Formation of Electric Field Spikes in Electron-Beam–Plasma Interaction

High-frequency waves, which are driven by a strong electron beam and propagate along a density gradient, can form spatially concentrated ``HF spikes'' which extend typically one wavelength (1 cm) in the direction along the beam. Experiments and computer simulations show that the spike is a standing wave with nodes at boundaries to regions with propagating waves. The spikes only form in a plasma density gradient, and attempts to produce them in homogeneous plasma have failed. They form without trapping of the waves in density cavities and remain stable after the formation, i.e., there is no tendency towards collapse of the structure.