Ponderomotive force of a high-frequency electromagnetic field in a dispersive plasma

Ponderomotive force density of an HF field in a dispersive cold collisionless plasma is calculated using two independent methods: energy-momentum-stress tensor with the Abraham choice of momentum density and two-fluid hydrodynamics. The results appear to coincide and show that, even in isotropic plasma, terms proportional to the time derivative of the electric field amplitude appear in the ponderomotive force. It is demonstrated that, in the presence of relativistic electron beams, spatial dispersion is not negligible and the related correction to the ponderomotive force is calculated.