Power limitations in helix travelling-wave tubes and the application of fluid cooling

Scaling equations are developed for broad-band helix travelling-wave amplifiers and are used to study the limitations due to cathode current density, beam interception and attenuation with cooling by radiation and conduction. The latter is more efficient but must be treated with reserve at the shorter wavelengths, owing to the added attenuation. By using the form of a theoretical equation to obtain correct dimensional behaviour and an experimentally determined factor, the attenuation calculations should be more reliable than hitherto. Limiting power/wavelength relationships for the various cases all show the allowable beam power to be proportional to a high power of the wavelength. Attenuation dissipation in particular imposes a rapid decrease in power level at wavelengths below the S-band. The possibilities of fluid-cooling small helices would from tube are then explored, a peculiarity of which is that higher-power valves at a given wavelength are favoured. The power in the beam must exceed a given level which rises as the wavelength is reduced, because the increases in helix size brought about by higher voltages, at a fixed wavelength, lower the flow impedance more rapidly than that required to cope with the increased power. Eventually, at 1-2 cm wavelength, the helix size cannot be increased without backward-wave oscillation. This method of cooling thus makes possible c.w. broad-band amplifiers with outputs of carcinotron magnitude using beam powers of the order of kilowatts at wavelengths of a few centimetres.