INTERACTING EIGENMODES OF A PLASMA DIODE WITH A DENSITY GRADIENT

The formation of narrow high frequency electric field spikes in plasma density gradients is investigated using one-dimensional particle in c ell simulations. It is found that the shape of the plasma density grad ient is very important for the spike formation. A coupling to the ion motion, is not necessary for the formation of hf spikes. However, the hf spike influences the ion motion, and ion waves are seen in the simu lations. Dispersion relations are calculated using realistic, non-Maxw ellian, distribution functions. The spike can be seen as a coupled sys tem of two eigenmodes of a plasma diode fed by a beam-plasma interacti on. Based on a simplified fluid description of such eigenmodes, explan ations for the localization of the spike, spatially and in frequency, are given. The amplitude of the oscillating density is comparable with the de level close to the cathode. This sets a limit of a wave amplit ude in the whole system. (C) 1998 American Institute of Physics.