Ponderomotive force and dc magnetic field generation induced by laser absorption

A new expression for the ponderomotive force induced by high frequency electromagnetic and electrostatic waves propagating in plasma including the effects of dissipation is derived. Besides the usual irrotational leading term, a new simpler expression for the leading rotational term, which can contribute to the generation of mega-Gauss dc magnetic fields is found. The leading irrotational term is proportional to the dissipation rate and vanishes for a standing wave. This expression is applied to the absorption of a planar electromagnetic wave, with polarization in (p) or out of (s) the plane of incidence, obliquely incident onto a half-space of plasma with a linear density profile. The result for the dc magnetic field source term compares very well with the result from a two and one-half dimensional particle simulation calculation for p polarization. Direct acceleration by the resonantly excited electrostatic waves is the dominant dissipation process in this case. At high power, the dominant dissipation process for s polarization is parametric decay into plasma waves which are Landau damped by the electrons. The magnetic field source term is found to be comparable to that of p polarization.