75 research outputs found
X-ray Thomson scattering spectra from DFT-MD simulations based on a modified Chihara formula
We study state-of-the-art approaches for calculating x-ray Thomson scattering
spectra from density functional theory molecular dynamics (DFT-MD) simulations
based on a modified Chihara formula that expresses the inelastic contribution
in terms of the dielectric function. We compare the electronic dynamic
structure factor computed from the Mermin dielectric function using an ab
initio electron-ion collision frequency to computations using a linear response
time dependent density functional theory (LR-TDDFT) framework for hydrogen and
beryllium and investigate the dispersion of free-free and bound-free
contributions to the scattering signal. A separate treatment of these
contributions in the Mermin dielectric function shows excellent agreement with
LR-TDDFT results for ambient-density beryllium, but breaks down for highly
compressed matter where the bound states become pressure ionized. LR-TDDFT is
used to reanalyze x-ray Thomson scattering experiments on beryllium
demonstrating strong deviations from the plasma conditions inferred with
traditional analytic models at small scattering angles.Comment: 14 pages, 10 figures, submitted to Physical Review
Bent crystal spectrometer for both frequency and wavenumber resolved x-ray scattering at a seeded free-electron laser
We present a cylindrically curved GaAs x-ray spectrometer with energy
resolution and wave-number resolution of
, allowing plasmon scattering at the resolution
limits of the Linac Coherent Light Source (LCLS) x-ray free-electron laser. It
spans scattering wavenumbers of 3.6 to \AA\ in 100 separate bins, with
only 0.34\% wavenumber blurring. The dispersion of 0.418~eV/m agrees
with predictions within 1.3\%. The reflection homogeneity over the entire
wavenumber range was measured and used to normalize the amplitude of scattering
spectra. The proposed spectrometer is superior to a mosaic HAPG spectrometer
when the energy resolution needs to be comparable to the LCLS seeded bandwidth
of 1~eV and a significant range of wavenumbers must be covered in one exposure
Carbon ionization at Gbar pressures: an ab initio perspective on astrophysical high-density plasmas
A realistic description of partially-ionized matter in extreme thermodynamic
states is critical to model the interior and evolution of the multiplicity of
high-density astrophysical objects. Current predictions of its essential
property, the ionization degree, rely widely on analytical approximations that
have been challenged recently by a series of experiments. Here, we propose a
novel ab initio approach to calculate the ionization degree directly from the
dynamic electrical conductivity using the Thomas-Reiche-Kuhn sum rule. This
Density Functional Theory framework captures genuinely the condensed matter
nature and quantum effects typical for strongly-correlated plasmas. We
demonstrate this new capability for carbon and hydrocarbon, which most notably
serve as ablator materials in inertial confinement fusion experiments aiming at
recreating stellar conditions. We find a significantly higher carbon ionization
degree than predicted by commonly used models, yet validating the qualitative
behavior of the average atom model Purgatorio. Additionally, we find the carbon
ionization state to remain unchanged in the environment of fully-ionized
hydrogen. Our results will not only serve as benchmark for traditional models,
but more importantly provide an experimentally accessible quantity in the form
of the electrical conductivity.Comment: accepted for publication in Physical Review Researc
Efficient laser-driven proton acceleration from cylindrical and planar cryogenic hydrogen jets.
We report on recent experimental results deploying a continuous cryogenic hydrogen jet as a debris-free, renewable laser-driven source of pure proton beams generated at the 150 TW ultrashort pulse laser Draco. Efficient proton acceleration reaching cut-off energies of up to 20 MeV with particle numbers exceeding 109 particles per MeV per steradian is demonstrated, showing for the first time that the acceleration performance is comparable to solid foil targets with thicknesses in the micrometer range. Two different target geometries are presented and their proton beam deliverance characterized: cylindrical (∅ 5 μm) and planar (20 μm × 2 μm). In both cases typical Target Normal Sheath Acceleration emission patterns with exponential proton energy spectra are detected. Significantly higher proton numbers in laser-forward direction are observed when deploying the planar jet as compared to the cylindrical jet case. This is confirmed by two-dimensional Particle-in-Cell (2D3V PIC) simulations, which demonstrate that the planar jet proves favorable as its geometry leads to more optimized acceleration conditions
A pulsed-laser calibration system for the laser backscatter diagnostics at the Omega laser
A calibration system has been developed that allows a direct determination of the sensitivity of the laser backscatter diagnostics at the Omega laser. A motorized mirror at the target location redirects individual pulses of a mJ-class laser onto the diagnostic to allow the in-situ measurement of the local point response of the backscatter diagnostics. Featuring dual wavelength capability at the 2nd and 3rd harmonic of the Nd:YAG laser, both spectral channels of the backscatter diagnostics can be directly calibrated. In addition, channel cross-talk and polarization sensitivity can be determined. The calibration system has been employed repeatedly over the last two years and has enabled precise backscatter measurements of both stimulated Brillouin scattering and stimulated Raman scattering in gas-filled hohlraum targets that emulate conditions relevant to those in inertial confinement fusion targets
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