Linear theory of a wide-band gyro-TWT amplifier using spiral waveguide

A novel electrodynamic system to realize high efficiency and a broad frequency band in a gyro-TWT amplifier at millimeter and microwave frequencies is described. A cylindrical waveguide having a helically grooved wall selectively couples an electromagnetic waveguide mode at a frequency close to cutoff to the forward travelling wave of a lower mode, resulting in an eigenwave with a constant positive group velocity and approximately zero axial wavenumber over a wide frequency band. For a travelling-wave gyroamplifier, such a dispersion characteristic permits operation with significantly increased bandwidth and reduced sensitivity to electron-beam axial velocity spread. Numerical calculations will be presented to show the beam-coupled dispersion characteristics, stability, and gain profiles of a simple amplifier, based upon a linearized coupled-wave theory.