, the plasma conditions are ...

Line profile measurements in a gas-liner pinch discharge

Abstract: We report on recent line shape measurements performed in a gas-liner pinch discharge. The 2s3s−2s3p singlet and triplet transitions in Ne VII were measured which are of interest for astrophysical line formation calculations. The measured widths of the Lorentzian line shapes are in good agreement with semi-classical calculations which include electron quadrupole broadening. The Stark widths of the 3s 2S−3p2 P0 transitions in lithiumlike N V and F VII were measured optically thin. At high plasma densities (⩽×1019 cm−3) the widths which are dominated by electron collisional broadening show deviations from linear scaling. Furthermore, we investigated spectral line shapes of transitions between levels of high principal quantum numbers of lithiumlike ions. The measurement of the n=4 to n=5 transition in F VII, where forbidden transitions and various (n−1,l)-(n,l′) fine structure components overlap, show excellent agreement with a suitable line shape code. Finally, we present measurements of the line shapes of P...

Investigation of a short-wavelength laser plasma of a gas-liner pinch discharge

Abstract: We studied the axial homogeneity of a gas-liner pinch plasma where amplification of line radiation at short wavelengths had been reported in an earlier study. For this purpose we measured temporally and axially resolved the Stark-broadened n=5 to n=4 transitions in F VII. From the linewidth electron densities are deduced employing spectral line shape calculations which are tested by Thomson scattering. We observe significant plasma inhomogeneities produced by Rayleigh-Taylor instabilities. Density gradients of {delta}n{sub e}/n{sub e}{approx}3 along the plasma axis on a scale length of z{lt}1 mm were found. They arise when compressing large amounts of a heavy gas (fluorine) of 5{endash}10{percent} total gas density in the discharge with a hydrogen pusher for short-wavelength laser studies. Reducing their amount to about 1{percent} of the total gas density results in homogeneous discharges. In this case no amplification can be observed. {copyright} {ital 1997} {ital The American Physical Society}

Observation of the saturation of Langmuir waves driven by ponderomotive force in a large scale plasma

Abstract: We report the observation of amplification of a probe laser beam (I {le} 1 {times} 10{sup 14} W/cm{sup 2}) in a large scale ({approximately} 1 mm) plasma by interaction with a pumping laser beam (I = 2 {times} 10{sup 15} W/cm{sup 2}) and a stimulated Langmuir wave. When the plasma density is adjusted to allow the Langmuir wave dispersion to match the difference frequency and wave number of the two beams, amplification factors as high as {times} 2.5 result. Interpretation of this amplification as scattering of pump beam energy by the Langmuir wave that is produced by the ponderomotive force of the two beams, allows the dependence of Langmuir wave amplitude on ponderomotive force to be measured. It is found that the Langmuir wave amplitude saturates at a level that depends on ion wave damping, and is generally consistent with secondary ion wave instabilities limiting its growth. 20 refs., 4 figs.