  It is demonstrated by three-dimensional quantum electrodynamics ---
particle-in-cell (QED-PIC) simulations that vacuum breakdown wave in the form
of QED cascade front can propagate in an extremely intense plane
electromagnetic wave. The result disproves the statement that the
self-sustained cascading is not possible in a plane wave configuration. In the
simulations the cascade initiates during laser-foil interaction in the light
sail regime. As a result, a constantly growing electron-positron plasma cushion
is formed between the foil and laser radiation. The cushion plasma efficiently
absorbs the laser energy and decouples the radiation from the moving foil
thereby interrupting the ion acceleration. The models describing propagation of
the cascade front and electrodynamics of the cushion plasma are presented and
their predictions are in a qualitative agreement with the results of numerical
simulations.
