Microwave phototube design considerations

Abstract

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 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P = \frac{1}{2}i^{2} R_{eq}</tex> where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</tex> is the peak value of the microwave current modulation at the cathode (i.e., the ac current) and R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eq</inf> is an "equivalent interaction resistance" given by <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R_{eq} = \pi^{2}N^{2}Z_{c}</tex> where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> is the helix length in electronic wavelengths and Z <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> is the longitudinal beam-circuit interaction impedance. Typical values of R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eq</inf> are from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> ohms. The bandwidth is determined by the product <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f^{2}Z_{c}</tex> , 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 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eq</inf> should be obtainable. However, in that case the frequency-dependent microwave interactions in the gun region may provide a serious bandwidth limitation.