Ponderomotive forces in a time-dependent plasma

Summary form only given. Forces acting on a plasma due to the presence of electric and magnetic fields are closely connected with energy and momentum densities of the fields. Although the corresponding expressions in the stationary case are well known, the problem for a time-dependent plasma is less trivial. The main point is that in a (weakly) time-dependent transparent medium an imaginary part of the dielectric function appears which depends on the slow time scale. The high-frequency electrostatic field in a nonstationary plasma creates three types of forces: the ponderomotive force, dissipative force (which necessarily includes terms because of the nonstationarity), and finally, the force due to change of momentum of the field. We find that for normal plasma modes, the general macroscopic consideration is sufficient to establish that only the pondermotive forces acts on the medium regardless of whether the system is open or closed.