Showing papers by "Siegfried Glenzer published in 2001"

Dense matter characterization by X-ray Thomson scattering

Abstract: We discuss the extension of the powerful technique of Thomson scattering to the X-ray regime for providing an independent measure of plasma parameters for dense plasmas. By spectrally resolving the scattering, the coherent (Rayleigh) unshifted scattering component can be separated from the incoherent Thomson component, which is both Compton and Doppler shifted. The free electron density and temperature can then be inferred from the spectral shape of the high-frequency Thomson scattering component. In addition, as the plasma temperature is decreased, the electron velocity distribution as measured by incoherent Thomson scattering will make a transition from the traditional Gaussian Boltzmann distribution to a density-dependent parabolic Fermi distribution. We also present a discussion for a proof-of-principle experiment appropriate for a high-energy laser facility.

Backscatter reduction using combined spatial, temporal, and polarization beam smoothing in a long-scale-length laser plasma.

Abstract: Spatial, temporal, and polarization smoothing schemes are combined for the first time to reduce to a few percent the total stimulated backscatter of a NIF-like probe laser beam $(2\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0ex}{0ex}}\mathrm{W}/\mathrm{cm}{}^{2},\phantom{\rule{0ex}{0ex}}351\phantom{\rule{0ex}{0ex}}\mathrm{nm},\phantom{\rule{0ex}{0ex}}f/8)$ in a long-scale-length laser plasma. Combining temporal and polarization smoothing reduces simulated Brillouin scattering and simulated Raman scattering (SRS) up to an order of magnitude although neither smoothing scheme by itself is uniformly effective. The results agree with trends observed in simulations performed with the laser-plasma interaction code F3D simulations [R. L. Berger et al., Phys. Plasma 6, 1043 (1999)].

Ionization balance in inertial confinement fusion hohlraums.

Abstract: We present the first x-ray spectroscopic measurements of the ionization balance in inertial confinement fusion hohlraums supported by $4\ensuremath{\omega}$ Thomson scattering diagnostics. The experimental data show agreement with non-LTE radiation-hydrodynamic calculations of the averaged Au charge state and electron temperatures. These findings are consistent with the successful integrated modeling of the hohlraum radiation fields. Comparisons with detailed synthetic spectra calculations show that the experimental ionization distribution is slightly shifted indicating nonsteady state kinetics.

Electron distribution function in laser heated plasmas

Abstract: A new electron distribution function has been found in laser heated homogeneous plasmas by an analytical solution to the kinetic equation and by particle simulations. The basic kinetic model describes inverse bremsstrahlung absorption and electron–electron collisions. The non-Maxwellian distribution function is comprised of a super-Gaussian bulk of slow electrons and a Maxwellian tail of energetic particles. The tails are heated due to electron–electron collisions and energy redistribution between superthermal particles and light absorbing slow electrons from the bulk of the distribution function. A practical fit is proposed to the new electron distribution function. Changes to the linear Landau damping of electron plasma waves are discussed. The first evidence for the existence of non-Maxwellian distribution functions has been found in the interpretation, which includes the new distribution function, of the Thomson scattering spectra in gold plasmas [Glenzer et al., Phys. Rev. Lett. 82, 97 (1999)].

Thomson scattering measurements of saturated ion waves in laser fusion plasmas.

Abstract: We have measured the characteristics of saturated ion-acoustic waves in inertial confinement fusion plasmas. A 263-nm probe laser has been applied to simultaneous Thomson scatter on both ion-acoustic waves excited by thermal electrostatic fluctuations and by stimulated Brillouin scattering of a kilojoule laser beam of varying intensity. The Thomson scattering spectra show saturated ion-wave amplitudes for intensities above 5x10(14) W cm(-2) consistent with three dimensional nonlinear wave modeling.

Status of our understanding and modeling of x-ray coupling efficiency in laser heated hohlraums

Abstract: Recent changes in the manner of performing hohlraum drive experiments have significantly advanced the ability to diagnose, understand and control the x-radiation flux (or drive) inside a laser heated hohlraum. Comparison of modeling and data from a very broad range of hohlraum experiments indicates that radiation hydrodynamics simulation codes reproduce measurements of time dependent x-radiation flux to about ±10%. This, in turn, indicates that x-ray production and capsule coupling in ignition hohlraums will be very close to expectations. This article discusses the changes to experimental procedures and the broad variety of measurements and tests leading to these findings.

Reduction of stimulated scattering losses from hohlraum plasmas with laser beam smoothing

Abstract: Laser beam smoothing by spectral dispersion and by polarization smoothing has been observed to significantly reduce the scattering losses by stimulated Brillouin and stimulated Raman scattering from inertial confinement fusion hohlraums. For these measurements, the laser beam smoothing and the high-Z hohlraum wall plasma parameters approach the conditions of future inertial confinement fusion experiments. The simultaneous application of the smoothing techniques has reduced the scattering losses by almost one order of magnitude down to the 1% level. The experimental scaling of the stimulated Brillouin reflectivity compares well to modeling assuming nonlinear damping on the ion acoustic waves in three-dimensional nonlinear wave simulations and calculated hohlraum plasma conditions from radiation-hydrodynamic modeling.

Detailed hydrodynamic and X-ray spectroscopic analysis of a laser-produced rapidly-expanding aluminium plasma

Abstract: We present a detailed analysis of K-shell emission from laser-produced rapidly-expanding Al plasmas. This work forms part of a series of experiments performed at the Vulcan laser facility of the Rutherford Appleton Laboratory, UK. 1-D planar expansion was obtained by over-illuminating Al-microdot targets supported on CH plastic foils. The small size of the Al-plasma ensured high spatial and frequency resolution of the spectra, obtained with a single crystal spectrometer, two vertical dispersion variant double crystal spectrometers, and a vertical dispersion variant Johann Spectrometer. The hydrodynamic properties of the plasma were measured independently by spatially and temporally resolved Thomson scattering, utilizing a 4ω probe beam. This enabled sub- and super-critical densities to be probed relative to the 1ω heater beams. The deduced plasma hydrodynamic conditions are compared with those generated from the 1-D hydro-code Medusa, and the significant differences found in the electron temperature discussed. Synthetic spectra generated from the detailed term collisional radiative non-LTE atomic physics code Fly are compared with the experimental spectra for the measured hydrodynamic parameters, and for those taken from Medusa. Excellent agreement is only found for both the H- and He-like Al series when careful account is taken of the temporal evolution of the electron temperature.

Ionization balance in inertial confinement fusion hohlraum plasmas

Abstract: We present X-ray spectroscopic measurements of the ionization balance in inertial confinement fusion hohlraums supported by 4 ω Thomson scattering diagnostics. The experimental data show agreement with non-LTE radiation-hydrodynamic calculations of the averaged Au charge state and electron temperatures. These findings are consistent with the successful integrated modeling of the hohlraum radiation fields. Comparisons with detailed synthetic spectra calculations show that the experimental ionization distribution is shifted indicating non-steady state kinetics.

A Thomson scattering post-processor for the MEDUSA hydrocode

Abstract: In order to understand the physical processes that occur in laser-produced plasmas it is necessary to diagnose the time-dependent hydrodynamic conditions. Thomson scattering is, in principle, an ideal diagnostic as it provides a non-intrusive method of measuring ion and electron temperature, electron density, plasma velocity, and heat flow. We describe here a post-processor for the MEDUSA hydrocode that simulates streak camera images of the Thomson spectra. The post-processor can be used in three ways: (1) creating simulated streak camera images that can be compared directly with experimental data, (2) evaluating experimental designs to determine the viability of the Thomson scattering diagnostic, and (3) as an automated data analysis routine for extracting hydrodynamic parameters from a calibrated experimental streak camera image.

Using high resolution x-ray spectroscopy of laser and EBIT plasma sources to test atomic models

Abstract: Understanding of the formation and dynamics of non-LTE plasmas is crucial to inertial and magnetic fusion energy research as well as astrophysical research. The intrinsic complexity of non-LTE systems at high density and the remote nature of x-ray emitting astrophysical plasmas present stringent challenges to our models. Confidence in our atomic kinetics models is gained through comparisons with experimental data from plasmas that are well diagnosed by techniques that are independent of spectroscopic models. To this end experiments with Au foils buried in Be targets have been designed to achieve steady-state, gradient free conditions on the Nova laser. Simulations of the experiments that use detailed atomic kinetics models have shown good agreement with measured x-ray spectra. Large sets of data and many detailed processes are included in these models; issues involving satellite configurations, accuracy of dielectronic recombination rates and non-zero optical depths for resonant lines have to be addressed. To test the detailed atomic data used in the models, experiments have been carried out on an electron beam ion trap (EBIT). The EBIT experiments measure directly the contribution of collisional excitation, dielectronic recombination and radiative recombination to the observed x-ray lines. Good agreement is found between the EBIT spectra and the HULLAC data used to interpret the Nova buried layer experiments.

Warm, Dense Plasma Characterization by X-ray Thomson Scattering

Abstract: We describe how the powerful technique of spectrally resolved Thomson scattering can be extended to the x-ray regime, for direct measurements of the ionization state, density, temperature, and the microscopic behavior of dense cool plasmas. Such a direct measurement of microscopic parameters of solid density plasmas could eventually be used to properly interpret laboratory measurements of material properties such as thermal and electrical conductivity, EOS and opacity. In addition, x-ray Thomson scattering will provide new information on the characteristics of hitherto difficult to diagnose Fermi degenerate and strongly coupled plasmas.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.