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. We refer to a key
quantity, the photoionization parameter, defined as xi = 4{\pi}F/n_e where F is
the x-ray flux and n_e the electron density. This is usually meaningful in a
steady state context, but is commonly used, in the literature, as a figure of
merit for laboratory experiments that are, of necessity, time dependent. We
demonstrate that we can achieve values of xi >100 erg-cm s-1 using laser-plasma
x-ray sources, in the regime of interest for several astrophysical scenarios.
In particular, we show that our use of a keV line source, rather than the
quasi-blackbody radiation fields normally employed in such experiments, has
allowed generation of a ratio of inner-shell to outer-shell photoionization
expected from a blackbody source with ~keV spectral temperature. This is a key
factor in allowing experiments to be compared to the predictions of codes
employed to model astrophysical sources. 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 of 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.Comment: 20 pages, 9 figure