Microwave phototube design considerations

The general behavior of the traveling-wave microwave phototube (TWP) as a broadband light demodulator has been described in previous publications. This paper presents specific theoretical and experimental analyses. It is shown that at low average currents the microwave power output of a TWP is given byP = frac{1}{2}i^{2} R_{eq}whereiis the peak value of the microwave current modulation at the cathode (i.e., the ac current) and Reqis an "equivalent interaction resistance" given byR_{eq} = pi^{2}N^{2}Z_{c}whereNis the helix length in electronic wavelengths and Zcis the longitudinal beam-circuit interaction impedance. Typical values of Reqare from 10⁵to 10⁷ohms. The bandwidth is determined by the productf^{2}Z_{c}, which can vary less than 3 db over an octave. The major noise contributions are found to be shot noise and thermal noise. The power output and signal-to-noise calculations are verified by experiments on an S-band TWP at low average current levels. Preliminary analysis of the TWP at high average currents shows that even higher values of Reqshould be obtainable. However, in that case the frequency-dependent microwave interactions in the gun region may provide a serious bandwidth limitation.