Generic model of an atom laser

We present a generic model of an atom laser by including a pump and loss te rm in the Gross-Pitaevskii equation. We show that there exists a threshold for the pump above which the mean matter field assumes a nonvanishing value in the steady state. We study the transient regime of this atom laser, and find oscillations around the stationary solution even in the presence of a loss term. These oscillations are damped away when we introduce a position -dependent loss term. For this case we present a modified Thomas-Fermi solu tion that takes into account the pump and loss. Our generic model of an ato m laser is analogous to the semiclassical theory of the laser. [S1050-2947( 98)07412-5].