3,044 research outputs found
Using the X-FEL to understand X-ray Thomson scattering for partially ionized plasmas
For the last decade numerous researchers have been trying to develop
experimental techniques to use X-ray Thomson scattering as a method to measure
the temperature, electron density, and ionization state of high energy density
plasmas such as those used in inertial confinement fusion. With the advent of
the X-ray free electron laser (X-FEL) at the SLAC Linac Coherent Light Source
(LCLS) we now have such a source available in the keV regime. One challenge
with X-ray Thomson scattering experiments is understanding how to model the
scattering for partially ionized plasmas. Most Thomson scattering codes used to
model experimental data greatly simplify or neglect the contributions of the
bound electrons to the scattered intensity. In this work we take the existing
models of Thomson scattering that include elastic ion-ion scattering and the
electron-electron plasmon scattering and add the contribution of the bound
electrons in the partially ionized plasmas. Except for hydrogen plasmas almost
every plasma that is studied today has bound electrons and it is important to
understand their contribution to the Thomson scattering, especially as new
X-ray sources such as the X-FEL will allow us to study much higher Z plasmas.
Currently most experiments have looked at hydrogen or beryllium. We will first
look at the bound electron contributions to beryllium by analysing existing
experimental data. We then consider several higher Z materials such as Cr and
predict the existence of additional peaks in the scattering spectrum that
requires new computational tools to understand. For a Sn plasma we show that
the bound contributions changes the shape of the scattered spectrum in a way
that would change the plasma temperature and density inferred by the
experiment.Comment: 13th International Conference on X-ray Lasers Paris, France June 10,
2012 through June 15, 201
Average-Atom Model for X-ray Scattering from Warm Dense Matter
A scheme for analyzing Thomson scattering of x-rays by warm dense matter,
based on the average-atom model, is developed. Emphasis is given to x-ray
scattering by bound electrons. Contributions to the scattered x-ray spectrum
from elastic scattering by electrons moving with the ions and from inelastic
scattering by free and bound electrons are evaluated using parameters (chemical
potential, average ionic charge, free electron density, bound and continuum
wave functions, and occupation numbers) taken from the average-atom model. The
resulting scheme provides a relatively simple diagnostic for use in connection
with x-ray scattering measurements. Applications are given to dense hydrogen,
beryllium, aluminum, titanium, and tin plasmas. At high momentum transfer,
contributions from inelastic scattering by bound electrons are dominant
features of the scattered x-ray spectrum for aluminum, titanium, and tin.Comment: 22 pages, 10 figures Presentation at Workshop IV: Computational
Challenges in Warm Dense Matter at IPAM (UCLA) May 21 - 25, 201
Shuttle Ku-band and S-band communications implementations study
The interfaces between the Ku-band system and the TDRSS, between the S-band system and the TDRSS, GSTDN and SGLS networks, and between the S-band payload communication equipment and the other Orbiter avionic equipment were investigated. The principal activities reported are: (1) performance analysis of the payload narrowband bent-pipe through the Ku-band communication system; (2) performance evaluation of the TDRSS user constraints placed on the S-band and Ku-band communication systems; (3) assessment of the shuttle-unique S-band TDRSS ground station false lock susceptibility; (4) development of procedure to make S-band antenna measurements during orbital flight; (5) development of procedure to make RFI measurements during orbital flight to assess the performance degradation to the TDRSS S-band communication link; and (6) analysis of the payload interface integration problem areas
X-ray Thomson scattering for partially ionized plasmas including the effect of bound levels
X-ray Thomson scattering is being developed as a method to measure the
temperature, electron density, and ionization state of high energy density
plasmas such as those used in inertial confinement fusion. Most experiments are
currently done at large laser facilities that can create bright X-ray sources,
however the advent of the X-ray free electron laser (X-FEL) provides a new
bright source to use in these experiments. One challenge with X-ray Thomson
scattering experiments is understanding how to model the scattering for
partially ionized plasmas in order to include the contributions of the bound
electrons in the scattered intensity. In this work we take the existing models
of Thomson scattering that include elastic ion-ion scattering and the
electron-electron plasmon scattering and add the contribution of the bound
electrons in the partially ionized plasmas. We validated our model by analyzing
existing beryllium experimental data. We then consider several higher Z
materials such as Cr and predict the existence of additional peaks in the
scattering spectrum that requires new computational tools to understand. We
also show examples of experiments in CH and Al that have bound contributions
that change the shape of the scattered spectra.Comment: SPIE 2013 Optics and Photonics, San Diego, CA, United States August
25, 2013 through August 29, 2013. arXiv admin note: substantial text overlap
with arXiv:1212.5972, arXiv:1207.507
Gyroscopic motion of superfluid trapped atomic condensates
The gyroscopic motion of a trapped Bose gas containing a vortex is studied.
We model the system as a classical top, as a superposition of coherent
hydrodynamic states, by solution of the Bogoliubov equations, and by
integration of the time-dependent Gross-Pitaevskii equation. The frequency
spectrum of Bogoliubov excitations, including quantum frequency shifts, is
calculated and the quantal precession frequency is found to be consistent with
experimental results, though a small discrepancy exists. The superfluid
precession is found to be well described by the classical and hydrodynamic
models. However the frequency shifts and helical oscillations associated with
vortex bending and twisting require a quantal treatment. In gyroscopic
precession, the vortex excitation modes are the dominant features
giving a vortex kink or bend, while the is found to be the dominant
Kelvin wave associated with vortex twisting.Comment: 18 pages, 7 figures, 1 tabl
Effective interactions and large-scale diagonalization for quantum dots
The widely used large-scale diagonalization method using harmonic oscillator
basis functions (an instance of the Rayleigh-Ritz method, also called a
spectral method, configuration-interaction method, or ``exact diagonalization''
method) is systematically analyzed using results for the convergence of Hermite
function series. We apply this theory to a Hamiltonian for a one-dimensional
model of a quantum dot. The method is shown to converge slowly, and the
non-smooth character of the interaction potential is identified as the main
problem with the chosen basis, while on the other hand its important advantages
are pointed out. An effective interaction obtained by a similarity
transformation is proposed for improving the convergence of the diagonalization
scheme, and numerical experiments are performed to demonstrate the improvement.
Generalizations to more particles and dimensions are discussed.Comment: 7 figures, submitted to Physical Review B Single reference error
fixe
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The European Climate Research Alliance (ECRA) is an association of leading European research institutions in the field of climate research (http://www.ecra-climate.eu/, last access: 6 December 2018). ECRA is a bottom-up initiative and helps to facilitate the development of climate change research, combining the capacities of national research institutions, and inducing closer ties between existing national research initiatives, projects and infrastructures. ECRA works as an open platform to bring together climate researchers, providing excellent scientific expertise for policy makers and of societal relevance. The ECRA Board consists of representatives of ECRA partners and decides on governance, scientific priorities, and organisational matters.
Currently organized into four Collaborative Programmes, climate scientists share their knowledge, experience and expertise to identify the most important research requirements for the future, thus developing a foresight approach. The CPs cover the topics: (1) Arctic variability and change, (2) Sea level changes and coastal impacts, (3) Changes in the hydrological cycle and (4) High impact events. The CP activities are planned in workshops and participation is open to all interested scientists from the relevant research fields. In particular, young researchers are actively encouraged to join the network. Each CP develops its joint research priorities for shaping European research into the future. Because scientific themes are interconnected, the four Collaborative Programmes interact with each other, e.g. through the organization of common workshops or joint applications. In addition, the Collaborative Programme leads attend the Board meetings.
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Fragmentation of UH Nuclei
We have measured the total charge changing cross sections as a function of energy for projectile _(36)Kr nuclei in a wide range of targets ranging from polyethylene to lead. These cross sections are energy dependent and the dependence increases as the target mass increases
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