The excitation of plasma waves by resonant absorption of an electromagnetic wave is studied experimentally near the top of a parabolic density profile. The experiment shows the development of a huge density cavity (2000${\lambda }_{\mathit{D}}$) inside which trapped plasma waves exhibit an oscillatory behavior, on a time scale smaller than the convection time.

• Received 20 February 1992

DOI:https://doi.org/10.1103/PhysRevLett.69.1197