Localization of the high frequency field in a beam-plasma diode due to density gradients

An experimental study of the high frequency (hf) interaction of a strong, w arm electron beam with an inhomogeneous, bounded plasma is presented and co mpared with particle-in-cell simulations. The density is highest in the cen ter of the diode and decreases towards the boundaries. The electric field i n the center is dominated by the unstable slow-space-charge wave, but exper imental evidence is found, that reflected waves are present near the beam i njection grid, where the oscillation frequency f is higher than the local p lasma frequency f(pe)(x). The strong field at the resonance point (f = f(pe )(x)) is proposed to excite the unstable beam mode. Simulations confirm thi s picture by showing a concentration of the spectral power of the beam wave s to the frequencies of the trapped Langmuir waves. Linear growth and Landa u damping concentrate the beam wave to a 1.5-2 wavelengths wide region. Tra pped Langmuir waves can accordingly determine the frequencies and position of the hf-field even for moderate density gradients and field strengths bel ow the threshold for Langmuir collapse.