Harmonic Gyrotron Research Articles

RF behavior of a second harmonic photonic band gap cavity gyrotron oscillator

RF behavior of a second harmonic photonic band gap cavity gyrotron oscillator

A Method for non-destructive testing of resonant frequency in the W-band second harmonic gyrotron complex cavity

A Method for non-destructive testing of resonant frequency in the W-band second harmonic gyrotron complex cavity

I/O system of V-band second harmonic gyrotron for 100/200 kW operation

This paper presents the design studies of the common input and output system of a continuous wave (CW), V-band gyrotron at second harmonic for plasma diagnostics application. Two different designs are carried out with design 1 operating at TE7,3 mode having same geometrical parameters for dual power operation of 100 kW and 200 kW and design 2 operating at TE8,3 mode has same geometry that can support dual power operation of 100 kW and 200 kW respectively. The selection of the initial design parameters has been done using our in-house computer code Gyrotron Design Studio Second Harmonic Version 2020 (GDS2H-2020). The electron guns along with magnetic guidance systems are designed to generate electron beam with the required beam parameters using software package EGUN. The design and optimization of the Non-Linear Taper (NLT), Quasi-Optical Launcher (QOL) and RF window are carried out for both the designs. NLTs are optimized for maximum transmission efficiency of the desired modes using Computer Simulation tool (CST). QOLs are designed for converting the operating modes into Gaussian-like mode using commercial software packages Launcher Optimization Tool (LOT) and SURF3D. The designs of the single-disk RF window for high-power output beam extraction are performed using CST.

Effect of Electron Beam Properties on a Second Harmonic Gyrotron

In harmonic gyrotrons, the narrow operating region of the harmonic mode is significantly restricted by the severe mode competition from the fundamental parasitic modes, which makes it important to study the nonlinear effects of beam-wave interaction in the case of a nonideal electron beam. In this article, the effect of the electron velocity spread and the beam radius spread on the mode excitation, interaction efficiency as well as mode competition are analyzed for the desired harmonic mode and the parasitic fundamental mode operating far from the cut-off frequency. The effect of velocity spread is interpreted via the electron cyclotron resonance condition as well as the electron beam susceptibility with respect to the resonator field. The results show that the velocity spread can greatly broaden the effective operating region of the harmonic mode and the efficiency degradation can be contained at a reasonable level by proper design consideration of the interaction circuit. Whereas, the beam radius spread is the main factor that accounts for the efficiency degradation and can aggregate the mode competition. The results are referential to the design and analysis of harmonic gyrotrons as well as continuously frequency tunable gyrotrons.

Forward-wave enhanced radiation in the terahertz electron cyclotron maser

Based on the principle of electron cyclotron maser (ECM), gyrotrons are among the most promising devices to generate powerful coherent terahertz (THz) radiation and play a vital role in numerous advanced THz applications. Unfortunately, THz ECM systems using a conventional high-Q cavity were theoretically and experimentally demonstrated to suffer from strong ohmic losses, and, accordingly, the wave output efficiency was significantly reduced. A scheme to alleviate such a challenging problem is systematically investigated in this paper. The traveling-wave operation concept is employed in a 1-THz third harmonic gyrotron oscillator, which strengthens electron-wave interaction efficiency and reduces the ohmic dissipation, simultaneously. A lossy belt is added in the interaction circuit to stably constitute the traveling-wave interaction, and a down-tapered magnetic field is employed to further amplify the forward-wave (FW) component. The results demonstrate that the proportion of ohmic losses is nearly halved, and output efficiency is nearly doubled, which is promising for further advancement of high-power continuous-wave operation of the ECM-based devices.

Development and Initial Experimental Results of a Terahertz Pulsed Field Gyrotron in the WHMFC

We report the design and initial experimental results of an 800-GHz second harmonic gyrotron prototype, in which a 15-T pulsed magnet is applied. This gyrotron prototype is developed at the Wuhan National High Magnetic Field Center (WHMFC). An in-house software for analyzing and designing terahertz gyrotrons and an experimental research platform are developed. In addition, alignment adjustments, eddy current effects, and radiation tests are discussed. The initial experimental results show that the prototype gyrotron can irradiate ~40 W at fundamental harmonic 383.7-GHz (TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,4</sub> ) waves at 14.14 T and a few milliwatts at second harmonic 0.8-THz (TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8,5</sub> ) waves at 14.86 T.

Design of a 330-GHz Frequency-Tunable Gyrotron With a Prebunched Circuit

The design of a 330-GHz second harmonic gyrotron with broadband tunability is presented in this article. Loaded with a prebunched cavity, the output power is almost doubled (~216 W, corresponding to the output efficiency of 3.6%) compared with a conventional open-cavity scheme, and the tuning range is ~1.85 GHz. To further reveal the feasibility of the prebunching section for the harmonic operations, it is theoretically demonstrated that the phase shift inside the prebunching Section plays an important role in improving the interaction performance. The dependence of the phase shift on the cavity dimension and propagation constant is explored, which instructs the circuit optimization. Moreover, the engineering designs of the magnetron injection gun, the output window, and the collector are presented under a 10-T super-conducting magnet. The overall configuration of the tube is exhibited at the end of the article.

Frequency-Tunable Second Harmonic Gyrotron With Selective Cavity: Design and Simulations

We show numerically that the gyrotron cavity with specially designed rectangular grooves provides a selective gyrotron excitation at the second cyclotron harmonic and allows frequency tuning. Simulations predict that the output power higher than 10 W can be obtained in the 0.3% band near the frequency of 400 GHz for a 15-kV, 0.5-A electron beam. Results obtained by analytical model calculations and particle-in-cell (PIC)-code are in good agreement.

Investigations on RF Behavior of a V-Band Second Harmonic Gyrotron for 100/200 kW Operation

This article presents the investigations on RF-behavioral aspects for the possible operation of a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> -band, continuous wave (CW) second harmonic gyrotron for plasma diagnostic application. Keeping in view the design goals and constraints, initial design studies for the mode selection and the computation of starting currents are carried out. From these studies, two possible modes, namely, TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7,3</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8,3</sub> are considered for the second harmonic operation. Later, the cold cavity design and self-consistent calculations are carried out for the selected operating modes. All the computations are performed using the latest version of our in-house code Gyrotron Design Studio Second Harmonic Version 2020 (GDS2H-2020) with Glidcop as the cavity material. The RF behavior studies confirm the feasible operation of such a second harmonic gyrotron with power levels in excess of 115.52/217.64 kW with the chosen modes of operation.

Phase-Locking of Second-Harmonic Gyrotrons for Providing MW-Level Output Power

We propose the method of providing megawatt (MW)-level output power in continuous wave (CW) or long-pulse second-harmonic (SH) gyrotrons using phase-locking by a weak external monochromatic signal. Typically, the power of harmonic gyrotrons is hindered due to excitation of spurious fundamental-harmonic modes at the front of the accelerating voltage and the beam current. Injection of a seeding signal at the operating SH mode (priming) can suppress the spurious generation during the startup process. Within the frame of a self-consistent multimode model, we demonstrate the applicability of this approach to provide &#x007E;1-MW radiation in a 230-GHz SH gyrotron with the high-order TE_34,14 operating mode which is standardly unattainable for harmonic operation.

A CW, 94 GHz Second Harmonic Gyrotron with a Continuous Operation Solenoid Cooled by Water

A CW, 94 GHz Second Harmonic Gyrotron with a Continuous Operation Solenoid Cooled by Water

Investigation of Mode Interaction in Harmonic Sub-THz Gyrotron

Investigation of Mode Interaction in Harmonic Sub-THz Gyrotron

Automodulation instability in gyrotrons operating at the second cyclotron harmonic

Interest in gyrotron operation at cyclotron harmonics is motivated by the users' desire to reduce the magnetic fields required for operation at a given frequency. However, operation at harmonics is more complicated than at the fundamental resonance because in harmonic gyrotrons there are many parasitic modes at the fundamental, which can be prone to excitation. The present study is devoted to the analysis of automodulation instability in harmonic gyrotrons. Such instability may occur, for example, when in the vicinity of the desired second harmonic mode there is a pair of parasitic modes at the fundamental, for the frequencies and azimuthal indices of which some specific conditions are met. In this paper, the equations describing this instability are derived in the cold-cavity approximation. The study is then focused on a second harmonic gyrotron with the parameters optimal for efficient operation. For such a gyrotron, the region of instability is determined in the plane of frequency mismatches between the operating and parasitic modes. This treatment performed within the framework of the general theory is complemented by consideration of some gyrotrons operating in specific modes, which are surrounded by specific sideband modes at the fundamental.

Terahertz gyrotrons with inhomogeneous magnetic fields to suppress mode competition and enhance efficiency

Gyrotrons are promising radiation sources for bridging the terahertz gap. They are based on the instability of electron cyclotron maser, where the harmonic operation is generally necessary to alleviate the need for a strong magnetic field. Unfortunately, the performance of a harmonic gyrotron is extremely sensitive to mode competition and magnetic tuning. In this study, to achieve highly efficient and mode-selective gyrotrons, inhomogeneous magnetic fields are applied to introduce a specified longitudinal distribution of the detuning frequency between the terahertz wave and the gyrating electron beam. This detuning frequency has different influences on the oppositely traveling forward wave (FW) and backward wave (BW) inside the cavity, from which optimized magnetic-field profiles for FW-favored and BW-favored interaction circuits are generalized accordingly. It is proposed that a negatively tapering magnetic field converts the energy-transfer rate of the FW interaction into a positive value, leading to highly efficient FW interaction. By contrast, a positively tapering magnetic field reduces the detuning frequency of BW interaction and extends its effective length. By controlling the detuning frequency, a scenario of suppressed mode competition is proposed in a 330-GHz second-harmonic gyrotron. A universal understanding of the influence of an inhomogeneous magnetic field—i.e., the detuning frequency—on the interaction dynamics would help to develop efficient and broadband tunable terahertz gyrotrons.

Investigation of electron optical gun and beam collector for 42 GHz, 200 kW second harmonic gyrotron

This paper presents a design investigation of the Electron Optical Gun (EOG) for a 42 GHz, 200 kW second harmonic gyrotron operating in the TE0, 3 mode. The initial design of MIG is done using established design equations and then optimized through numerical simulations. The different software, such as 2D-Numerical codes EGUN, TRAK and 3D-CST Particle Studio, are used to optimize the design and found good agreement in results obtained from these codes. The sensitivity analysis has also been carried out for the sake of complete design. Furthermore in brief, a design has been done for the un-depressed and Single-Stage Depressed Collector (SDC). In addition, the uneven or uniform beam distribution on collector wall is presented with result comparison of both the collectors. The optimized beam voltage and single-stage depressed collector voltage are 65 kV, 30 kV respectively.

A High-Current-Density Terahertz Electron-Optical System Based on Carbon Nanotube Cold Cathode

A high-current-density terahertz electronoptical system based on a carbon nanotube (CNT) cold cathode has been investigated in order to solve notable mode competition in high-order harmonic gyrotrons. Simulation results show that a near-axis electron beam can be generated, with a beam current of 160 mA at an accelerating voltage of 23.5 kV. The source supports the electron beam with a velocity ratio of 1.1 and a guiding center radius of 57 μm. A narrow beam spatial distribution is evidenced by adjusting control anode voltage, satisfying the need for high operating current density in slow wave devices. The current density of the electron beam is up to 620 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at the output port and the velocity ratio is 0.44, with parallel energy accounting for 84% of the total energy.

Investigations on W-Band Second Harmonic Gyrotron for 50/100-kW Operation

In this article, RF behavior studies on a $W$ -band continuous wave (CW) gyrotron for industrial, scientific, and medical applications are presented. Two different designs are carried out for the output power level of 50 and 100 kW. The mode competition with the first and second harmonic neighboring modes is carefully investigated, and TE6,2 and TE10,4 are chosen as the operating modes for these designs. The cold cavity design and self-consistent computations for power and efficiency are done to determine the geometrical and beam parameters of the desired modes using the in-house code, Gyrotron Design Suite Second Harmonic Version 2018 (GDS2H-V. 2018). The desired output power is obtained for two different designs at the chosen cavity modes. Additionally, electron guns along with magnetic guidance systems are designed to generate an electron beam with the required beam parameters for the two designs.

The Nonlinear Design and Experiment of a 0.68-THz Second Harmonic Gyrotron

One of biggest obstacles for the development of terahertz (THz) science and technology is the lack of the THz sources, especially, when the requirement of the power reaches to kilowatts. Among all the existing sources, the gyrotron is the most promising one which can generate high-power THz radiation. Recently, a THz detection system is under development in the THz Research Center, University of Electronic Science and Technology of China, Chengdu, China, which is applied for detecting radioactive material. Thus, a 0.68-THz second harmonic gyrotron has been simulated and tested in the center. In this article, we present the theoretical study on the designed gyrotron based on the linear and nonlinear theories. The influences of the different electron beam parameters on the beam–wave interaction are analyzed. The mode competition based on the time-dependent multimode nonlinear theory has also been investigated in the designed gyrotron; it is shown that the operating mode can dominate the mode competition when the gyrotron is working at the optimized operating point. The experiment results show that the output power can reach 3.25 kW with the efficiency 9.26%, at frequency 0.679 THz, when the beam voltage is 39 kV, the beam current is 0.9 A, and the magnetic field is 12.84 T.

Nonlinear excitation of parasitic modes in harmonic gyrotrons

The investigation of interaction between modes resonant with electrons at different cyclotron harmonics is presented. The study is focused on mode interaction in the region of currents lower than the start current of the parasitic mode at the fundamental. In this region, the excitation of such parasitic modes is possible due to nonlinear excitation of them by the harmonic modes only. It is shown that, when the beam current increases, various oscillation regimes replace each other: when the current exceeds the start current of the harmonic mode only slightly, the single-frequency oscillations of this mode are stable. At higher currents, due to nonlinear excitation, the oscillations of both modes with constant amplitudes are possible. Then, this regime is replaced by the auto-modulation of two modes with periodically varying mode amplitudes. Finally, at even higher currents, the parasitic mode at the fundamental suppresses the parental second harmonic mode. The role of two saturation mechanisms (the orbital phase bunching and the depletion of the free rotational energy in the process of beam-wave interaction) in the nonlinear excitation of parasitic modes at the fundamental is analyzed. The parallels between this set of events and events that take place in the prey–predator theory are discussed.

On applicability of absorbing rectilinear electron beams in high-frequency gyrotrons operating at cyclotron harmonics

This paper presents the studies of electron–wave interaction in a high-frequency harmonic gyrotron with an additional absorbing rectilinear electron beam. The expression for the starting current of the gyrotron in the presence of an absorbing beam is obtained within the model of a fixed axial field structure. We investigate the effectiveness of mode selection by absorbing beam in gyrotrons with different resonator lengths to wavelength ratios on the basis of simple theory, as well as the results of numerical simulation within a self-consistent model.