Dependence of parasitic modes on geometry and attenuation in gyrotron beam tunnels

Recent high-power gyrotrons, capable of producing radio-frequency power above 1 MW, often suffer from parasitic oscillations in the beam tunnel, despite the presence of dielectric loading materials intended to prevent the growth of such modes. Such oscillations affect the operation and the efficiency of these devices significantly. Lately, a variety of dielectric materials have been used, with limited success, in ring-loaded beam tunnels to suppress unwanted oscillations. In this paper, we perform an extended parametric study of the effects of beam-tunnel geometry and lossy material properties on the damping of such oscillations. © 2012 IEEE

Hybrid field/transmission-line model for the study of coaxial corrugated waveguides

A transmission-line model is reformulated and combined with field theory to study the propagation characteristics in coaxial waveguides with wedge-shaped corrugations, either on the outer wall or the inner conductor. Numerical results show that this equivalent-circuit approach is in agreement with conventional full-wave methods presented in the literature. Additionally, this formulation overcomes numerical issues in the calculation of higher order Bessel functions, which usually conscript sophisticated expansion techniques. © 1963-2012 IEEE

Open-ended Coaxial Cavities with Corrugated Inner and Outer Walls

In this work an open-ended coaxial cavity with a corrugated insert and a relatively small number of corrugations on the outer wall is studied. In particular, the Spatial Harmonics Method (SHM) is employed in order to derive the TE modes characteristic equation, which is then solved by truncation for the calculation of the corresponding eigenvalues. Special care is given in the expansion functions used in order to avoid numerical instabilities in the calculation of high-order spatial terms. Various cases of outer wall corrugations are studied numerically in order to identify the effect of the outer corrugations and understand the mode coupling mechanism. © 2015, Springer Science+Business Media New York

Linear and non-linear inserts for genuinely wideband continuous frequency tunable coaxial gyrotron cavities

We consider two continuous frequency tunable CW coaxial gyrotron oscillators, one 330 GHz with 3 GHz bandwidth and output power 50 - 400 W for scientific applications and one 30 GHz with 0.4 GHz bandwidth and output power 40 - 140 kW for industrial applications. The continuous tuning of both gyrotrons is achieved by moving the linearly tapered inner conductor in the axial direction in combination with the proper adjustment of the operating magnetic field. We consider also a non-linear tapering, which makes it possible to reduce the length of the insert and to improve efficiency of the device. © 2008 Springer Science+Business Media, LLC

Selectivity Properties of Coaxial Gyrotron Cavities With Mode Converting Corrugations

Longitudinally corrugated inserts have been proposed as an additional mean to enhance the selectivity properties of coaxial gyrotron cavities. However, when increasing the frequency and the output power, the corrugated insert may not be sufficient to ensure mode stability. Vane loaded cavities with corrugated inserts seem to have the potential for superior mode selectivity and could be employed to facilitate the development of multimegawatt gyrotrons. In this paper, a numerical code based on the spatial harmonics method is used to study the spectrum rearrangement of a typical coaxial cavity with corrugated insert in which additional wedge-shaped corrugations are introduced on the outer wall. A general design procedure for the upgrade of the mode selectivity is presented. Its merits are demonstrated for a 140-GHz gyrotron cavity. © 2016 IEEE

Calculations on the beam-wave interactions in coaxial gyrotron beam tunnels

We present the mathematical formulation and numerical results for the beam-wave interaction in a coaxial waveguide with rectangular slots filled with a lossy dielectric material. The formalism is based on a full-wave analysis and the linearized Vlasov equation. Numerical results are given for all kinds of modes in simplified gyrotron beam tunnels, and the effect of the losses to the parasitic oscillations is examined and discussed. © 2010 IEEE