Formation of Shock Waves in an Expanding Plasma into The Vacuum in The Field of an Electromagnetic Wave

In the frame work of fluid theory, in spite of the compressional shocks, it is commonly believed that the rarefacting shocks can propagate only in the regime of anomalous thermodynamics. Plasma a charged fluid is not an exception; so in the one dimensional plasma expansion accompany with the propagation of an electromagnetic (E. M.) field, in opposite direction. In this paper it is shown that the pressure of E. M wave through pondermotive force can modulate plasma density and as a result, the condition for propagation of a rarefaction shock preserved.