Measurement of the evolution in phase space of an electron beam interacting with a plasma

The result of a measurement of the locus of the beam electrons with reference to energy, position and wave phase, is described. The experiment follows theory until saturation. Subsequently the beam and wave do not follow prediction in that the wave decays and the beam particles become trapped at velocity smaller than the wave velocity. Two candidates for the untrapping are the transverse electric field and cyclotron damping, both of which are present in this wave. The damping transfers energy into the plasma, while the transverse electric field transfers energy into transverse motion of the beam electrons. Thus, it is possible that longitudinal beam energy is lost finally to the plasma and/or transverse beam energy.