Abstract <p>An electromagnetic wave amplifier based on the forced Cherenkov effect of a relativistic thin tubular electron beam in a coaxial waveguide with a plasma-dielectric filling is considered. In the linear approximation, a dispersion equation is derived, amplified frequency domains are defined, and wave amplification factors are calculated. Two maximum amplification modes were studied depending on the beam-dielectric gap and frequency. The nonlinear dynamics of the amplification process was investigated. Amplification efficiencies for different beam currents and different frequencies of the amplified signal were determined. The effect of plasma on wave amplification in a coaxial waveguide with a dielectric insert was studied in detail. The interaction of the beam with the potential Langmuir wave in plasma was shown to affect but slightly the amplification of electromagnetic waves.</p>