A. E. Fedotov

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.

New sources of coherent submillimeter-wave radiation

Large orbit gyrotron (LOG) provides output power of 40 kW at the frequency of 240 GHz. Orotrons radiate at frequencies of 100-380 GHz with power of 50-200 mW and high frequency stability. In a LOG, electron-wave interaction at high cyclotron harmonics is more selective than in conventional gyrotrons that allows achieving higher frequencies. Higher output power and higher frequency stability can be obtained in orotrons in comparison with conventional BWOs due to use of open cavities.

Smooth Wideband Frequency Tuning in Low-Voltage Gyrotron With Cathode-End Power Output

The possibility of a smooth electronic frequency tuning for medium-power gyrotrons operated at the fundamental cyclotron resonance has been demonstrated in the numerical simulations. The use of a low operating voltage allows a drastic increase in the gyrotron operation efficiency at the higher axial modes and, hence, in the bandwidth of continuous frequency tuning. A gyrotron cavity with power output in the direction of the cathode end can provide a smoother power versus frequency dependence in the gyro-backward wave oscillator regime than a cavity with the conventional output in the direction of the collector. Simulations for a 2-kV gyrotron with a frequency of about 264-GHz demonstrate 3-GHz full-width at half-maximum frequency tuning with a power level of 15 to 30 W, which are attractive parameters for spectroscopic applications.

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.

Operation of a sub-terahertz CW gyrotron with an extremely low voltage

Decreasing the operating voltage for medium-power sub-terahertz gyrotrons aimed at industrial and scientific applications is highly attractive, since it allows size and cost reduction of the tubes and power supply units. In this paper, we examine such an opportunity both numerically and experimentally for the fundamental cyclotron resonance operation of an existing gyrotron initially designed for operation at the second cyclotron harmonic with a relatively high voltage. Simulations predict that output power higher than 10 W can be produced at the fundamental harmonic at voltages less than 2 kV. To form a low-voltage helical electron beam with a sufficiently large pitch-factor, a positive voltage was applied to the first anode of the gyrotron three-electrode magnetron-injection gun with a negative voltage at the cathode. CW gyrotron operation at voltages down to 1.5 kV has been demonstrated at a frequency about of 256 GHz.

Gyrotron generation of broadband chaotic radiation under overlapping of high- and low-frequency resonances

Significant (up to 10%) broadening of the band of noise-like radiation in gyrotrons is possible due to specific tuning of gyrofrequency relative to the critical frequency of the working mode when the high- and low-frequency cyclotron resonances are substantially separated at the given translation and transverse velocities of electrons. The generation bands at the resonances are overlapped under operation above the threshold. The analysis with allowance for finiteness of the electron transit time in the interaction space and, hence, different slopes of the dispersion characteristics of electron beam and wave is critically important for correct investigation of the generation regimes in the framework of the average approach. The results are verified using direct 3D particle-in-cell simulation of the chaotic generation regimes of the gyrotron generation in the 8-mm-wavelength range.