Grigoriy G. Denisov

Based on numerical simulation, the parameters of a megawatt gyrotron with operating frequency of 170 GHz and the operating mode TE25,10 were optimized to increase the output power up to 1.2 MW while maintaining an acceptable level of peak Ohmic loads on the cavity wall. This result is achieved by increasing the beam current while shortening the length of the regular part of the resonator. At the same time, the thermal load in the resonator remains at an acceptable level. The optimal relation between the resonator length and the operating current was found, taking into account the limitations of the cathode emission capacity. The obtained data can be used to implement a gyrotron with increased power, corresponding to the new requirements for the ITER plasma heating system, while maintaining the configuration of the cathode unit and quasi-optical converter.

We propose a gyrotron scheme with a two-mirror echelette-type resonator to obtain high-power continuous wave terahertz (THz) radiation, in which a corrugated profile applied to one of the mirrors is specified in the transverse plane (with respect to an electron's translation velocity). The gyrotron has been driven by a sheet helical electron beam (2 A/20 keV) with a pitch-factor of 1.2 and acceptable velocity spread formed by a planar magnetron-injection gun. The theoretical analysis was carried out both within the framework of a self-consistent theory and on the basis of 3D particle-in-cell simulations. The linear and nonlinear stages of electron–wave interaction were analyzed. Simulations demonstrate the operability of the proposed gyrotron circuit in the frequency range of 0.79 THz, operating at the second cyclotron harmonic with a magnetic field strength of 15 T at the high-order TE0,50,1 mode. The output power level in the steady-state single-mode oscillation regime reached several kilowatts with an Ohmic loss value of less than 20%. Proof-of-principle experiment was carried out to study the selective characteristics of a two-mirror echelette-type resonator in the G-band, demonstrating the feasibility of the proposed gyrotron design.

We discuss the possibility of significantly increasing the operating frequency of cyclotron resonant rectifiers for advanced systems of microwave wireless power transmission. Such systems, also called as cyclotron energy converters, are based on the absorption of a microwave signal with subsequent recovery of the accumulated electron energy. Increasing the frequency of cyclotron energy converter can be reached by using the interaction of a large-radius cylindrical electron beam with a high-order transverse waveguide mode. This principle is well known for gyrotrons; thus, the cyclotron energy converter can be considered as an “inverted” gyrotron that do not generate, but absorb radiation. The paper contains estimations of the efficiency of the cyclotron energy converters with an operating frequency of 35 GHz, as well as a principal design of its main components, including an electron optical system and a two-directional quasi-optical converter for input/output of microwave radiation.

The design and results of the first hot-test experiment on a gyrotron backward-wave oscillator (gyro-BWO) using a newly proposed microwave circuit are presented. The gyro-BWO employs an interaction circuit based on a quasi-optical transmission line that directs a Gaussian wave beam along a zigzag path with vertical and oblique segments periodically spaced along the longitudinal axis. A resonant cyclotron beam-wave interaction occurs in the regions where the beam intersects the wave perpendicularly, resulting in low sensitivity to particle velocity spread, similar to that of a gyrotron. The device was hot-tested in a continuous wave (CW) mode, generating microwave radiation in the form of a Gaussian beam with powers of 0.2-1.2 kW and frequencies in the 90-195 GHz range. 15 bands were measured, within which the generation frequency linearly and continuously changed as the magnetic field or accelerating voltage were varied. Due to the unique dispersion properties of the zigzag gyro-BWO, these bands cover the entire operating range with a fill-factor of approximately 70%. This is significantly greater than that of any other experimentally tested gyrotron-type device.

A 20 kW gyrotron-driver has been used to control the RF generation of a 170 GHz/1 MW ITER prototype gyrotron. The frequency-locked operation regime has been obtained, accompanied by an increase in power and efficiency as well as stabilization of the frequency of the slave MW-level tube.

For the development of complexes of coherently operating gyrotrons, we propose a scheme with serial connection when the generation frequency and phase of each gyrotron are locked by a previous one. In contrast with a more basic parallel connection, such a scheme does not imply the use of a master-oscillator by dividing its power into many channels, that is, the number of connected gyrotrons is not limited. Besides, a significant part of the power of each generator can be used to synchronize the next element in the series; thus, the locking bandwidth is expanded. In accordance with the numerical simulations, the drop in efficiency when a strong external signal is applied can be negated by varying the value of the guiding magnetic field.

We propose the concept of high-power THz radiation sources based on five-fold frequency multiplication in gyrotrons intended for plasma applications. The efficient excitation at the 5th cyclotron harmonic is due to the specific property of the eigenmodes of cylindrical waveguides, as a result of which, the conditions of simultaneous electrodynamic resonance at two selected TE modes are satisfied asymptotically with very high accuracy. Previously, we have verified this principle in experiments with a low-frequency kilowatt-level gyrotron in which, due to the low-density spectrum, the operating mode is excited with no competition from parasitic oscillations. The novel concept is a development of this idea applied to the systems with a denser spectrum, which is inevitable in higher frequency and power devices. Simulations within the averaged time-domain model demonstrate that, despite the mode competition, it is possible to excite Watt-level 1.25 THz 5th cyclotron harmonic in a recently developed sub-MW 0.25 THz gyrotron with TE_19,8 operating mode. The obtained results open a possibility for implementation of radiation sources with output power/frequency combination, practically inaccessible using other THz generation methods and highly sought for a number of applications, including high-resolution molecular spectroscopy.