EU is developing a 1 MW cylindrical cavity gyrotron. In the last year the design of the components of the new gyrotron has been finalized while the technological design of the new tube has been defined. In the present paper, the main characteristics of the new EU gyrotron for ITER are presented.

High-power high-frequency gyrotrons are required for several applications, including plasma heating, current drive, and plasmas stabilization for controlled thermonuclear fusion. Despite the use of dumping dielectric material in these gyrotrons, parasitic oscillations often appear, which significantly affect their operation and efficiency. In this paper, we perform an extended parametric study on the effect of the beam-tunnel characteristics, i.e., the outer radius of the inner waveguide and the width of the grooves, as well as the lossy material properties and the beam-radial position on the dumping of these oscillations.

Summary form only given. In high-power gyrotrons sweeping systems are used in order to distribute the energy of the spent beam in a larger area of the collector1. Currently, there are two common types of sweeping systems, the axial and transversal sweeping system2. In the first case, the sweeping coils surround the collector creating a harmonically varying magnetic field that is coaxial to the externally applied one. In the second one, the coils are distributed symmetrically around the collector creating a rotating magnetic field normal to the external one. In both cases the periodic variation of the magnetic field induces eddy currents in the conductive walls of the collector, which tend to cancel the magnetic field of the sweeping coils, reducing in this way the efficiency of the sweeping system.

Summary form only given. Coaxial gyrotrons offer the potential to generate microwave power in the multi-megawatt levels at frequencies well above 100 GHz1, since very high-order volume modes can be used. The presence of the coaxial insert reduces the voltage depression and eliminates the restrictions of mode selectivity, making it possible to maintain the cavity ohmic losses at a reasonable low level (<2kW/cm2) allowing the use of very high-order volume modes.

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.

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.

Summary form only given. Megawatt gyrotrons are found to suffer from various RF oscillations in the beam tunnel prior to the desired interaction zone (the cavity). The development of such parasitics degrades the beam quality reducing the efficiency of the cyclotron interaction and the overall produced power. Furthermore, it affects the stability of operation. Several design approaches have been implemented in order to suppress the parasitic modes; most of them involve the increase of the dissipation in the beam tunnel by means of lossy ceramic materials. However, despite the increased losses, parasitic oscillations still appear in several new designs, especially when the produced power and/or operating frequency is increased. In this work, we employ the numerical code FISHBONE, as well as the commercial simulation software CST STUDIO SUITE in order to study the parasitic modes which appear in a gyrotron beam tunnel. Furthermore, the effect of the geometry of the structure as well as the various design parameters on the parasitic modes is studied, in order to identify the origin and properties of the parasitic oscillations.

Summary form only given. Coaxial cavity gyrotrons with axial corrugated insert are able to provide high-frequency microwave power in the MW region. The insert reduces the voltage depression and enhances the mode-selectivity of the cavity. Choosing the insert's radius and corrugation parameters properly, the parasitic modes are more easily suppressed. A major constraint in the design of such resonators is the heating of the structure due to the dissipation of part of the generated RF power of 2-3kW/cm2, whereas for the inner one this limit is up to ten times lower. The EU 2 MW, 170 GHz coaxial gyrotron cavity has been designed using the Surface Impedance Model (SIM) to calculate the ohmic loading of the insert. However, comparisons of SIM results with those obtained by the full-wave Space Harmonics Method (SHM) revealed significant discrepancies in the calculated ohmic loading. In this work, we perform a parametric study of the ohmic loading using SHM and SIM. Possible optimization, by means of minimization of ohmic losses and mode competition, of the EU coaxial gyrotron cavity is investigated.

Coaxial resonant cavities with longitudinal corrugations on the inner conductor are used in high-frequency high-power gyrotrons as means to reduce the number of possible competing modes. For a sufficiently large number of corrugations, the analytical approach usually treats the surface corrugation as a homogeneous surface impedance to obtain simple formulas for the characteristic equation and field components. These formulas can be introduced to interaction codes in a quite straightforward way. Full-wave approaches that account for the azimuthal periodicity of the structure and consider azimuthal spatial harmonics to describe the field distributions have been also employed, increasing though the complexity of the solution and the effort given in numerical calculations. In this paper, a full-wave code is used in an attempt to identify the way that the azimuthal spatial terms contribute to the reformation of the eigenvalue spectrum and propose a criterion for the selection of the spatial terms that should be taken into account for accurate enough calculations.

The Spatial Harmonics Method has been used to study TE modes in a coaxial corrugated cavity. A numerical code has been developed to calculate eigenvalues, fields and ohmic losses on the walls. Numerical results are presented to clarify the influence of the higher-order spatial harmonics to these calculations.

Using the numerical code Fishbone, developed to study the parasitic oscillations in gyrotron beam tunnels, a parametric study is performed on the effect of the dielectric material as well as of the slot geometry on the growth rate of the excited parasitic modes.

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.

We present the mathematical formulation and numerical results for the resonance characteristics of the TMmp modes in a coaxial cavity with a longitudinally corrugated insert. Based on the spatial harmonics method, the eigenvalues of the transverse plane are calculated, and then, the Vlasov approximation is used to calculate the resonance frequency, diffractive quality factor, and longitudinal field profile of every mode. The numerical results are presented and compared with those found in the literature.

We present the design of two continuous tunable CW coaxial cavity gyrotron oscillators, one of 330 GHz - 200 W with a bandwidth of 3 GHz for scientific applications and one of 30 GHz-150 kW with a bandwidth 0.4 GHz for industrial applications. The tuning of both gyrotrons is achieved by moving the tapered inner conductor in the axial direction and by adjusting the operating magnetic field.

The scattering properties for both TE and TM modes of an abruptly ended two-layered slab waveguide with anisotropic core and isolated substrate are examined by an improved iteration technique, which is based on the integral equation method with accelerating parameters. The relative dielectric constants of the core for the three Cartesian directions are considered to be different, but cases with isotropic core are also considered. The electric field distribution on the terminal plane and the reflection coefficients of the dominant TE and TM guided modes, as well as the near-field distribution and the far-field radiation pattern, are computed, while numerical results are presented for several cases of the core anisotropy.

The authors present the mathematical analysis for the calculation of the dispersion relation, the field distributions, and the ohmic losses for TEm,p modes in an infinite coaxial waveguide with a longitudinally corrugated insert. The method employed is based on an appropriate eigenfunction expansion, and its main advantage is the very fast convergence with a few spatial harmonics. The analysis is properly extended to include tapered cavities with varying, in respect to the z-coordinate, outer and/or inner radius. Numerical results are presented for several tapered cavity geometries and compared with already published methods

The dispersion characteristics and the field distributions for all kind of waves which can propagate in surface corrugated waveguides are calculated by a method based on the Floquet theorem. Numerical results are presented for several corrugated structures with rectangular and circular cross-section and a comparison is made with already established codes.

We study the dispersion characteristics of a rectangular waveguide grating for microwave amplifier applications. The Floquet theorem and an appropriate standing waves expansion is employed to express the fields in the vacuum region and inside the grooves, respectively. The application of the boundary conditions leads to an infinite system of equations, which is solved numerically by truncation. The main advantage of the procedure employed is that it gives directly and with a few spatial harmonics the dispersion relation. Furthermore, an adequate procedure (simulation tool) has been introduced in order to distinguish the real roots from spurious solutions and it has been found to work effectively for all cases presented in this work. Numerical results are presented for both shallow and deep grooves and comparison with previously published works is made.