At power levels exceeding the 100 MW level the spectrum of the output signal from a traveling wave tube amplifier contains a significant amount of power in frequencies other than the input frequency. The structure of the spectrum is determined by the noise in the system and the interference of the two waves bouncing due to finite reflections at the two ends of the waveguide. There are cases when the power in these selected frequencies is actually equal or even larger than in the initial frequency. Consequently, the device operates more like an oscillator than as an amplifier. In order to overcome this problem we have designed a narrow band periodic structure in which the electrons, whether they are bunched or not, can emit radiation only in a range of less than 50 MHz—in comparison to 1.5 GHz in the old structure. In this structure the group velocity is very low (&bgr;gr<0.01) and therefore the gain per unit length (for the same current) is about 5 times larger than in the previous structure. The low group velocity practically eliminates also the reflection problem since we can design the system such that by the time the first reflected wave reaches the input, the electron pulse has ended.