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It is found that a cavity design strategy proposed recently to enhance the continuous frequency tuning band of a sub-terahertz second-harmonic gyrotron aggravates the problem of competition between the operating mode and the first-cyclotron-harmonic (fundamental) modes. A stepped coaxial inner conductor is proposed in this article as a means for a selective suppression of the most dangerous fundamental competitors. It is shown that, with a proper choice of the operating mode, the effect of the inner conductor is only distinct for unwanted competing modes. In this case, the gyrotron performance is slightly sensitive to the longitudinal profile of the coaxial insert. Therefore, this profile can be selected in such a way as to reduce the effective cavity length for the competing modes, together with their diffractive quality factors. As a consequence, starting currents of these modes can be selectively increased. This is illustrated by a realistic design of a 0.4-THz second-harmonic gyrotron with 3-GHz continuous frequency tuning. [ABSTRACT FROM AUTHOR]

In the fixed-field (cold) approximation, a generalized formula is derived for the starting currents of eigenmodes excited by an electron beam in a gyrotron cavity with the mode conversion. Tapered gyrotron cavities with gradual and abrupt transitions are considered as examples. It is shown that the effect of the mode conversion on the starting current consists mainly in the change of the diffractive quality factor and, therefore, is small for cavities with low diffractive losses. For such cavities, our formula is in good agreement with single-mode, self-consistent (hot) calculations of the electron beam–wave interaction. The agreement degrades for high-order axial modes having clear-cut distinctions of cold and hot fields. The derived formula is used to test the validity of the single-mode approximation for a complex gyrotron cavity with the abrupt change in radius. [ABSTRACT FROM AUTHOR]

Cavity modification is proposed with the aim of enhancing the continuous frequency tunability of a gyrotron. The modification can be applied to both uniform and tapered gyrotron cavities and involves just using an additional cavity section. It ensures larger effective cavity length and thus lower starting current for higher order axial modes. As a consequence, the frequency tuning band of the gyrotron can be increased significantly. Moreover, the proposed cavity modification is beneficial for smoothing the variations of output power within the tuning band. [ABSTRACT FROM AUTHOR]

The problem of power degradation due to the dominant ohmic losses in gyrotron cavity is considered. As an example, the operating performances of the FU CW III gyrotron of Fukui University are investigated numerically. It is found that, for this gyrotron, there is a significant divergence between the classical and the self-consistent computations of the loss-induced power degradation. The reason is that the former computations ignore the effect of the cavity ohmic losses on beam-wave interaction efficiency. [ABSTRACT FROM PUBLISHER]