Mode Competition In Gyrotron Research Articles

Influence of reflections on mode competition in gyrotrons

A general theory is presented for studying the influence of reflections on mode competition in gyrotrons. Strong effect is found in the illustrative example: the coaxial cavity TE/sub 28,16/ gyrotron operating close to the optimal point in the general parameter spare. In some cases, the fixed-field approximation and the self-consistent approach lead to qualitatively different predictions, which emphasizes the need for self-consistent calculations in the final stage of design of a gyrotron.

Nonstationary theory of mode competition in gyrotrons with allowance for small inhomogeneity of the guiding magnetic field

High-power millimeter wave generators can induce electron cyclotron waves in a plasma which can fulfill many functions (plasma breakdown, heating, current drive) in fusion plasma reactors including both tokamak and stellarator concepts. All these applications require tens of megawatts of power in the frequency range 100–300 GHz and power 0.5–1.5 MW per unit. Among various microwave sources gyrotrons are the most developed source. Because of the heat losses in resonator walls at such high frequencies and powers oversized cavities must be used. In such cavities the mode competition becomes a severe problem. The most advanced theory of mode competition in gyrotrons is the nonstationary nonlinear theory. In the present article we extend this theory by allowing for inhomogeneity of the guiding magnetic field. The importance of this generalized theory is illustrated in the case of a 3 MW 140 GHz gyrotron with a coaxial cavity working in the TE21,13 mode.

Cold-cavity and self-consistent approaches in the theory of mode competition in gyrotrons

On the basis of a generalized theory, the cold-cavity and the self-consistent methods of calculation of mode competition in gyrotrons are compared. This comparison enables the conditions when the cold cavity approximation is justified to be determined. Specific examples are presented and numerical problems are discussed. It is concluded that mode competition can be studied by neglecting the interaction of the electrons and the RF field in the output taper in those cases when the parameter of irregularity of the output taper is >15. Here, the results obtained in the cold-cavity approximation overestimate the starting current of the parasitic mode by approximately 25% in comparison with the self-consistent approach. For values of the parameter of irregularity >

Self-consistent theory of mode competition for gyrotrons

Abstract The equations for a fully self-consistent theory of mode competition in gyrotrons are derived and some possible methods of solution are discussed. The importance of including interaction in the resonator output taper is discussed in connection with the validity of linearizing the equations in the amplitude of the parasitic mode.

Self-consistent field model for the complex cavity gyrotron

Mode competition in gyrotrons with highly overmoded cavities can degrade efficiency and output mode purity. One approach to mode selection which has been demonstrated experimentally involves a sudden change in the cavity radius. Cavities with this feature are called ‘complex-’ or ‘step-cavities’. This paper presents a theoretical model for the step-cavity gyrotron. The model applies to the steady state operation. The axial variation of the cavity rf fields is determined self-consistently with the electron beam motion by integration of the wave equation simultaneously with the nonlinear electron equations of motion. The model accounts for mode conversion effects at the step as well as beam loading effects such as cavity Q modification and frequency pulling. The model is applied to the analysis of a TE01/TE04 complex cavity gyrotron