  Generalizations of Onsager reciprocity relations are established for the
nonlinear response coefficients of ballistic transport in the effusion of
gaseous mixtures. These generalizations, which have been established on the
basis of the fluctuation theorem for the currents, are here considered for mass
separation by effusion. In this kinetic process, the mean values of the
currents depend nonlinearly on the affinities or thermodynamic forces
controlling the nonequilibrium constraints. These nonlinear transport effects
are shown to play an important role in the process of mass separation. In
particular, the entropy efficiency turns out to be significantly larger than it
would be the case if the currents were supposed to depend linearly on the
affinities.
