An application of the spatial-harmonic wave on a large-diameter helix

The paper describes the characteristics of the large-diameter helix as a travelling-wave tube and gives experimental results obtained on a scale model operating at 4 Gc/s. For a helix whose circumference is greater than one wavelength (2¿a/¿0> 1), the first spatial-harmonic wave operates as a forward wave and is equivalent, as regards the phase velocity and coupling impedance, to the fundamental wave of the usual helix of diameter (2¿a ¿ ¿0)/¿. Although the bandwidth of the operational frequency then becomes narrow, the application of such a spatial-harmonic wave to travelling-wave amplifiers permits the use of a considerably larger helix and a large beam current. The helix used for this experiment is wound round the outside of a polished quartz tube. The mean diameter, the pitch, the wire diameter and the effective length of this helix are 25, 1.5, 0.5 and 220 mm, respectively. The inside of the quartz tube is coated with an Aquadag film to prevent backward-wave oscillation. Two fine solid electron beams are made to flow along the helix for the measurement of interaction with it. A net gain of 7 dB was obtained at a beam current of 3.5 mA, and this result agrees with the theoretical calculation. From the correspondence of experimental results with theoretical values, an increase of power output at higher frequencies is expected to result from the use of a hollow beam instead of the two solid ones.