Space-time structure of ion beam-plasma turbulence

The turbulence excited by an ion beam penetrating an unmagnetized plasma is investigated. One-dimensional linear kinetic theory predicts the ion acoustic mode to be driven unstable when the beam velocity v/subb/ ranges from c/subs/ to 2c/subs/. Numerical calculations are reported for the three dimensional problem, in which attention is given to unstable waves. The direction of propagation, the phase velocities, and the growth rate contours in the (k/sub parallel/, k/sub perpendicular/) plane, are given as a function of v/subb/. This instability is created in a double plasma device, and investigated by means of space and time correlations. Turbulence is excited, consisting of modes propagating at a phase velocity which agrees with theory. As v/subb/ increases, an abrupt transition from one-dimensional to three-dimensional structure occurs as expected