Frequency-broadband gyro-devices with a novel microwave system in the form of a helically corrugated waveguide are currently under development. The helical corrugation ensures the appearance of an eigenwave with unique properties: namely it possesses a nearly constant group velocity in the region of close-to-zero axial wavenumbers. The use of this operating eigenwave in a gyro-TWT and a gyro-BWO substantially improves their output parameters as compared to analogous gyro-devices based on smooth waveguides [1].

The main theoretical predictions were first confirmed in proof-of-principle experiments exploiting moderately relativistic (200-300 keV) short-pulse (20-100 ns) field-emission electron beams [2]. An efficiency of 29% and an instantaneous frequency amplification band of 21% were simultaneously obtained in these experiments.

Important progress has recently been achieved in a low-relativistic (80 kV) second-harmonic Ka-band helical-waveguide gyro-TWT which uses a thermionic cathode. This amplifier provides stable amplification with a maximum electronic efficiency of 27%, maximum output power of 180 kW, saturated gain of 25 dB and instantaneous -3 dB bandwidth of nearly 10%.

A helical-waveguide frequency-tunable gyro-BWO, attractive for some technological applications, is also under investigation. In the experiment a pulsed prototype of a 20-kV CW oscillator was developed which generated an output power of about 5 kW with smooth frequency tuning within a frequency range of 23.5-25 GHz.

1. Denisov G.G., Bratman V.L., Phelps A.D.R. and Samsonov S.V., IEEE Trans. on Plasma Science, 1998, 26, 508.
2. Bratman V.L., Cross A.W., Denisov G.G., et al., Phys. Rev. Lett., 2000, 84, 2746.