Cavity Gyrotron Research Articles

A model of electron beam neutralization for gyrotron simulations

A numerical model is presented to describe the electron beam neutralization during long-pulse and continuous-wave operation in gyrotrons. This model has been implemented in the electrostatic, self-consistent electron optics code Ariadne. Using this model, the electron beam parameters in the cavity could be determined as a function of the level of neutralization. The electron beam was shown to be partially neutralized in the cavity, and the effect of neutralization on the beam properties was investigated for a cylindrical gyrotron. This study was extended to a coaxial cavity gyrotron, identifying two ion trapping regimes, which only had a minor impact on the beam parameters.

High-Frequency MW-class Coaxial Gyrotron Cavities Operating at the Second Cyclotron Harmonic

High-Frequency MW-class Coaxial Gyrotron Cavities Operating at the Second Cyclotron Harmonic

Nonadiabatic effects in the electron-optical system of a relativistic millimeter-wave gyrotron

Gyrotrons based on relativistic electron flows generated by thermionic magnetron injection guns are promising pulsed sources of millimeter and submillimeter radiation with multi-megawatt output power. Obviously, to increase the output power of such devices, it is necessary to provide a high beam current of more than 100 A. At the selected emission density, this forces the use of an emitter with a sufficiently large area. As is well known, wide emitters produce beams with a large velocity spread, which reduces the efficiency of the electron-wave interaction. Therefore, it is necessary to increase the diameter and, accordingly, increase the distance from the cathode to the gyrotron cavity, which leads to an increase in the magnetization reversal coefficient. Thus, the magnetic field in the region of beam formation becomes weak, and the Larmor radius and the turn pitch of the electron trajectory become large compared to the geometry of the surrounding electrodes, which can cause nonadiabatic effects. In this work, we consider one of these effects, which consists in the emergence of a range of anode voltage values in which the monotonic growth of the pitch factor stops. The results of trajectory analysis using current tube and discrete source methods based on the ANGEL software package are presented, as well as an assessment of the influence of a magnetic lens on the properties of an electron beam based on a simple analytical model.

Spectral approach with iterative clarification of a radiation boundary conditions for modeling of quasimodes of a gyrotrons open cavities

Purpose. The article presents a new method for numerical simulation of quasi-eigenmode oscillations in open resonators of gyrotrons — powerful vacuum generators of electromagnetic waves in the millimeter and submillimeter ranges. The gyrotron cavity has the shape of a weakly inhomogeneous hollow circular metal waveguide. Methods. The proposed approach uses the inhomogeneous string equation with radiation boundary conditions to formulate a nonlinear spectral boundary value problem describing oscillations in a resonator, neglecting the couplings of waves with different radial indices. By linearizing with respect to frequency the radiation boundary conditions, the boundary value problem is reduced to a linear boundary value problem. To discretize this boundary value problem, the finite difference method is used and a linear generalized matrix eigenvalue problem is formulated. This problem is solved by the Arnoldi method with eigenvalues calculation in a shift-invert mode. An iterative algorithm is proposed that makes it possible to sequentially calculate a given number of frequencies and quality factors of quasi-eigenmodes of oscillations. Results. The computer program was developed written in the Wolfram Language and Fortran using the algorithms proposed in the work. The results of test calculations for real gyrotron resonators are presented, which demonstrate the high accuracy of the obtained values of frequencies, quality factors and field distributions of quasi-eigenmode oscillations in the studied resonators. Conclusion. The methods, algorithms and created program proposed in the article can significantly facilitate the process of developing gyrotrons intended for various practical applications and operating in new frequency ranges. The method of iterative refinement of boundary conditions can be generalized to the case of equations of the linear theory of a gyrotron and used to develop new methods for analyzing the starting conditions for the soft self-excitation in gyrotrons — generators.

Fabrication and assembly of the gyrotron multi-stage depressed collector prototype at KIT

In this paper details of the fabrication and assembly of the first Multi-stage Depressed Collector (MDC) prototype developed at KIT for megawatt-class gyrotrons are presented. Utilizing the E×B drift concept for electron trajectory separation, the cylindrical Short-Pulse (SP) MDC prototype features a design compatible with applications in different fusion gyrotrons at KIT, for W7-X, ITER, and DEMO. The fabrication process includes inner electrodes out of copper-chromium-zirconium (CuCr1Zr) with a triple helix isolation design, modular vacuum housing consisting of four parts, and additional external coils for electron beam confinement. Detailed assembly procedures are provided for two configurations: the 170 GHz 2 MW coaxial cavity gyrotron and the W7-X upgrade SP gyrotron. The successful manufacturing of the modular collector and a robust design for vacuum tightness are demonstrated. The prototype is now primed for verification in the KIT FULGOR test stand with the W7-X gyrotron for validation of the E×B drift concept and improvement of the gyrotron efficiency. This comprehensive work bridges theoretical concepts with practical implementation, offering insights crucial for refining and advancing MDCs for megawattclass gyrotrons for fusion applications.

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

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

Design of MW-Class Coaxial Gyrotron Cavities With Mode-Converting Corrugation Operating at the Second Cyclotron Harmonic

Design of MW-Class Coaxial Gyrotron Cavities With Mode-Converting Corrugation Operating at the Second Cyclotron Harmonic

Design and Experiments of a 0.66-THz Short-Pulse TE14,5 Mode Second-Harmonic Gyrotron

Experimental investigations on a 0.66-THz gyrotron have been accomplished. Meanwhile, the magnetic injection gun and the gyrotron beam–wave interaction cavity are designed and optimized. The gyrotron cavity is a traditional three-section cavity. The experimental results demonstrate that this gyrotron is under the stable operation of the second cyclotron harmonic. The operating mode is TE <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{14, \text{5}}}$</tex-math> </inline-formula> mode and the pulselength is 80 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> s. By tuning the beam energy and the operating magnetic field, the maximum frequency of this gyrotron reaches 656.95 GHz and the output power is up to 282 W. Furthermore, this gyrotron can be utilized as a frequency-tunable source, the frequency-tunable range is about 70 MHz. This gyrotron is specially designed to meet the requirement of the experiment of irradiating THz waves on living bodies.

Simulation of Parasitic Backward-Wave Excitation in High-Power Gyrotron Cavities

The possibility of parasitic excitation of backward waves directly in the gyrotron cavity is demonstrated, by simulation, for two existing high-power gyrotrons. These are the 140-GHz 1-MW TE <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{28},{8}}$ </tex-math></inline-formula> -mode gyrotron for the stellarator W7-X and the 140-GHz 1.5-MW TE <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{28},{10}}$ </tex-math></inline-formula> -mode gyrotron, also for W7-X. The parasitic backward waves, namely, the TE <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{23},{7}}$ </tex-math></inline-formula> mode in the 1-MW gyrotron and the TE <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{-{24},{10}}}$ </tex-math></inline-formula> mode in the 1.5-MW gyrotron, are excited at high frequencies (RF), which are of the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim $ </tex-math></inline-formula> 10% lower than the nominal operating frequency and which can lead to significant performance degradation, with respect to the output power, efficiency, and stability of the tube. This finding offers an additional possibility, besides parasitic mode excitation in the gyrotron beam tunnel or after the gyrotron cavity, for the origin of experimentally observed RF parasitic oscillations in high-power, high-frequency gyrotrons, operating in high-order modes. To strengthen the confidence in the simulation, the results of two codes, each using different modeling of the interaction between the electron beam and the RF wave, are compared and the appropriateness of the modeling with respect to the accurate simulation of backward waves is discussed in detail.

Gyrotron Cavity with an Azimuthally Asymmetric, Mechanically Variable Cross Section

Gyrotron Cavity with an Azimuthally Asymmetric, Mechanically Variable Cross Section

Ultimate transverse power of pulsed low-voltage gyrotron beam

Low operating voltage is highly attractive for medium-power millimeter-wave gyrotrons since it can reduce their size and cost, increase their safety, and, thus, improve usability for applications. However, at low voltages, the voltage depression caused by DC space-charge fields significantly limits the electron current and transverse power in the beam. Moreover, this current limitation is more pronounced for a beam with a higher pitch factor. As a result, for a given anode voltage, there is a pitch factor at which the transverse beam power in the gyrotron cavity is the maximum. This ultimate transverse power is found analytically in the non-relativistic approximation. Such a power is reached when the pitch factor calculated without taking into account voltage depression is only 0.82; voltage depression decreases the axial electron velocities, thus, increasing the actual pitch factor value in the cavity up to 1.4. As a result of this effect, high power and high efficiency cannot be obtained simultaneously in a low-voltage gyrotron. Using particle-in-cell simulations, two variants of low-voltage (5 kV) gyrotrons have been designed, namely, a device with higher power and an optimal pitch factor of 0.82 in the cavity and a device with a high pitch factor and high efficiency, but lower power.

Design of a High-Q Diamond-Loaded Cavity for a Third-Harmonic Subterahertz Gyrotron Driven by a Low-Power Electron Beam

A continuous-wave (CW) high-harmonic gyrotron driven by a low-power electron beam is a compact radiation source demanded by terahertz applications. Its physical feasibility, however, is hampered by ohmic losses and mode competition in the gyrotron cavity. An ultralow-loss diamond loading of the cavity can give a clue to this problem. This article is concerned with theoretical aspects of mode selection and design for a gyrotron cavity loaded with coaxial rod made of chemical vapor deposition (CVD) diamond. As an example, the design of a high-Q diamond-loaded cavity for a third-harmonic 658-GHz gyrotron powered by a 0.1-A, 15-kV electron beam is presented. It is shown that the designed cavity enables the gyrotron to produce up to 116-W output power in a single oscillating mode.

Optimization of the flow distribution in a gyrotron cavity using evolutionary CFD simulations driven by a genetic algorithm

The steadily increasing performance requested to gyrotrons, to comply with their function of heating and current drive in fusion reactors, put a progressively increasing burden on the heat removal from the interaction cavity, where the heat flux can easily reach 20 MW/m2 on its inner surface. The cavity is actively cooled by subcooled water in forced flow in an annular region, and the water flow typically enters and exits through a single inlet and outlet. The high-speed flow entering from the inlet potentially drives an inhomogeneous flow azimuthally in the cavity region, but helps in locally bursting the heat removal due to the impinging effect of the cold-water flow. Here a new design of the water inlet in the cavity region is performed through a simplified genetic algorithm, in such a way that the flow homogeneity in the gyrotron cavity is maximized, without reducing the beneficial cooling due to the impact of the water jet on the cavity wall. The presence of multiple holes feeding the cavity, with their azimuthal location driven by the genetic algorithm, is analyzed through Computational Fluid Dynamics (CFD) simulations. Once the multi-inlet location is optimized, the dimension of the inlet holes is tuned to burst the heat transfer effect, reaching a high level of temperature and flow homogeneity in the cavity.

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.

Optimization of the Flow Distribution in a Gyrotron Cavity Using Evolutionary Cfd Simulations Driven by a Genetic Algorithm

Optimization of the Flow Distribution in a Gyrotron Cavity Using Evolutionary Cfd Simulations Driven by a Genetic Algorithm

Gyrotron Cavity With an Azimuthally Asymmetric, Mechanically Variable Cross Section

Gyrotron Cavity With an Azimuthally Asymmetric, Mechanically Variable Cross Section

Particle-in-Cell Algorithm-Based Computation of Time-Dependent-Multimode Behavior of 42-GHz Gyrotron

The analysis of the multimode behavior of the interaction cavity for 42-GHz gyrotron is presented here. The in-house developed codes single-mode cold cavity solver (SMCCS) and GYCAD and commercial particle-in-cell (PIC) simulator MAGIC are used in the design and analysis of 42-GHz gyrotron cavity. SMCCS is used for the calculation of cold cavity parameters such as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> factor, resonant frequency, and the axial electric field profile, which are further used in the beam–wave interaction computation. The PIC code is used here for the parametric optimization and time-dependent-multimode beam–wave interaction simulations of the interaction cavity. The mode purity in terms of the electromagnetic power in individual modes is also calculated using the data generated in the beam–wave interaction simulations. The single-mode analysis of the cavity is also performed using the analytical code GYCAD based on the generalized nonlinear theory. The results obtained by the PIC simulator and GYCAD are discussed and compared here. The analysis of the electron beam misalignment, which may occur due to the radial shifting or tilting of the different components during the fabrication of the device, is also performed and the effects on the efficiency, mode competition, and mode stability are discussed in detail.

Stable Excitation of Higher Axial Modes in the Traveling-Wave-Tube Regime in Gyrotron Cavities With Additional Loss Elements

Weakness of the electron-wave coupling in compact low-power terahertz-frequency-range gyrotrons due to relatively low electron beam currents and operation at high cyclotron harmonics results in the use of long-length cavities with high diffraction Q-factors, and, therefore, with a high share of the ohmic loss of the radiated wave power. In this work, we demonstrate a possibility for stable operation at higher axial modes possessing relatively low diffraction Q-factors. Enhanced efficiency of the electron-wave interaction is provided due to the excitation of higher axial modes in the traveling-wave-tube regime.

RF behavior of a 35 GHz conventional cavity gyrotron using multimode analysis and PIC simulation

ABSTRACT In this paper, the RF behavior of a 35 GHz, TE 04 mode cylindrical cavity gyrotron is investigated using the nonlinear multimode theory and PIC code “CST Particle Studio”. A code is developed to realize the beam-wave interaction mechanism using time-dependent multimode theory. The cold cavity behavior is observed for the cavity operation in the designed TE 04 mode and 35 GHz frequency. The magnetic field profile is kept uniform along the length of the interaction structure. The output power is estimated for the designed mode as well as all nearby competing modes using the multimode analysis and PIC simulation. This yields the output power more than 200 kW with 33% efficiency in the designed TE 04 mode, whereas the smaller output power in the nearby competing modes. Power sustainability is also observed for the stable operation of the device. The effects of beam velocity spread and detuning of magnetic field on its performance are also observed.

Calibration of the KIT test setup for the cooling tests of a gyrotron cavity full-size mock-up equipped with mini-channels

In high-power fusion gyrotrons, the maximum heat-load on the wall of the interaction section is in the order of 2 kW/cm2, which is the major limiting technological factor for output power and pulse-length of the tube. The ongoing gyrotron development demands a very effective cavity cooling system for optimum gyrotron operation. In this work, the experimental investigation of a mini-channel cavity cooling using a mock-up test set-up is described. The mock-up test set-up will be used to experimentally validate the predictive simulation results and verify the mini-channel cooling performance. It is crucial for validation of the mini-channel cooling properties to determine the amount of the heat load introduced in the cavity wall by an induction heater. In order to estimate that heat load, full 3D electromagnetic simulations have been performed using the CST Studio Suite® software. A suitable calibration factor for the load deposited in the mock-up inner wall is identified after numerical investigation by a 3D thermal model. Calorimetry measurements are performed and the experimental results are compared with the simulation results obtained with a 3D thermal-hydraulic model, using the commercial software STAR-CCM+. When the calibration factor is applied, the experimental calorimetry is well reproduced by the simulations.