Experimental study of turbulent electron beam--plasma interaction with an auxiliary probing E-beam and a high frequency correlation technique

The linear, nonlinear, and turbulent regimes of the interaction of an electron beam with a collisionless plasma in a high magnetic field is experimentally studied by means of high frequency correlation techniques and an auxiliary probing electron beam. Theoretical predictions on wave growth, saturation, and the time required for the onset of a quasi-stationary state are verified. In addition, the theoretically predicted occurrence of a phase mixing regime is observed and found to be the most important effect in randomization of the initially narrow spectra of the wave electric fields. The velocity space diffusion constant D(v) is measured by a monoenergetic electron probe and the result is compared with weak and strong turbulence theories. Furthermore, anomalous damping of externally excited Langmuir waves with phase velocities much larger than the plasma electron thermal velocity (v/sub p/ greater than or equal to 4v/sub T/) is observed. An analytical model is developed which interprets this phenomena as a phase mixing decay (transient decay) due to those electrons which are trapped in the excited wave packet