Steady ion momentum in nonlinear plasma waves.

The analysis of a one-dimensional two-fluid hydrodynamic model with relativistic electrons and nonrelativistic ions shows that the propagation of a nonlinear plasma wave is accompanied by a steady currentless plasma drift. Ions, due to their larger mass, appear to be the main carriers of the average momentum of the plasma wave. Two examples of nonlinear plasma waves generated by moving sources (short laser pulses and electron bunches) are analyzed to show details of the energy and momentum conservation laws.