Design of cuccia couplers for quadrupole amplifiers

The electron coupler devised by Cuccia has recently been used to couple r.f. signals to and from the electron beam in the low-noise electron-beam parametric or quadrupole amplifier described by Adler. In this role, its design determines the minimum noise factor, bandwidth and dynamic range of the amplifier. The paper describes the detailed design of Cuccia couplers for use in quadrupole amplifiers. Expressions for the above performance parameters are derived, the features of coupler and tube design that influence them are discussed and useful design criteria established. In particular, it is shown that the maximum fractional amplification bandwidth of a quadrupole amplifier with identical input and output couplers is approximately equal to 1/N, N being the length of the coupler in electron wavelengths. The paper concludes with the application of these design criteria to the coupler design for a low-noise tube to operate at a frequency of 400 Mc/s. Excellent agreement between the predicted and actual performance is, in general, obtained.