The ionosphere plays a prominent role in the performance of critical civilian and military communication systems. The properties of the ionosphere can be affected by Ionospheric Modification (IM). The key instrument in IM research is a powerful, ground-based, high frequency source of electromagnetic waves known as a heater. Existing heaters operate with large, fixed location antenna arrays. With a mobile heater, investigators would be able to conduct IM research at different latitudes without building a costly permanent installation. For developing a mobile heater with a much smaller antenna array, a new highly efficient megawatt-class Radio Frequency (RF) source is required to reduce the overall power demands on a fully deployable system. The concept of such a source has been described previously [Beaudoin et al., J. Electromagn. Waves Appl. 31(17), 1786-1801 (2017)]. Here, experimental results using an electron beam produced by a gridded thermionic electron gun to drive an external lumped element circuit for a high efficiency RF generation are reported. The gun produces an electron beam bunched at the driving frequency with a narrow phase angle spread that is then collected by an external circuit for resonant impedance matching to the load. The results showed that effects, such as the internal resistance of the inductor and deflection of the beam electrons by the induced RF voltages on the beam collector, are important considerations to be included in the design of a practical device using this configuration for high efficiency RF generation. Published by AIP Publishing.

For gyrotron applications in plasma installations, one of the most important factors is the gyrotron efficiency. To maximize the interaction efficiency, it is necessary not only to optimize such operating parameters as the magnetic field, beam voltage, and current but also the axial profile of the electromagnetic (EM) field in the interaction space. The present paper describes a study of the effect of the profile of an irregular waveguide serving as a resonator on the axial structure of the EM field. Specific attention is paid to the profile of the uptaper connecting the regular part of a resonator to the output waveguide. Conditions of applicability of the nonuniform string equation, which is widely used in gyrotron designs for finding the axial structure of the EM field, are discussed. Also discussed are the occurrence of reflections from a smooth uptaper and the analogy between the nonuniform string equation and the stationary Schrodinger equation.

It is shown that gyrotrons operating at cyclotron harmonics can be designed for operation in symmetric TE0,p-modes. Such operation in fundamental harmonic gyrotrons is possible only at small radial indices (p <=3D 3) because of the severe mode competition with TE2,p-modes, which are equally coupled to annular beams as the symmetric modes. At cyclotron harmonics, however, this "degeneracy" of coupling is absent, and there is a region in the parameter space where harmonic gyrotrons can steadily operate in symmetric modes. This fact is especially important for sub-THz and THz-range gyrotrons where ohmic losses limit the power achievable in continuous-wave and high duty cycle regimes. (C) 2015 AIP Publishing LLC.

It is shown that gyrotrons operating at cyclotron harmonics can be designed for operation in symmetric TE0,p-modes. Such operation in fundamental harmonic gyrotrons is possible only at small radial indices (p <= 3) because of the severe mode competition with TE2,p-modes, which are equally coupled to annular beams as the symmetric modes. At cyclotron harmonics, however, this "degeneracy" of coupling is absent, and there is a region in the parameter space where harmonic gyrotrons can steadily operate in symmetric modes. This fact is especially important for sub-THz and THz-range gyrotrons where ohmic losses limit the power achievable in continuous-wave and high duty cycle regimes. (C) 2015 AIP Publishing LLC.

It is shown that electrons moving along helical trajectories in an external uniform magnetic field and propagating in periodic slow-wave circuits can simultaneously interact with several space harmonics of such waves. The linear theory describing combined resonances in such systems is developed. Our treatment is limited by consideration of four resonances: 1) two cyclotron resonances at the fundamental and second cyclotron harmonics; 2) the Cherenkov resonance at the zero space and zero cyclotron harmonics; and 3) the cyclotron resonance under the anomalous Doppler effect condition at the minus first cyclotron harmonic. It is shown that the growth rate and the bandwidth in the cases of such a combined resonance, in which the dominant role is, as a rule, played by the cyclotron resonance at the fundamental and the Cherenkov resonance, can be much larger than in the case of single resonances.

Gyrotrons form a specific group of devices in the class of fast-wave vacuum electronic sources of coherent electromagnetic wave radiation known as electron cyclotron masers (ECMs) or cyclotron resonance masers (CRMs). The operation of CRMs is based on the cyclotron maser instability which originates from the relativistic dependence of the electron cyclotron frequency on the electron energy. This relativistic effect can be pronounced even at low voltages when the electron kinetic energy is small in comparison with the rest energy. The free energy for generation of electromagnetic (EM) waves is the energy of electron gyration in an external magnetic field. As in any fast-wave device, the EM field in a gyrotron interaction space is not localized near a circuit wall (like in slow-wave devices), but can occupy large volumes. Due to possibilities of using various methods of mode selection (electrodynamical and electronic ones), gyrotrons can operate in very high order modes. Since the use of large, oversized cavities and waveguides reduces the role of ohmic wall losses and breakdown limitations, gyrotrons are capable of producing very high power radiation at millimeter and submillimeter wavelengths. The present review is restricted primarily by the description of the development and the present state-of-the-art of gyrotrons for controlled thermonuclear fusion plasma applications. The first gyrotron was invented, designed and tested in Gorky, USSR (now Nizhny Novgorod, Russia), in 1964.

A mobile heater for ionospheric modification studies requires a new megawatt (MW) class radio frequency (RF) source operating with an antenna array 1/20 the area of the High-Frequency Active Auroral Research Program (HAARP). To deliver an effective power density comparable to HAARP, the total source power must be in the range of 16MW, thus demanding highly efficient sources. While the development of a whole multi-megawatt system for mobile ionospheric heaters is a complex engineering problem, in the present paper we describe only the work of our group on studying main features of a prototype MW-class vacuum electronics RF source for such system. The source design we are currently pursuing assumes class D operation using a modified version of the inductive output tube. The electron beam is a thin annular beam, switched on and off by a mod-anode as opposed to a grid. The beam is then passed through a decelerating gap, and its kinetic energy is extracted using a tunable resonant circuit that presents a constant impedance in the range of 3-10MHz. With this design the beam is almost completely decelerated at all frequencies, thus achieving high efficiency.

A 300-kW 670-GHz gyrotron, operating with a pulsed coil at the fundamental cyclotron harmonic, is designed at the University of Maryland for an application of detecting concealed radioactive materials. The design of a low-spread diode-type magnetron injection gun for this gyrotron is presented. Constraints due to the pulsed coil design and the limitation of maximum electric field at the cathode result in a steep tilting angle of the cathode surface, 74 degrees, along with a Pierce-type focusing section and a high magnetic compression ratio larger than 170. A pitch ratio of 1.34 and a low pitch ratio spread of 2.5% for a cold beam and 9.2% due to emitter surface temperature of 0.1 eV and 1-mu m roughness were obtained. The results were benchmarked with three simulation codes: EGUN, TRAK, and MICHELLE. Numerical results were calculated for beam currents up to 19 A, accelerating voltage of 50-90 kV, and magnetic field of 25-30 T. A sensitivity analysis with respect to critical parameters such as the depth of the focusing section and the internal simulation parameters is provided.

The starting conditions for excitation of backward waves in the beam tunnel between an electron gun and a cavity of a high-power gyrotron are studied. The excitation of these waves leads to electron energy spread that spoils the beam quality and, hence, degrades the gyrotron efficiency. The suppression of these modes by a resistive coating on the wall of a smooth beam tunnel is examined. The guiding magnetic field and the tunnel wall radius vary along the axis, and thus, the theory is essentially the small-signal theory of a gyro-backward-wave oscillator (gyro-BWO) with tapered parameters. The results obtained are applied to a sample gyrotron.

In this paper the effect of the spread in the radii of electron guiding centers on the gyrotron efficiency is studied. First, the analytical theory is developed for describing this effect in gyrotrons operating in regimes of soft and hard self-excitations. Then, study of this effect in a 670 GHz gyrotron is carried out both analytically and numerically with the use of the available nonstationary self-consistent code. Comparison of results demonstrates very reasonable agreement between numerical data and analytical predictions. It was found that in the case of beam injection at the inner peak of the function describing the beam coupling to the wave the spread in electron guiding center radii can be significantly larger than is usually allowed. This fact greatly alleviates the design of electron guns for terahertz-range gyrotrons. (C) 2010 American Institute of Physics. [doi:10.1063/1.3467036]

Spectroscopic applications have stimulated strong interest in frequency tunable gyrotrons. One of the possibilities to tune gyrotron frequency is known as frequency pulling, which is based on the effect of the electron beam on the oscillation frequency. Recently, the linear theory of this effect was developed, and the results were obtained for a sequence of modes with different axial indices. This paper is intended to analyze saturation effects in such gyrotrons. The nonlinear theory describing the frequency pulling and the efficiency in gyrotrons is developed in the cold-cavity approximation. The results are obtained for the modes with different axial indices (from 1 to 4). The frequency shift and the efficiency are calculated in a wide range of the dimensionless parameters characterizing the beam current and the external magnetic field. These results related to the frequency pulling are compared with those of the linear theory. Also, comparison is made with the results of the self-consistent theory and experiments.

To increase the power level of the sources of coherent electromagnetic radiation at frequencies from 100 GHz up to the terahertz range it makes sense to develop devices with a spatially extended interaction space. Sheet-beam and multiple-beam devices belong to the category. In the present paper the small-signal theory of traveling-wave tubes with sheet-beam and multiple sheet-beam configurations is developed. It is shown that in such tubes the wave coupling on electron beams may occur even in small-signal regimes. The wave coupling and its role for amplification of forward and excitation of backward waves in such amplifiers is studied. Also the effect of transverse nonuniformity of the electromagnetic field on the device operation is analyzed and illustrated by several examples.