  Laser-generated plasma gratings are dynamic optical elements for the
manipulation of coherent light at high intensities, beyond the damage threshold
of solid-stated based materials. Their formation, evolution and final collapse
require a detailed understanding. In this paper, we present a model to explain
the nonlinear dynamics of high amplitude plasma gratings in the spatially
periodic ponderomotive potential generated by two identical counter-propagating
lasers. Both, fluid and kinetic aspects of the grating dynamics are analyzed.
It is shown that the adiabatic electron compression plays a crucial role as the
electron pressure may reflect the ions from the grating and induce the grating
to break in an X-type manner. A single parameter is found to determine the
behaviour of the grating and distinguish three fundamentally different regimes
for the ion dynamics: completely reflecting, partially reflecting/partially
passing, and crossing. Criteria for saturation and life-time of the grating as
well as the effect of finite ion temperature are presented.
