It is well known that wave packets can be excited in an electron-beam-plasma system. The packets have three developing processes at the time scale of electron response: growing, saturation and damping. It was found that the packets in the growing process obey beam mode properties described by linear theories. But, in regimes of the saturation and the damping, further study and argument are required. The authors[1] observed the phenomenon that one wave packet continuously emits some wave packets in a saturation process. But it has been necessary for particle-trapping effects[2] to be considered as the mechanism of such phenomenon. In this study, the presence of particle-trapping in this system is experimentally investigated by using a coaxial probe and an energy analyzer to detect potentials of wave packets and velocity distribution functions of electron beams, respectively.
In a cylinder chamber made of stainless steel filled with argon gas, plasma can be produced by DC-discharges between four heated filaments and the chamber wall. Pulsed electron beams are repeatedly injected into the plasma. They travel one-dimensionally along an axial magnetic field into the chamber. The authors generate test wave packets synchronized with the beams into the plasma.
It is shown that the packets after growing are saturated and finally damp, as propagating from the upstream to the downstream, and that the beam distributions extend into the velocity space along the axis. As the beam density increases, the deformation of the packets in the saturation enhances and the distributions begin to split clearly at the low velocity side. It is found that the center of this splitting distribution almost corresponds to the position and velocity of the packets. Accordingly, the authors conclude that above results imply that particle-trapping, inducing the interaction between the beam and the packets, can occur.

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