Generation of photoionized plasmas in the laboratory of relevance to accretion-powered x-ray sources using keV line radiation
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Published version
Author(s)
Type
Journal Article
Abstract
We describe laboratory experiments to generate x-ray photoionized plasmas of relevance to accretion-powered xray sources such as neutron star binaries and quasars, with significant improvements over previous work. A key
quantity is referenced, namely the photoionization parameter, defined as ξ = 4πF/ne where F is the x-ray flux and
ne the electron density. This is normally meaningful in an astrophysical steady-state context, but is also
commonly used in the literature as a figure of merit for laboratory experiments that are, of necessity, timedependent. We demonstrate emission-weighted values of ξ > 50 erg-cm s
− 1 using laser-plasma x-ray sources,
with higher results at the centre of the plasma which are in the regime of interest for several astrophysical
scenarios. Comparisons of laboratory experiments with astrophysical codes are always limited, principally by the
many orders of magnitude differences in time and spatial scales, but also other plasma parameters. However
useful checks on performance can often be made for a limited range of parameters. For example, we show that
our use of a keV line source, rather than the quasi-blackbody radiation fields normally employed in such experiments, has allowed the generation of the ratio of inner-shell to outer-shell photoionization expected from a
blackbody source with ~keV spectral temperature. We compare calculations from our in-house plasma modelling
code with those from Cloudy and find moderately good agreement for the time evolution of both electron
temperature and average ionisation. However, a comparison of code predictions for a K-β argon X-ray spectrum
with experimental data reveals that our Cloudy simulation overestimates the intensities of more highly ionised
argon species. This is not totally surprising as the Cloudy model was generated for a single set of plasma conditions, while the experimental data are spatially integrated.
quantity is referenced, namely the photoionization parameter, defined as ξ = 4πF/ne where F is the x-ray flux and
ne the electron density. This is normally meaningful in an astrophysical steady-state context, but is also
commonly used in the literature as a figure of merit for laboratory experiments that are, of necessity, timedependent. We demonstrate emission-weighted values of ξ > 50 erg-cm s
− 1 using laser-plasma x-ray sources,
with higher results at the centre of the plasma which are in the regime of interest for several astrophysical
scenarios. Comparisons of laboratory experiments with astrophysical codes are always limited, principally by the
many orders of magnitude differences in time and spatial scales, but also other plasma parameters. However
useful checks on performance can often be made for a limited range of parameters. For example, we show that
our use of a keV line source, rather than the quasi-blackbody radiation fields normally employed in such experiments, has allowed the generation of the ratio of inner-shell to outer-shell photoionization expected from a
blackbody source with ~keV spectral temperature. We compare calculations from our in-house plasma modelling
code with those from Cloudy and find moderately good agreement for the time evolution of both electron
temperature and average ionisation. However, a comparison of code predictions for a K-β argon X-ray spectrum
with experimental data reveals that our Cloudy simulation overestimates the intensities of more highly ionised
argon species. This is not totally surprising as the Cloudy model was generated for a single set of plasma conditions, while the experimental data are spatially integrated.
Date Issued
2024-06
Date Acceptance
2024-03-18
Citation
High Energy Density Physics, 2024, 51
ISSN
1574-1818
Publisher
Elsevier
Journal / Book Title
High Energy Density Physics
Volume
51
Copyright Statement
© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
License URL
Identifier
http://dx.doi.org/10.1016/j.hedp.2024.101097
Publication Status
Published
Article Number
101097
Date Publish Online
2024-03-19