  In the framework of mean-field approximation the dynamics of Bose-Einstein
condensates can be described by the hydrodynamic-like equations. These
equations are analyzed here with account of mutual interaction between
condensate and non-condensate atoms. The Lagrange invariants and freezing-in
invariants of such a system have been found. This allows to get some necessary
conditions for creation of an atom laser with controlled parameters of the
beam. Particularly, the atom laser beam can carry quite well defined angular
momentum. This can be practically realized in the most simple case, when the
vorticity of condensate appears to be a freezing-in field. The optimal
conditions for a source mode regime are found out in the paper.
