  It is shown in an electron-beam-plasma system that phase-space holes evolve
dynamically in electron time scales from appearance to collapse. The holes are
synchronized with a wave packet dominated by a beam mode, and their velocity
radii depend on the packet crest amplitudes. This suggests that the holes are
induced by a self-trapping phenomenon. It seems from the observed images that
the trapped-beam is hard to be detrapped while the packet grows linearly, but
easy to be done after that.
