Nowadays gyrotrons are the powerful, relatively compact and highly efficient sources of terahertz (THz) electromagnetic radiation. Their application include: electron spin resonance spectrometry of magnetic materials, the detection of hidden weapons, explosives and radioactive materials, remote high-resolution imaging, plasma diagnostics, material processing (in particular, ceramic sintering), deep space exploration, non-destructive methods of material testing, nuclear magnetic resonance spectrometry of biomolecules, medical technologies (for example, treatment of cancer tumors) and others.

Most of the above-mentioned applications require continuous terahertz radiation, which can be produced by gyrotrons based on the permanent superconducting magnets. However, the development of such gyrotrons is a rather complicated technical task. This is because the available permanent magnets are characterized by the relatively small (in comparison with pulsed solenoids) field strength and are bulky, energy-consuming and expensive.

The limitation on the magnetic field strength can be partially eliminated for waves generated at high cyclotron harmonics (s > 1). For this reason, gyrotrons operating at high cyclotron harmonics look very attractive for use in THz technologies.

In practice, however, their implementation often faces the problem of severe competition between the operating gyrotron mode and one of the fundamental (s = 1) cyclotron modes. The reason is the close location of the starting currents (oscillation thresholds) of the competing modes. Such mode competition narrows the operating region and reduces both the gyrotron efficiency and output power. Thus, for the successful realization of efficient harmonic gyrotrons it is vital to discriminate against the fundamental parasitic modes.

Electrodynamic methods can be used for this purpose. They imply optimization of the gyrotron cavity with the aim to prevent selectively the leakage of the operating mode. Such cavity optimization makes it possible to reduce the diffraction losses and the starting current of the operating mode with respect to analogous values for the parasitic modes. This expands the region of single-mode operation for the harmonic gyrotron.

A detailed investigation on the operation of terahertz harmonic gyrotron has been performed on the basis of the self-consistent gyrotron theory developed in the SPE RESST.

Two types of a gyrotron cavity were considered (Fig.1) and their optimization aimed at selective suppression of fundamental parasitic modes has been performed. The first type is a conventional cylindrical gyrotron cavity with uniform central (main) section, which is connected at the input and output ends to weakly nonuniform tapered sections. Its optimization has been done with respect to both the length of the main section and the output taper angle. Ohmic cavity losses related to the finite wall conductivity have been incorporated into numerical calculations. The second type of a gyrotron cavity is a cylindrical cavity loaded with iris. Its optimization has been performed with respect to both the width and depth of the iris. In addition to ohmic wall losses, the mode conversion associated with the cavity nonuniformity has been taken into consideration. The 50-kW 400-GHz TE_8,9-mode second (s = 2) harmonic gyrotron was considered as an example. Such gyrotrons find applications in the systems of plasma diagnostics based on the collective Thomson scattering.

 The main results of our study are as follows:

 analytic expression for the starting current of cavity modes has been obtained in the fixed-field approximation, which takes into account the effect of mode conversion;

 optimization has been performed for the cavity of the 50-kW 0.4-GHz second harmonic gyrotron and parameters of efficient gyrotron operation have been found out for two types (conventional and iris-loaded) of the cavity (Fig. 2). The cavity parameters that ensure the minimal ratio between the starting currents of the operating and parasitic modes at the required level of gyrotron power have been also determined (Fig. 3);

 it is revealed that the iris-loaded gyrotron cavity of optimal shape provides the most favorable conditions of stable operation for the 50-kW 0.4-THz second harmonic gyrotron. However, the maximum gyrotron power (56 kW) is almost half that of the gyrotron based on the conventional cavity of optimum shape. This is explained by the increased fraction of the ohmic losses in the total losses for the gyrotron cavity loaded with iris.