George P. Latsas

Experimental Investigations and Analysis of Parasitic RF Oscillations in High-Power Gyrotrons

Megawatt gyrotrons are found to suffer from various parasitic oscillations, in particular, RF oscillations in the beam tunnel prior to the desired interaction zone (the cavity). This paper describes the experimental results from a gyrotron experiment which was dedicated to investigate parasitic oscillations in the beam tunnel and to verify improved beam-tunnel structures. A system for improved spectral measurements and a new analysis method are presented. The results verify theoretical predictions on the parasitic oscillations, and in effect validate the corresponding improved beam-tunnel structure. In addition, other types of parasitic oscillations were observed and explained.

From Series Production of Gyrotrons for W7-X Toward EU-1 MW Gyrotrons for ITER

Europe is devoting significant joint efforts to develop and to manufacture MW-level gyrotrons for electron cyclotron heating and current drive of future plasma experiments. The two most important ones are the stellarator Wendelstein W7-X at Greifswald and the Tokamak ITER at Cadarache. While the series production of the 140 GHz, 1 MW, CW gyrotrons for the 10-MW electron cyclotron resonance heating system of stellarator W7-X is proceeding, the European GYrotron Consortium is presently developing the EU-1 MW, 170 GHz, CW gyrotron for ITER. The initial design had already been initiated in 2007, as a risk mitigation measure during the development of the advanced ITER EU-2-MW coaxial-cavity gyrotron. The target of the ITER EU-1-MW conventional-cavity design is to benefit as much as possible from the experiences made during the development and series production of the W7-X gyrotron and of the experiences gained from the earlier EU-2-MW coaxial-cavity gyrotron design. Hence, the similarity of the construction will be made visible in this paper. During 2012, the scientific design of the ITER EU-1-MW gyrotron components has been finalized. In collaboration with the industrial partner Thales electron devices, Vélizy, France, the industrial design of the technological parts of the gyrotron is being completed. A short-pulse prototype is under development to support the design of the CW prototype tube. The technological path toward the EU ITER-1 MW gyrotron and the final design will be presented.

Calculation of the electromagnetic waves in nonperiodic corrugated waveguides With Dielectric loading

We present the mathematical analysis, based on an eigenfunction expansion, and the main steps of the corresponding numerical code developed to calculate the dispersion relation and the field distributions for all kind of waves, which can propagate in a nonperiodic circumferentially corrugated circular waveguide with losses. The code offers fast computations with accuracy, which can be chosen by the user. Numerical results are given for several geometries and comparison with already established codes is made.

Azimuthal Mode Coupling in Coaxial Waveguides and Cavities With Longitudinally Corrugated Insert

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.

New Algorithm for Great Elliptic Sailing (GES)

An analytical method and algorithm for great elliptic sailing (GES) calculations is presented. The method solves the complete GES problem calculating not only the great elliptic arc distance, but also other elements of the sailing such as the geodetic coordinates of intermediate points along the great elliptic arc. The proposed formulas provide extremely high accuracies and are straightforward to be exploited immediately in the development of navigational software, without the requirement to use advanced numerical methods. Their validity and effectiveness have been verified with numerical tests and comparisons to extremely accurate geodetic methods for the direct and inverse geodetic problem.

Beam–Wave Interaction in Corrugated Structures in the Small-Signal Regime

A method based on an eigenfunction expansion and the linearized Vlasov equation to study the beam-wave interaction in cylindrical surface corrugated waveguides with losses is presented. The mathematical formulation is described in detail, and numerical results are given in comparison with the literature.

Design of the EU-1MW gyrotron for ITER

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.

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.

Investigations on parasitic oscillations in megawatt gyrotrons

Recently megawatt gyrotrons are found to suffer from parasitic oscillations induced by the electron beam prior to the desired interaction zone (the cavity). This paper describes experimental and theoretical investigations on both high and low frequency parasitic oscillations. A new measurement method is introduced to determine and characterize these undesired oscillations, and a dedicated experimental study is presented.

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.