High-frequency waves, which are driven by a strong electron beam and p
ropagate along a density gradient, can form spatially concentrated ''H
F spikes'' which extend typically one wavelength (1 cm) in the directi
on along the beam. Experiments and computer simulations show that the
spike is a standing wave with nodes at boundaries to regions with prop
agating 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 af
ter the formation, i.e., there is no tendency towards collapse of the
structure.