On the ponderomotive force of a high-frequency field in a cold plasma

It is shown that in the presence of the high-frequency electromagnetic field the pressure tensor has an averaged part which does not vanish in the cold plasma limit. It leads to the change of expressions for the ponderomotive force and for the stress tensor. It is found also that in the case of a warm plasma the inclusion into consideration only the scalar pressure seems to be incorrect. A correct approach should be based on the general statistical definition of the pressure tensor. Thermal effects can be excluded only after the calculation of the averaged part of the pressure tensor, which exists in both cold and warm plasma. The expression for the electromagnetic stress tensor obtained this way has additional terms explicitly representing the effects of dispersion of the medium (K.O.Kang and N.C.Lee, 1990).