The theory of slow backward-wave amplifications is developed based on electron cyclotron maser(ECM) mechanism employing an initially rectilinear beam. A nonlinear evolution equation is derived to describe the electron energy. Numerical calculations show that the saturated interaction e?ciency in this system may exceed 20%, and the saturated interaction length spans 3-6 centimeters. The distinctive interaction mechanism is promising for the design of compact backward microwave amplification devices. Numerical studies are also presented for the slow-wave ECM e?ciency with inclusion of Gaussian beam electron velocity spread. It is shown that the velocity spread reduces the interaction efficiency.
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