Optical-field-ionization effects on the propagation of an ultraintense laser pulse in high- Z gas jets.

Interaction of an ultraintense, a(0) >>1, laser pulse with an underdense Ar plasma is analyzed via a two-dimensional particle-in-cell simulation which self-consistently includes optical-field ionization. In spite of rapid growth of ion charge Z and, hence, electron density at the laser front, relativistic self-focusing is shown to persist owing to a reduction of the expected plasma defocusing resulting from the weak radial dependence of the ion charge on laser intensity (even for Z/gamma>1 where gamma is the electron relativistic factor).