Novel linear analysis for a gyrotron oscillator based on a spectral approach

Abstract

With the aim of getting a better physical insight in linear regimes in gyrotron, a new linear model was developed. This model is based on a spectral approach for solving the self-consistent system of equations describing the wave-particle interaction in the cavity of a gyrotron oscillator. Taking into account the walllosses self-consistently and including the main system inhomogeneities in the cavity geometry and in the magnetic field, the model is appropriate to consider real system parameters. The main advantage of the spectral approach, compared to a time-dependent approach, is the possibility to describe all the stable and unstable modes, respectively with negative and positive growth rate. This permits to reveal the existence of a new set of eigenmodes, in addition to the usual eigenmodes issued from cold-cavity modes. The proposed model can be used for studying other instabilities such as for instance backward waves potentially excited in gyrotron beam tunnels.

Parasitic Oscillations in Smooth-Wall Circular Symmetric Gyrotron Beam Ducts

“…The models used to study the parasitic oscillations in smooth-wall metallic beam ducts are derived from the monomode nonlinear, self-consistent TWANG model [16], based on the common slow time scale formulation. Two complementary self-consistent codes are used, TWANGlinspec [17] and TWANG-PIC [23]. The first one is a linear and spectral model [17].…”

“…Two complementary self-consistent codes are used, TWANGlinspec [17] and TWANG-PIC [23]. The first one is a linear and spectral model [17]. The linearization has been done considering a moment-approach [18], allowing to reduce the system of N+1 equations, N equations for the N electrons and one wave equation, to a system of three Partial Differential Equations for three moments.…”

“…It is important to stress that the momentum approach used in this paper does not allow to treat effects associated to electron beam velocity and or energy spread. The details of the derivation as well as the numerical discretization can be found in [17,18]. In [17], the model has been validated by comparing its results with experimental measurements involving high and low power gyrotrons.…”

“…The details of the derivation as well as the numerical discretization can be found in [17,18]. In [17], the model has been validated by comparing its results with experimental measurements involving high and low power gyrotrons. The aim of this paper is to extend the model to the beam duct region.…”

“…In figure 1 is shown a longitudinal cross section of an "extended" geometry including the cavity, the usual interaction zone, the spacer and the beam duct. Nevertheless, as will be explained later, for taking into account the large inhomogeneities compared to [17], a new numerical scheme based on hybrid finite elements has been implemented.…”