Based on the steady-state multimode theory,the instability and stabilization of a TE_(11) mode fundamental GyroTWT is discussed through increasing the power of input signal,changing the guiding center radius of electron beam and coating the microwave absorption material on the inner surface of interaction structure to suppress the TE_(21) mode second harmonic backward wave oscillation and obtain the higher amplifying gain and the larger output power for the TE_(11) mode fundamental operation with relatively...