Structure and apparent dispersion of ballistic plasma wave echoes of second order

The spatial structure of electron plasma wave echoes is investigated with special regard to the apparent dispersion, i.e. the relation between echo wavelength and frequency. The apparent dispersion is obtained by separately recording echo phase and amplitude, using an r.f.-interferometer combined with a network analyzer. Comparison with theory reveals the ballistic nature of the second order echoes investigated; due to the dispersion as a quantitative measure of the velocity diffusion, it yields furthermore a velocity-independent diffusion coefficient D2 = (1.5 +- 0.5) x 1017 m2 s-3, two orders of magnitude larger than expected from equilibrium theory. The enhanced velocity diffusion can be related to non-equilibrium fluctuations due to ion-acoustic turbulence present in the target plasma. (author)