Observation of Plasma Wave Echoes

Experimental observation of a new nonlinear plasma phenomenon, the plasma wave echo, is reported. A recent theory predicts that if a longitudinal electron plasma wave is excited at one position in a collisionless plasma and Landau damps away, and a second wave is excited at another position and also damps away, then a third wave (i.e., the echo) will spontaneously appear at a third position. The existence of various echoes, associated with various orders of the perturbation theory, is demonstrated experimentally. The echoes appear at the predicted position in the plasma. The frequency of the echo wave and the dependence of its amplitude on the amplitude of the initial wave are also in agreement with the theory. Experimental data on the saturation of echo amplitude with increasing initial wave amplitude and increasing distance are given. Data are also presented on another new, but related, phenomenon "the sheath echo."