P. B. Makhalov

Terahertz Orotrons and Oromultipliers

The capabilities of two types of terahertz sources based on stimulated Smith-Purcell radiation of electrons in open cavities are discussed. A series of developed pulsed orotrons provides coherent radiation with an output power of 1-0.1 W in the frequency range of 90-410 GHz, with high frequency stability and broad-band electromechanical frequency tuning. A promising alternative source with smaller currents than that in orotrons is a frequency multiplier based on excitation of a surface wave of a periodic structure and Smith-Purcell radiation of arising electron bunches inside the orotron cavity. In contrast to a number of works where only an open grating is used, exploiting a cavity enables a stimulated radiation process with a much higher power. The developed theory demonstrates the possibility of single-mode generation in such multipliers with wide electron beams.

Experimental demonstration of Smith–Purcell radiation enhancement by frequency multiplication in open cavity

The excitation of a low-frequency surface wave can provide coherent high-frequency Smith–Purcell radiation due to the frequency multiplication effect. The use of an open cavity allows an additional power increase due to the induced nature of the radiation. A radiation source based on these effects is experimentally demonstrated at a frequency of about 190 GHz.

Smith–Purcell frequency multiplier with synchronization of radiation from a wide electron beam

An orotronlike feedback can provide a significant increase in the selectivity and power of frequency-multiplied Smith–Purcell radiation of the electron bunches formed in the course of self-excitation of a grating surface eigenmode. This method looks promising for efficient terahertz generation from both weakly and mildly relativistic electron beams.

Design and Numerical Analysis of W-band Oscillators With Hollow Electron Beam

The use of hollow electron beams permits a significant increase in the diameter of the beam tunnel in comparison with the conventional pencil-beam slow-wave electron devices. As a result, the current density and the heating of the microwave structure are decreased, which allows increasing the average radiation power at high frequencies. To demonstrate this capability, two versions of W-band oscillators, namely, an axisymmetric orotron and a backward-wave oscillator (BWO), have been designed. The electron beam with an outer diameter of 1.6 mm and a 0.1-mm thick wall could be produced in a 30 kV/1 A Pierce-like electron gun with the hundredfold magnetic compression. Oscillators use microwave structures with an azimuthally symmetric corrugation, sinusoidal for the orotron and rectangular for the BWO. Simulations based on the averaged equations and 3-D PIC-code predict an output power up to 1 kW for the orotron and 0.7 kW for the BWO, whereas the thermal regime in the BWO is easier for continuous-wave operation. Wideband frequency tuning of the BWO is simulated. To reduce ohmic losses, a smooth downtaper of the corrugation in the orotron and an abrupt cutoff with a specially optimized last tooth in the BWO were designed.

Numerical Study of a Low-Voltage Gyrotron (“Gyrotrino”) for DNP/NMR Spectroscopy

Feasibility of a gyrotron for the dynamic nuclear polarization (DNP) purpose, integrated with nuclear magnetic resonance (NMR) spectrometer inside a single cryomagnet, is analyzed on the basis of numerical simulations. The necessary condition for DNP is matching of the gyrotrino and DNP frequencies. This imposes a strong restriction on the gyrotron operating voltage, which should be less than 2 kV. The most part of the uniform magnetic field region in the cryomagnet is occupied by a sample with NMR probe, so there is a very limited space for the gyrotron cavity. This dictates a number of peculiarities for the gyrotrino design, in particular, the diffraction power output from the cathode end of the cavity and collecting of a thin electron beam in a strong magnetic field. According to simulations, the gyrotrino operating at the fundamental cyclotron resonance with a voltage of 1.5 kV can provide an output power of 10–20 W at a frequency of 264 GHz, which is suitable for many NMR-DNP experiments.

Simulations of Sectioned Cavity for High-Harmonic Gyrotron

A recently proposed method of decreasing the ohmic losses in long gyrotron cavities is thoroughly studied. Its workability is proved using various simulations. A fourth-cyclotron-harmonic submillimeter-wavelength gyrotron, which utilizes this method, is designed, and its ability to operate with high efficiency is numerically demonstrated.

Frequency Tuning in a Subterahertz Gyrotron With a Variable Cavity

According to calculations, electromechanical frequency tuning by 8% relative to a central frequency of 316 GHz can be provided in a high-power gyrotron with a slit and variable cavity. A monoaxis helical electron beam (configuration of large orbit gyrotron) is proposed for the use in such an oscillator instead of the conventional polyaxis beam, which was used in earlier long-wavelength prototypes. This modification increases gyrotron efficiency and frequency tuning range, as well as stability of the generation regime. A mechanism for the precise parallel movement of the cavity halves and mode converter, which transforms an azimuthally standing cavity mode into a wavebeam, have been designed. Peculiarities of frequency tuning associated with the coupling of the operating and parasitic modes in the slit cavity have been studied.

Development of the Orotrons at Millimeter and Submillimeter Wavelength Range

In this paper, the low-voltage pulse orotron generators of the short millimeter and long submillimeter waves with power level up to 1 W have been developed. Some methods of frequency and power enhancement are proposed.

Method of Providing the High Cyclotron Harmonic Operation Selectivity in a Gyrotron With a Spatially Developed Operating Mode

An electrodynamic method for suppressing low-frequency oscillations at the fundamental cyclotron resonance in a gyrotron at a high cyclotron harmonic is proposed and theoretically investigated. According to simulations, the use of this method in the terahertz gyrotron at the second cyclotron harmonic operating at the spatially developed mode TE63,15 can provide a multiple excess of the starting currents of the parasitic modes over the starting current of the desired oscillations.

Design and Modeling of a Slow-Wave 260 GHz Tripler

The extended-interaction klystron frequency multiplier with a hollow electron beam has been studied numerically as a source of subterahertz radiation with controlled phase. The designs of electrodynamic and electro-optical systems are discussed for the frequency tripler with an output frequency of 260 GHz. The simulations of electron-wave interaction take into account the electron velocity spread, beam interception by a slow-wave structure, and the space charge effects. According to simulations, a two-cavity frequency tripler fed by a 1 W input signal at 86.67 GHz can deliver a maximum output power of about 15 W at a frequency of 260 GHz with a 90 MHz bandwidth (FWHM). Such a device is aimed to pulsed methods of the dynamic nuclear polarization in nuclear magnetic resonance spectroscopy.