Phase mixing and echoes in a pure electron plasma

The two-dimensional s2Dd fluid echo is a spontaneous appearance of a diocotron wave after two externally excited waves have damped away, explicitly demonstrating the reversible nature of spatial Landau damping. The inviscid damping, or phase mixing, is directly imaged by a low-noise charge-coupled device camera, which shows the spiral wind-up of the density perturbation. Surprisingly, the basic echo characteristics agree with a simple nonlinear ballistic theory that neglects all collective si.e., moded effects. Also, the simple 2D picture is violated by end confinement fields that cause vz-dependent u drifts, so the observed echo must be interpreted as a superposition of separately damping and separately echoing velocity classes. The maximal echo lifetimes agree with a theory describing weak collisional velocity scattering between velocity classes. In addition, large second wave excitations degrade the echo up to 53 faster than collisions. © 2005 American Institute of Physics. fDOI: 10.1063/1.1885006g