Twystrode experiments with tapered and untapered helices

A series of experiments has been performed at NRL to determine the saturation behavior and maximum conversion efficiency of emission gated electron beams in slow wave circuits. Simulations suggest that single-pass conversion efficiencies exceeding 50% to the fundamental harmonic may be achieved with moderately tight bunches in a properly tapered helix. Such an efficiency, achieved in a circuit only two to three slow-wavelengths long, points the way toward significant advances of microwave power tubes into new markets and applications. Emission gated devices, including klystrodes and twystrodes, have potential as compact and efficient power booster amplifiers. The NRL Twystrode Experiment is a flexible instrumented amplifier operating in a wide parameter space, allowing thorough verification of analytical and simulation models. The electron beam (from a gridded thermionic cathode) can be modulated in any degree, from DC, to 100% amplitude modulation, to discrete electron bunches only 90/spl deg/ wide. The beam voltage is variable from below to well above synchronism, and the beam current from the ballistic to the spacecharge regimes. We find that moderate bunching (270/spl deg/ or greater width) and velocity tapering of the output circuit improve beam conversion efficiency at the cost of an increased saturation circuit length.<>