Abstract: A relativistic electron beam is injected into a plasma whose density is lower than the beam’s density. The head of the electron beam pushes out the plasma electrons, leaving an ion channel. The ion focusing force is similar to the magnetic field in binding electrons. The dispersion equation of corrugated waveguide filled with plasma is derived. It is analyzed ty numerical computation that the performance of traveling wave tube (RTWT) amplifier is affected by geometry parameters of slow-wave structure, such as periods, depth of ripple and average radius of waveguide. The results are helpful for designing RTWT.