Generation of photoionized plasmas in the laboratory of relevance to accretion-powered x-ray sources using keV line radiation
High Energy Density Physics(2023)
摘要
We describe laboratory experiments to generate X-ray photoionized plasmas of
relevance to accretion-powered X-ray sources such as neutron star binaries and
quasars, with significant improvements over previous work. A key quantity is
referenced, namely the photoionization parameter. 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, time-dependent. We demonstrate emission-weighted values of ξ >
50 ergcm/s 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-beta
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.
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