Evolution of highly anisotropic plasma distribution functions in strong laser fields

The Boltzmann equation is solved to obtain a two-dimensional electron distr ibution function (2D EDF) to investigate the relaxation of highly anisotrop ic EDFs in plasmas acted by a strong laser field. Two kinds of anisotropic EDFs are considered: (i) elongated along the external field polarization di rection, and (ii) elongated perpendicularly to it. Two kinds of external fi elds are considered: (i) a linearly polarized field with constant amplitude , and (ii) a linearly polarized ultrashort-pulsed field. Basic results of t he calculations are (i) the EDFs initially elongated perpendicularly to the laser field polarization relax considerably faster than EDF's elongated al ong the field polarization; (ii) an initially highly anisotropic EDF, of an y shape, in a strong field relaxes slower than in a weak field, or in absen ce of any field; (iii) in cases of fast relaxation, an EDF initially elonga ted perpendicularly to field polarization approaches isotropization after h aving acquired a shape elongated parallel to the field polarization; (iv) d uring its relaxation, the calculated EDF is approximated rather well by a b i-Maxwellian with the same effective temperatures.