SUMMARYWhen an electron beam is injected into a low-pressure gas (about 10 4 Torr), various waves are produced by the beamplasma interactions. These waves are expressed by TrivelpieceGould modes that consider the boundary conditions. The growth rate has almost never been sought until now. In this paper, we explain which modes are strongly excited when the gas pressure is changed in a beamplasma system based on the results of improving the linear dispersion relation used by Trivelpiece and Gould and performing the calculations in complex numbers. The properties of the waves found by this calculation are compared with our experimental results. © 2001 Scripta Technica, Electron Comm Jpn Pt 2, 84(4): 915, 2001