The formation of narrow high frequency electric field spikes in plasma density gradients is investigated using one-dimensional particle in cell simulations. It is found that the shape of the plasma density gradient 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 simulations. Dispersion relations are calculated using realistic, non-Maxwellian, distribution functions. The spike can be seen as a coupled system of two eigenmodes of a plasma diode fed by a beam-plasma interaction. Based on a simplified fluid description of such eigenmodes, explanations for the localization of the spike, spatially and in frequency, are given. The amplitude of the oscillating density is comparable with the dc level close to the cathode. This sets a limit of a wave amplitude in the whole system.

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