Collision frequencies and electron distributions in a strong laser field

Strong field calculations are reported on subjects of large interest in radiation-matter interactions, such as the collision frequencies and the time evolution of the electron velocity distribution function of a fully ionized plasma. The collision frequencies calculations are carried out within both nonrelativistic and relativistic treatments. In the domain of high intensities, with a linearly polarized field, the easiest conditions under which the collision frequency shows a behavior similar to v(e)(-3), with v(e) being the peak quiver velocity, are met by an electron beam having parallel velocity to the field polarization. For all the other cases, including electrons having a Maxwellian distribution, considerably higher intensities are required to observe the v, behavior. Concerning the electron distribution function (EDF) calculations, a two-dimensional kinetic equation is solved without restrictions as to the degree of anisotropy. For initial values of the ratio v(e)/v(T)(0) up to values slightly larger than 4, an initially isotropic EDF is found to evolve rapidly towards an oblate shape (i.e., elongated in the field polarization direction), maintaining such a shape also in later times, when the relaxation stage is entered. V-T(0) is the initial thermal velocity. For values of v(e)/v(T)(0) larger than 4, the initially isotropic EDF is found to first evolve rapidly towards an oblate shape (i.e. elongated perpendicularly to the field polarization direction), and then to change to an oblate form. The final stage of relaxation is entered with the oblate form. The larger the initial value of the ratio v(e)/v(T)(0), the longer times are required for EDF to pass through all the stages of its evolution.