F. A. Starikov*
Russian Federal Nuclear Center 3/4 All-Russia Scientific-Research Institute of Experimental Physics,
Sarov, Nizhegorod oblast, 607190 Russia
*e-mail: starikov@otd13.vniief.ru
Received June 22, 1999
Abstract—A physical model, based on the solution of the quasi-optics equation for the transverse correlation
function (TCF) of the field amplitude, is developed for investigating the brightness, angular divergence, and
spatial coherence of the amplified spontaneous emission in a laboratory X-ray laser. The model takes account
of the spontaneous source of radiation, diffraction, regular refraction, regular amplification taking account of
saturation, nonresonance absorption, scattering by small-scale fluctuations of the electron density and the gain,
and scattering by random hose-like deviations of the extended plasma medium of the X-ray laser. It is estab-
lished that the TCF method makes it possible to obtain the final result much more quickly than the basic Monte Carlo
method for the parabolic equation for the field amplitude. As a result of the statistical linearization of the equation for
the transverse correlation function in the presence of gain saturation, this method overestimates the absolute values
of the average intensity of the amplified spontaneous radiation, but the maximum overestimation does not exceed
10%. It is found that fluctuations of the optical parameters of the medium of the X-ray laser degrade the quality
of the amplified spontaneous radiation beam, and they are the analog of the nonresonance absorption from the
standpoint of the effect on the brightness of the laser and therefore decrease the observed gain. For the charac-
teristic conditions of an X-ray laser with a quasistationary generation scheme, the contribution of small-scale
gain fluctuations and random hose-like deviations of the plasma filament of the laser to the scattering of the
amplified spontaneous radiation is much smaller than the contribution of small-scale density fluctuations. Cal-
culations of the amplified spontaneous radiation in an X-ray laser, which is produced by unilateral irradiation
of a curved target and possesses an asymmetric plasma electron density profile in the gain zone, are performed.
It is shown that in the gain saturation regime the coherence length and the coherent power of the amplified spon-
taneous radiation can be substantially increased, realizing in the gain zone a convex electron density profile
instead of a typical concave profile. It is found that this improvement of the coherence occurs only under con-
ditions such that the characteristic depth of the small-scale density fluctuations does not exceed several percent
of the typical regular values of the density in the gain zone. © 2000 MAIK “Nauka/Interperiodica”.
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