Undulator Amplification of a Long Sub-Terahertz Wave Signal by a Short Dense Electron Bunch

Amplification of a long THz-frequency-range wave pulse coming from a continuous-wave gyrotron by a short electron bunch is studied in the situation, when the axial electron velocity is close to the wave group velocity. The possibility of realization of two different regimes of the electron-wave interaction at different slippages between the electron bunch and the wave is shown. In the case of a short bunch and the zero slippage, a quasi-regular amplification is possible; in this regime, the amplitude increases in a region around the moving bunch and, therefore, a wave pulse with a relatively long length (as compared to the electron bunch length) is formed. The second regime is realized in the case when the bunch is relatively long, and it slightly slips with respect to the wave. In this case, a short (as compared to the bunch length) wave pulse is formed on one of fronts of the bunch and amplifies as it "runs" along the bunch. The peak power of this wave pulse can be several orders of magnitude higher than the saturated power achieved in the quasi-regular amplification regime.