Linear mode conversion in coaxial gyrotrons and its influence on beam-wave instability and gyrotron efficiency :: SPE RESST

Linear mode conversion in coaxial gyrotrons and its influence on beam-wave instability and gyrotron efficiency


At present, powerful millimeter and submillimeter radiation is widely used for scientific, technological and industrial purposes. The most promising applications of such radiation are spectroscopic investigations, monitoring of biomolecules, telecommunication technologies, development and research of the novel functional (micro- and nanostructured) materials, the improved means for plasma heating and diagnostics in controlled thermonuclear fusion devices, infrared astronomy, medicine (oncology, stomatology) and others.

In the submillimeter wave range, conventional electrovacuum sources (traveling and backward wave tubes, orotrons, etc.) are extremely low-power, while free electron lasers are very expensive and cumbersome because of the need to use high-energy electron accelerators. Contrary to this, gyrotrons, which operate with low-energy electron beams and have compact structure and moderate weight, are capable of providing the required level of radiation power. That is why coaxial gyrotrons show promise for use in the above-mentioned applications.

One of the most important and urgent tasks in this area is to increase the efficiency of gyrotrons. A high level of energy dissipation in the terahertz gyrotron cavities limits their areas of application. In fusion-relevant coaxial gyrotrons, the mechanisms of power dissipation have not been sufficiently investigated, which makes it impossible to expand their long-pulse operation to very high powers (more than 2 MW).

With the aim of improving the mathematical methods for analysis of the operating efficiency of coaxial gyrotrons, we have developed the general theory of mode conversion in inhomogeneous waveguides with impedance walls.

	It has been shown that such a theory can be formulated on the basis of the field expansion in terms of the eigenfunctions of the self-adjoint Dirichlet and Neumann problems for a two-dimensional domain. This domain is formed by the waveguide cross-section and depends on the longitudinal coordinate.

	Using such scalar expansions for the longitudinal components of the electric and the magnetic fields, we obtain a system of ordinary differential equations for the unknown expansion coefficients. This system of equations was solved for the typical parameters of gyrotron cavities.

The influence of the mode conversion on the resonant frequency, the Q-factor, the efficiency, the starting current and the generation region for the coaxial gyrotron with operating ТЕ_34,19 mode has been investigated numerically.

	The mode conversion leads to a slight (of about few tens of megahertz) increase of the resonant frequency for the operating mode. This effect magnifies with increasing taper angle of the output cavity section.

	The mode conversion leads to the decreased Q factor for the operating mode of the coaxial gyrotron at the nominal value of the output taper angle. The Q factor starts to grow with the increase of this angle and can become 10% higher.

	The inclusion of mode conversion changes only slightly the output power and the efficiency of coaxial gyrotron. However, this conclusion is true for the idealized design parameters of the gyrotron (the corrugations of the inner conductor and the coupling between TE and TM cavity modes were ignored).

	The mode conversion leads to a small shift of the generation region for the operating mode and to a slight change in its starting current (which can either increase or decrease depending on the accelerating voltage, see Fig. 1).

Figure 1 - Starting current as a function of accelerating voltage for the operating mode of the 170 GHz coaxial gyrotron with consideration for the mode conversion