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Languages : en
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Book Description
Modeling X-ray Thomson Scattering Spectra of Warm Dense Matter
Author: Using X-ray Thomson Scattering to Measure Plasma Conditions in Warm Dense Matter Experiments on the OMEGA Laser
Author: Alison Marie-Anne SaundersPublisher:
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Languages : en
Pages : 143
Book Description
High energy density physics (HEDP) is an emerging field that seeks to investigate the properties of matter at extreme conditions. High energy density conditions occur in materials with pressures exceeding 1 Mbar, or pressures that exceed Earth’s atmospheric pressure by a factor of more than a million. A regime of HEDP of particular interest is warm dense matter (WDM) physics, which describes the behavior of materials at near solid densities and 10’s of eV temperatures. WDM occurs in astrophysical objects, such as giant planets and brown dwarfs, and is also generated in inertial confinement fusion (ICF) experiments. X-ray Thomson scattering (XRTS) offers a powerful tool to probe the equation of state of WDM. XRTS spectra consist of two components: elastically scattered photons with the frequency of the original x-ray source and inelastically scattered photons that are down- shifted in frequency. The Compton-shifted profile of inelastically scattered x-rays can be analyzed to return the sample’s electron density and electron temperature. The ratio of elastically to inelastically scattered x-rays relates to the number of tightly bound versus free electrons, and thus reflects the ionization state. This thesis discusses the results of XRTS experiments on WDM performed at the OMEGA Laser facility. The first experiment presents and discusses XRTS results from 1 mm diamond spheres. The scattering spectra show evidence of higher ionization than predicted by several commonly-applied ionization models. A second experiment analyzed the contributions to elastic scattering from a small argon impurity in imploding beryllium capsules. The exper- iment found that less than 1 at.% of argon significantly affects the elastic scattering signal strength, and concluded that impurities in a sample should be considered before drawing conclusions from elastic scattering signals. The final experiment uses XRTS to measure the electron temperature and ionization state in isochorically heated materials used in ion stopping power experiments. The results from these experiments demonstrate the power of XRTS to measure ionization in WDM to benchmark theoretical modeling.
Theory of X-ray Thomson Scattering in Warm Dense Matter
Author: Kathrin WünschPublisher:
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Languages : en
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This thesis presents the theoretical framework required to apply spectrally resolved x-ray Thomson scattering (XRTS) as a diagnostic method for warm dense matter. In particular, the theory is generalised to allow for the description of systems with multiple ion species where all mutual correlations are taken into account within the new approach. Supplemented with the theory presented, XRTS is now a promising diagnostics for high-energy-density matter containing different chemical elements or mixtures of different materials. The signal measured at XRTS contains the unshifted Rayleigh peak and frequency-shifted features. The first is related to elastic scattering from electrons co-moving with the ions whilst the second occurs due to scattering from free electrons and excitation/ionisation events. The focus of this thesis lies on the elastic scattering feature which requires the ion structure and the electron density around the ion as input for the theoretical modelling. The ion structure is obtained from quantum simulations (DFT-MD) and classical hypernetted-chain (HNC) equations. The analysis of the DTF-MD simulation data reveals that partial ionisation yields strong modifications of the ion-ion interactions. Similar effects are found for the form of the electron screening cloud around an ion. On the basis of the newly developed theory and structural models, multicomponent effects on the XRTS signal are studied. It is shown that the Rayleigh feature is very sensitive to the ratio of the elements in the scattering volume and their mutual correlations. These results indicate that XRTS is well-suited to probe the properties of complex materials and the process of mixing in the WDM regime. The advanced theories are finally applied to experimental spectra. The procedure allows for both extracting the basic plasma parameters and assessing the quality of the theoretical models applied. Comparisons with several experiments demonstrated that the non-collective regime (large scattering angle) is reasonably well understood whereas the collective regime (small scattering angle/long wavelength limit) still holds challenges. The collective regime is problematic as here strong correlations and screening are highly relevant and, thus, a yet unknown description for fully coupled quantum systems needs to be applied.
Simulations of X-ray Thomson Scattering from Warm Dense Matter
Author: Ghulam Shabbir NazUsing Collective X-ray Thomson Scattering to Measure Temperature and Density of Warm Dense Matter
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Languages : en
Pages : 11
Book Description
Collective x-ray Thomson scattering allows measuring plasmons, i.e electron plasma oscillations (Langmuir waves). This is manifest in the appearance of spectrally up- and down-shifted spectral features in addition to the Rayleigh signal. The ratio of the up- and down-shifted signals is directly related to detailed balance, allowing to determine the plasma temperature from first principles. The spectral shift of the plasmon signals is sensitive to temperature and electron density. We discuss the experimental considerations that have to be fulfilled to observe plasmon signals with x-ray Thomson scattering. As an example, we describe an experiment that used the Cl Ly-[alpha] x-ray line at 2.96 keV to measure collective Thomson scattering from solid beryllium, isochorically heated to 18 eV. Since temperature measurement based on detailed balance is based on first principles, this method is important to validate models that, for example, calculate the static ion-ion structure factor S{sub ii}(k).
Simulations of X-ray Thomson Scattering From Warm Dense Matter
Author: G. S. N. NazX-ray Thomson Scattering Measurements of Warm Dense Matter
Author: Demonstration of Space-resolved X-ray Thomson Scattering Capability for Warm Dense Matter Experiments on the Z Accelerator
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Languages : en
Pages : 12
Book Description
Experiments on the Sandia Z pulsed-power accelerator demonstrated the ability to produce warm dense matter (WDM) states with unprecedented uniformity, duration, and size, which are ideal for investigations of fundamental WDM properties. For the first time, space-resolved x-ray Thomson scattering (XRTS) spectra from shocked carbon foams were recorded on Z. The large (> 20 MA) electrical current produced by Z was used to launch Al flyer plates up to 25 km/s. The impact of the flyer plate on a CH2 foam target produced a shocked state with an estimated pressure of 0.75 Mbar, density of 0.52 g/cm3, and temperature of 4.3 eV. Both unshocked and shocked portions of the foam target were probed with 6.2 keV x-rays produced by focusing the Z-Beamlet laser onto a nearby Mn foil. The data is composed of three spatially distinct spectra that were simultaneously captured with a single spectrometer with high spectral (4.8 eV) and spatial (190 [mu]m) resolutions. Furthermore, these spectra provide detailed information on three target locations: the laser spot, the unshocked foam, and the shocked foam.
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