Efficiency enhancement of coupled-cavity TWT´s through cavity resonance tapering

Beam-wave resynchronization through circuit velocity reduction is a well-known method of traveling-wave tube (TWT) efficiency enhancement. Circuit-velocity reduction in coupled-cavity TWT´s is usually accomplished through period tapering, that is, the periodic length is reduced with the circuit resonant frequencies kept more or less unchanged. However, the amount of period tapering permitted is limited by stability considerations. Beyond a certain critical value of velocity reduction, the tube may be subject to zero drive oscillations originating in the velocity taper region at frequencies near the low-frequency cutoff of the circuit (lower band-edge oscillations). An alternate approach to circuit-velocity reduction in coupled-cavity TWT´s is suggested. This approach, referred to as cavity resonance tapering, allows the velocity reduction to continue beyond the limit of stable period tapering. Cavity resonance tapering is accomplished by a gradual reduction in the cavity resonant frequency with the period kept unchanged and the circuit bandwidth kept more or less unchanged. For any frequency that remains in the circuit passband, there is a gradual increase in phase shift per period and, hence, a gradual decrease in phase velocity. The advantages and disadvantages of cavity resonance tapering vis-a-vis period tapering are discussed. Cold test data and the results of large-signal computer calculations are presented for illustrative purposes. Results indicate that, when applicable, cavity resonance tapering can produce efficiencies as high as can period tapering without incurring the same risk of lower band-edge oscillations.