Waveguide cross section and background magnetic field tapers for broadbanding a gyro-TWT

Abstract

The gyro-TWT in a cylindrical waveguide of linearly-tapered cross section was analyzed for the gain-frequency response, using the Pierce-type gain equation. The taper in the waveguide cross section was adjusted for wide device bandwidths, either by changing the taper angle, while keeping the interaction length to be constant, or by changing the interaction length, while keeping the initial and final radii of the waveguide constant. Tapering led to the prediction of wide bandwidths, though at the cost of gain, as compared to a non-tapered device. The range of the DC background magnetic flux density relative to its grazing-point value was identified as a crucial parameter for large gains, with appreciable bandwidths, and minimum mode mixing in a tapered device, the latter in general facing more mode competition than a non-tapered device.