Numerical study of a CO(2) laser beam focused on a theta-pinch plasma in the axial direction.

Using two different empirical density profiles for the end region of a theta-pinch plasma, one with a maximum density on the axis (radiation-dispersing profile) the other with a pronounced axial minimum (radiation- trapping profile), the trajectory of the CO(2) laser beam (10.6 microm) focused axially on such a plasma was studied numerically. This calculation is used to evaluate the optical influence of the plasma, since the maximum power density in the focal plane can be reduced by several orders of magnitude owing to the presence of the plasma. This influence can be substantial even for very subcritical electron densities (n(e) << 10(19) cm(-3)). In cases of large dispersion, the characteristics of a multifocal lens capable of producing perfect focusing are found, and it is shown that the solution is not unique. The radial distribution of the laser beam power density is also calculated and shows numerous irregularities and discontinuities due to the nonuniform beam dispersion.