ELECTRIC-FIELD SPIKES FORMED BY ELECTRON BEAM-PLASMA INTERACTION IN PLASMA-DENSITY GRADIENTS

In the electron beam-plasma interaction at an electric double layer th e beam density is much higher than in the classical beam-plasma experi ments. The wave propagation takes place along the density gradient tha t is present at the high potential side of the double layer. Such a ca se is studied experimentally by injecting the electron beam from a pla ne cathode, without any grids suppressing the gradient, and by particl e simulations. The high frequency field concentrates in a sharp ''spik e'' with a half width of the order of one wavelength. The spike is fou nd to be a standing wave surrounded by regions dominated by propagatin g waves. It forms at a position where its frequency is close to the lo cal plasma frequency. The spike forms also when the electric field is well below the threshold for modulational instability, and long before a density cavity is formed in the simulations. Particle simulations r eveal that, at the spike, there is a backward traveling wave that, whe n it is strongly damped, accelerates electrons back towards the cathod e. In a simulation of a homogeneous plasma without the density gradien t no spike is seen, and the wave is purely travelling instead of stand ing. (C) 1997 American Institute of Physics.