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 by
P = \frac{1}{2}i^{2} R_{eq}
where
i
is the peak value of the microwave current modulation at the cathode (i.e., the ac current) and R
eq
is an "equivalent interaction resistance" given by
R_{eq} = \pi^{2}N^{2}Z_{c}
where
N
is the helix length in electronic wavelengths and Z
c
is the longitudinal beam-circuit interaction impedance. Typical values of R
eq
are from 10
⁵
to 10
⁷
ohms. The bandwidth is determined by the product
f^{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 R
eq
should be obtainable. However, in that case the frequency-dependent microwave interactions in the gun region may provide a serious bandwidth limitation.