Evolution of highly anisotropic plasma distribution functions in strong laser fields

The Boltzmann equation is solved to obtain a two-dimensional electron distribution function (2D EDF) to investigate the relaxation of highly anisotropic EDFs in plasmas acted by a strong laser field. Two kinds of anisotropic EDFs are considered: (i) elongated along the external field polarization direction, and (ii) elongated perpendicularly to it. Two kinds of external fields are considered: (i) a linearly polarized field with constant amplitude, and (ii) a linearly polarized ultrashort-pulsed field. Basic results of the calculations are (i) the EDFs initially elongated perpendicularly to the laser field polarization relax considerably faster than EDF's elongated along the field polarization; (ii) an initially highly anisotropic EDF, of any shape, in a strong field relaxes slower than in a weak field, or in absence of any field; (iii) in cases of fast relaxation, an EDF initially elongated perpendicularly to field polarization approaches isotropization after having acquired a shape elongated parallel to the field polarization; (iv) during its relaxation, the calculated EDF is approximated rather well by a bi-Maxwellian with the same effective temperatures.