Narrow, stationary and stable electric field spikes produced by an electron beam in an inhomogeneous plasma

Summary form only given, as follows. In a laboratory experiment it is found that high frequency waves, which are driven by a strong electron beam propagating along a density gradient, can form stationary, spatially concentrated "HF spikes" which extend typically one wavelength (in our case 1 cm) in the direction along the electron beam. The frequency is of the order of the plasma frequency. Both the experiment and closely matched computer simulations show that the spike is a standing wave with nodes at the boundaries to regions with propagating waves. Judging from the simulations, this type of standing narrow HF spike only forms in plasma density gradients. The same electron beam in a homogeneous plasma excited propagating waves in the same frequency range, but with an amplitude which varied much more slowly along the beam.