Multimode nonlinear nonstationary theory of a coaxial gyrotron :: SPE RESST

Multimode nonlinear nonstationary theory of a coaxial gyrotron


A complex SIE code has been developed. The code is based on the multimode nonlinear nonstationary theory of a coaxial gyrotron and make it possible to calculate and optimize the gyrotron performances (ohmic losses, efficiency, resonant frequencies, diffractive Q-factors), and mode competition scenarios. Contrary to the available numerical codes, our code accounts for the higher spatial harmonics in the coaxial cavity with corrugated inner conductor. There are three main aspects related to the effect of higher spatial harmonics in such cavity:

	field modification near the corrugated surface, which results in the reduced power losses inside the inner conductor of the coaxial cavity;

	change of local transverse wave number or local cutoff frequency;

	change of the beam-wave coupling coefficients for competing modes.

Numerical simulations of the mode competition scenario for the preprototype tube of the industrial coaxial gyrotron show that, within the single-mode approximation, the higher spatial harmonics have only a slight effect on the excitation process inside coaxial cavity. In this case our results are in good agreement with computations of the Karlsruhe code and also with the experimental data (Fig. 1). However, the mode interaction in the coaxial gyrotron cavity modifies substantially the mode competition scenario (Fig. 2). This holds true for the mode sequence exited during start-up, the operating region and the maximum net power of the operating mode.

	Fig. 1. The output power of ТЕТЕ_34,19 mode of the coaxial gyrotron versus accelerating voltage. The results have been obtained in the single-mode approximation

	Fig. 2. Gyrotron startup scenario for the nominal operating parameters, where (1) – SPE RESST code (SIE), (2) – Karlsruhe code (SELFT), (3) – experimental data

The results of the multimode analysis (6 modes have been considered) are consistent with the experimental data, if the effective mode attenuation of about 12% is included (this also true for other codes). Therefore, an important task is to study new attenuation mechanisms in a coaxial gyrotron cavity.

For the inner conductor, the reduction of the power losses due to higher spatial harmonics makes it possible to increase the conductor radius with the aim to enhance the mode selection in coaxial cavity[61(2010)]. Moreover, the corrugation depth can be optimized further in order to keep these losses as small as possible.

The results obtained depend notably on the number of the competing modes under consideration and their type.