The heating of a completely ionized plasma by inverse Bremsstrahlung absorption (IB) is studied by numerically integrating the time varying kinetic equation for electrons, and the resulting distribution functions are used to calculate the continuum X-ray emission. The shape of the distribution functions is determined by the competition between IB and electron4ectron collisions, and, in all our uniform plasma simulations, we find that the distributions are well fitted by the formula Cwhere m is between 2 and 5 . We find the dependence of m on a = Z2u,,,/v:h, where Z is the atomic number, v,,, is the oscillation velocity in the laser field and vth is the thermal velocity: m(a) = 2 f 3/(1+ 1.66/a0 724). More importantly, in the realistic case of a space and time dependent kinetic simulation of a Be plasma heated by a Nd laser, including heat flow and ion motion, this fit remains valid for electron densities below O.75Nc (with constant m), although other effects come into play closer to the critical surface. Thus, these simple results can be used to determine the continuum X-ray spectrum due to Bremsstrahlung (BR) and to direct radiative recombination (DRR). The contributions of BR (valid for average and large atomic numbers) and DRR into each shell are given in a parametric form as functions of m and T,. The implications for the plasma diagnostics based on continuum X-ray emission are discussed.
The Maxwell-Bloch code COLAX has been upgraded to use detailed hydrodynamical and collisional-radiative simulations of a soft x-ray laser plasma with traveling-wave pumping. The seeding of short pulses of high-order harmonics of the pump laser into the x-ray laser medium has been simulated. The amplification is shown to be a dynamic, two-stage process: the atomic dipoles of the lasing ions are first coherently excited by the short pulse, and subsequently generate a radiation wake which is amplified along its path through the plasma, with consequences on the experimentally recorded spectra.
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