Suppression of spurious modes in high-power traveling-wave tubes

In order to establish a suitable means to suppress band-edge oscillations in coupled-cavity broad-band traveling-wave tubes, the fields in the Slow wave structure at the band edge (i.e., upper cutoff frequency) and in the operating range were analyzed theoretically. By conformal mapping the folded waveguide was transformed into a straight waveguide, for which the boundary conditions can be more easily formulated in spite of an inhomogeneous medium. Maxwell´s equations were converted into a system of coupled differential equations, which could be solved numerically using a computer program. As an essential result we have recognized in the corner of the cavity the stimulation of E-type waves at the upper band edge, whereas in the operating range only H-type (TE) waves are existent, as is usual in rectangular waveguides at low frequencies. To suppress the spurious oscillation, attenuated E010- or H111- resonators can be inserted into the outer or cross walls of the slow wave structure, respectively. These resonators are dimensioned for the spurious frequency and tightly coupled to the E-type wave. They act as a selective loss for the oscillation mode, whereas the operating range is affected only little. To reduce the dimensions of the resonators, they are filled with a material of high dielectric constant. Because of disadvantages in the first case (E010-resonators in the outer wall enlarge the outer diameter; thereby the magnetic field is reduced), the second type (H111) was actually tested in a 30-GHz TWT. Excluding the selective loss, a beam current of only 300 mA maximum was possible, corresponding to an output power of 800 W. H111-resonators in the cross wall allowed a beam current of more than 500 mA without spurious oscillations; the output power was over 2 kW. As an additional result, by using an automatic microfilm plotter a trick film was produced which shows the formation of the upper cut-off frequency in moving pictures very evidently.