  Spectral properties of the x-ray pulses, generated by perfectly uniform
cylindrical implosions of tungsten plasma with parameters typical of wire array
z-pinches, are investigated under the simplifying assumption that the final
stage of the kinetic-to-radiant energy conversion is not affected by the
magnetic field. The x-ray emission is shown to be generated within a narrow
(sub-micron) radiation-dominated stagnation shock front with a "supercritical"
amplitude. The structure of the stagnation shock is investigated by using two
independent radiation-hydrodynamics codes, and by constructing an approximate
analytical model. The x-ray spectra are calculated for two values of the plasma
column mass, 0.3 mg/cm and 6 mg/cm, with a newly developed two-dimensional
radiation-hydrodynamics code RALEF-2D. The hard component of the spectrum (with
a blackbody-fit temperature of 0.5-0.6 keV for the 6-mg/cm mass) originates
from a narrow peak of the electron temperature inside the stagnation shock. The
softer main component emerges from an extended halo, where the primary shock
radiation is reemitted by colder layers of the imploding plasma. Our calculated
x-ray spectrum for the 6-mg/cm tungsten column agrees well with the published
Sandia experimental data (Foord et al., 2004, Phys. Rev. Lett., vol. 93,
055002).
