In superintense laser beams collisional absorption exhibits a large amplitude modulation over a laser cycle. In this paper formulas for the time-dependent electron-ion collision frequency nu(ei)(t) are presented. On the basis of a ballistic interaction model we deduce an expression for nu(ei)(t) which holds for an arbitrary isotropic distribution function and arbitrary anharmonic oscillatory electron motion [Eq. (4)]. For a Maxwellian we present compact formulas for the various ratios v(os)(t)/v(th). It is shown that the strong time dependence over one laser cycle leads to the generation of intense odd harmonics. The cycle-averaged collision frequency nu(ei);(t) is compared with expressions derived from the more complex dielectric model. It is shown that the correct choice of cutoffs as a consequence of dynamical screening is essential.

The stationary expansion of an ionized uranium vapor, created in an electron-beam evaporator, is studied in this article. Using an electrostatic probe, the electron temperature, the electron density, and the plasma potential have been measured along the vertical axis in the evaporator. The analysis, made by moving the probe along the vertical axis, first shows that the plasma is thermalized during its expansion in the vacuum chamber and, second, that the plasma density decreases exponentially. The plasma expansion is theoretically described using a fluid model which takes into account the charge exchange process. In solving the system, consisting of three equations (continuity, momentum conservation, and energy conservation), good agreement with the experimental data is obtained.

In femtosecond-laser-matter interaction, collisional absorption plays an important role during the early stages of the interaction, when the laser intensity and the plasma temperature still have moderate values. We propose a cutoff impact parameter b(max) for the Coulomb logarithm lambda=ln Lambda in the electron-ion collision rate which takes into account, for an overdense plasma, the crystalline structure of the ion background. Calculations are presented for a bcc lattice and generalized to sc and fcc lattices. The results are applicable for laser intensities I(l)< or =10(17) W cm(-2) and when the Debye theory is valid, that is to say, when the Landau length is distinctly less than the minimum ion distance, and when the Debye screening is unaffected by the electron quiver motion in the laser field. We present the space angle dependence of the cutoff b(max) as also its spatial average, and we discuss the ensuing corrections to the Coulomb logarithm due to the improved values of b(max). Finally, we present the pertinent values, averaged over a Maxwellian distribution function, which are needed for applications of the ballistic model for collisions.

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