Gyrotron Operation Research Articles

Fault prediction of gyrotron system on test bench using a deep learning algorithm

The gyrotron system is an essential component of the Electron Cyclotron Resonance Heating (ECRH) system, generating high-power millimeter waves that can be utilized for plasma heating and current drive in magnetically confined nuclear fusion. During high-power long-pulse operation, the gyrotron is prone to faults such as arcing, mode jump, and overcurrent, leading to interruptions in continuous system output. These faults not only endanger the safety of the gyrotron but also reduce the overall efficiency and performance of the system. Addressing this issue, we have employed deep learning methods to predict faults that may occur during the gyrotron’s long pulse operation. Using experimental data collected from the megawatt-level continuous wave (CW) gyrotron test bench since 2019, we constructed a dataset comprising 52,772 data points, including 23,241 instances of faulty data. We trained a model based on the Transformer encoder architecture and compared its performance against classical CNN and LSTM models. The results demonstrated that the Transformer model exhibited superior performance in this context, achieving an AUC value of 0.869. The True Positive Rate (TPR) and False Positive Rate (FPR) reached 85.2% and 18.2%, respectively. Subsequent offline testing verified that the trained model could successfully predict faults in advance with second-level accuracy, laying the groundwork for future gyrotron operation management. In conclusion, deep learning has demonstrated its potential application in the prediction of operational faults in the gyrotron system.

Design and Preliminary Experiment of Room-Temperature Bitter Magnet for Compact Gyrotron

Magnetic field system is one of the most crucial components for gyrotron operation. A compact gyrotron with a custom-designed magnet would be a promising continuous radiation source for millimeter-wave power applications. In this article, a design and preliminary experiment of a room-temperature Bitter magnet for developing compact gyrotron are presented. To lengthen the magnetic field homogeneity region without dramatically improving power consumption, a special design that every Bitter segment with different outer radii is adopted. Both in simulation and experimental measurement, the length of the homogeneity region is 120 mm, and homogeneity <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \textit{B}$</tex-math> </inline-formula> / <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{B}$</tex-math> </inline-formula> is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm$</tex-math> </inline-formula> 0.9% is obtained. A 5 kV magnetron injection gun (MIG) was designed based on the experimental measurement of this Bitter magnet, in which a velocity ratio of 1.3 and transverse velocity spread of 12% were obtained at current 0.5 A in simulation.

Long Pulse and High Duty Operation of a W-Band Gyrotron Traveling Wave Tube

In this letter, we reported a gyrotron traveling wave tube (gyro-TWT) featured broadband and high power at the University Electronic Science and Technology of China (UESTC), which had achieved a long pulse width up to 2 ms and a maximum duty cycle of up to 40%. Two typical operation states, i.e. high voltage/current (60.5 kV/9.0 A) and low voltage/current (46 kV/2.6~2.9 A), are respectively hot tested for high pulse and average power. With a duty cycle of 40%, the measured maximum average power is up to 20.1 kW.

High-harmonic large-orbit terahertz gyrotrons for physical applications

An overview is given of works on two experimental installations that implement large-orbit gyrotrons (LOG) operating in the sub-terahertz frequency range at high harmonics of the electronic cyclotron frequency. At the installation of a pulsed LOG (80–100 keV/0.7–1.0 A), a radiation source with a frequency of 1 THz with a kilowatt output power level is being developed, which is planned to be used in plasma applications. A continuous subterahertz LOG (30 keV/0.7 A) is being created as a prototype of a universal multi-frequency source for spectroscopic applications. Complex electrodynamic systems are also described, designed to increase the selectivity and efficiency of excitation of high cyclotron harmonics in these devices, as well as to ensure the tuning of the generation frequency.

Evaluation of a newly developed low reflection dummy load for high power millimeter waves

A low reflection dummy load (LRD) has been newly developed for gyrotron development and operation. This LRD is designed for 0.5 s of high power (∼1 MW) millimeter wave injection with a low reflection. In order to reduce the reflection, the LRD consists of an inner cylinder, a main chamber, and a conical mirror, which results in the efficient absorption by long pass length between the inner cylinder and the main chamber. The characterization of low reflection has been demonstrated by the water temperature rise of the DC break (DCB) section inside the gyrotron, which works as an indicator of the reflected stray millimeter wave from the dummy load (DL). The water temperature rise of DCB with LRD was decreased by 1%, 15% and 17% at three tested frequencies of 82 GHz, 110 GHz and 138 GHz, respectively, compared with the conventional DLs, and the power absorption of LRD is 4–7% larger than the conventional DLs. A thermal analysis by a finite element method (FEM) with experimentally obtained heat load distribution and cooling time constant showed that the maximum temperature at 0.5 MJ injection is below the melting point of the material but it is not acceptable for the cycle operation considering the allowable stress of the aluminum-alloy used in the present design. FEM analysis suggested that changing its material to CuCrZr enable to use for 0.5 MJ injection. The result of FEM analysis also shows that the CW injection with less than 0.15 MW is acceptable in the present design.

Frequency multiplication in a high-current relativistic gyrotron for obtaining high-power THz-band radiation

Using averaged equations and 3D particle-in-cell simulations, we investigate the frequency multiplication regime in a high-current relativistic gyrotron operating in 0.1 THz frequency band. We demonstrate that the ratio of the 3rd harmonic power and the fundamental cyclotron resonance power can reach 0.4–0.8%, which aloows for obtaining the 0.3 THz radiation with sub-MW output power level.

Boosted excitation of the fifth cyclotron harmonic based on frequency multiplication in conventional gyrotrons.

We discovered a specific property of the eigenmodes of a cylindrical waveguide, due to which efficient mode excitation at ultrahigh (s=4n+1, n is integer) multiples of the gyrofrequency can be provided by a polyhelical weakly relativistic electron beam, standardly used for gyrotron operation. In the proof-of-principle experiment with a V-band gyrotron driven by a 25-keV, 2-A beam, about 100 mW radiation power at the fifth cyclotron harmonic (0.22 THz) has been detected in the cw regime. Based on the gyrotron theory, we demonstrate in simulations that, using parameters of existing gyrotrons, this method can be applied for producing cw radiation at a power level of up to several watts in the terahertz gap.

Detailed Investigation on Nonstationary Behavior in a Frequency-Tunable Gyrotron

As a terahertz (THz) radiation source, the frequency-tunable gyrotron is promising in application and attractive to researchers. In this article, the nonstationary phenomenon in a frequency-tunable gyrotron operating in different regimes&#x2014;gyromonotron, transition, and gyro-backward-wave oscillator (BWO)&#x2014;has been investigated in detail by 3-D particle in cell (PIC) simulation. Simulated results demonstrate that among the three regimes, gyromonotron has the strongest capability to keep stationary, and nonstationary oscillations are most likely to occur in the transition regime. Moreover, in the gyro-BWO regime, the capability of the system to maintain steady-state operation gets slightly improved with the increase in the axial mode. The corresponding interpretation is based on detuning frequency and feedback mechanism. Finally, the effect of the velocity spread on the nonstationary behavior has also been studied. Investigations reveal that the stability of stationary oscillations would be weakened when velocity spread is considered. The results in this article may be of benefit of a deeper insight into the nonstationary phenomenon in a frequency-tunable gyrotron.

Assessment of different RANS turbulence models in mini-channels for the cooling of MW-class gyrotron resonators

The adoption of high-speed water flow in mini-channels is a viable option for the cooling system of the resonant cavity in gyrotrons, which are a candidate technology for the external plasma heating in nuclear fusion reactors. The evaluation of the performance of such mini-channel cooling system is a combined fluid dynamics and heat transfer phenomenon which seeks more attention to a highly accurate computational analysis. In this study, a computational-based comparative platform is proposed to evaluate the performance and fidelity of the applied turbulence models which are utilized to study the mini-channel cavity cooling systems in gyrotrons. A full-size mock-up of the gyrotron resonator equipped with mini-channels has been realized and tested in 2019 by THALES to check its total pressure drop applying a wide range of water flow rates, including that available for the gyrotron operation. In parallel, a numerical model of the mock-up has been developed using the commercial software STAR-CCM+, and simulations have been performed using different RANS turbulence closures, and namely: SST k−ω, realizable k−ε and Lag EB k−ε. The detailed comparison of the computed hydraulic characteristics (i.e., a range of pressure drop measurements at different flow rates) to the set of measured values has been addressed using a multivariate metric to assess the performance of different turbulence models in pure hydraulic simulations. This comparative platform reveals a significant clarified difference in fidelity among the RANS models. Based on the performed comparative studies against the entire set of available experimental data, the Lag EB k−ε closure provides the best performance among the other turbulence models and can be applied for the future studies of the mini-channel cavity cooling systems of the gyrotron resonators.

Experimental Testing of the European TH1509U 170-GHz 1-MW CW Industrial Gyrotron—Long Pulse Operation

The upgraded European 1-MW, 170-GHz continuous wave (CW) industrial prototype gyrotron (TH1509U) for electron cyclotron resonance heating and current drive in ITER was tested at the Karlsruhe Institute of Technology (KIT). In this work, we report on the major achievements during the experimental campaigns that took place intermittently between October 2020 and July 2021. The upgraded gyrotron clearly surpassed the performance of the previous TH1509 tube. In particular, TH1509U delivered (i) 0.9 MW in 180 s pulses (maximum possible pulse length with the KIT test stand) and (ii) more than 1 MW at a pulse length limited to 40 s, due to an unforeseen problem with the test stand cooling circuit at that time. In addition, it was possible to also demonstrate gyrotron operation at (iii) 0.5 MW in 1600 s pulses. The experiments will be continued at the FALCON test stand at the École Polytechnique Fédérale de Lausanne (EPFL).

Concept design of the megawatt power level gyrotron stabilized by a low-power signal for DEMO project

The specific features of the main components of the new powerful 230 GHz/80 kV/40 A gyrotron aimed to use in the future control fusion facility DEMO are described. The gyrotron design provides a stable output power generation of more than 1 MW using a superconducting magnet with a moderate size warm bore. Furthermore, the new original quasi-optical converter providing the gyrotron operation in three possible regimes—two free oscillation regimes with co-rotating TE33,13 or counter-rotating TE33,−13 mode, and the regime with frequency locking by the stable input signal—is suggested and optimized. The Gaussian content in the output wave-beam in all above-mentioned regimes is about 98%.

Frequency and mode measurement techniques for megawatt-class gyrotrons

Abstract State-of-the-art vacuum electron tubes such as gyrotrons, deliver RF output powers up to more than 2 MW at frequencies up to 170 GHz. In terms of the very high power levels, a proper verification of the gyrotron components itself and measurements during gyrotron operation are vital to prevent possible fatal errors. Several basic RF measurement setups, which are used at IHM/KIT, are discussed. Currently, their upper frequency limit is 175 GHz. In terms of future gyrotron operation above 200 GHz, upgrades of the measurement setups for operation up to 260–330 GHz are prepared. The experimental devices discussed herein are a quasi-optical mode generator for the verification of the quasi-optical gyrotron output system, the window measurement test stand to verify the ceramic gyrotron output window and the frequency diagnostic system to measure the operating frequency and thereby the excited mode.

Influence of Misalignment on the Behavior of Electron Beam of an 800 GHz Gyrotron

The operation of gyrotrons highly depends on the alignment between the magnet and the tube, which will cause instability problems or degrade the output efficiency, especially for high-frequency gyrotrons with a small cavity size. In this letter, the influence of misalignment, including parallel shift, arbitrary tilt, and their co-existence, on the behavior of the electron beam was studied. A prototype of an 800 GHz gyrotron with a sapphire window was developed to study the correlation between the misalignment and the beam trajectory. A method to determine the tilt angle and shift distance was proposed. The feasibility of the misalignment correction method was verified in the experiment.

A 250-Watts, 0.5-THz Continuous-Wave Second-Harmonic Gyrotron

To cover the so-called terahertz gap in powerful sources of coherent electromagnetic waves, a second-harmonic gyrotron operating in a 10 T liquid helium-free superconducting magnet has been designed, manufactured, and tested. With an operating voltage of 15 kV and a beam current of 0.6 A, the gyrotron generated coherent continuous-wave radiation at a frequency of 0.526 Terahertz with an output power of 250 Watts and an efficiency of 2.7%.

Gyrotron Development at High Order Modes

The present work has been characterized by higher order modes in the cavities of the Gyrotron; they are capable of producing RF plasma by developments of it. It uses for fusion systems. We choose the TE31,8 mode in our study. The main problem of gyrotron is the device of the thermal cavity loading. The problem of the thermal loading is solved when any parasitic modes suppress, absence of desired modes; the thermal loading is increased when the high power tube of gyrotron operation is unstable. The mathematical interaction model contains equations that describe the electron motion and the field profiles of the transferred electric modes of the resonator, these are interacting with electrons based on the finite difference method that has been designed to study the starting current, the frequency, quality factor and calculates the roots of the Bessel function by the program we designed in Fortran language. They are used to calculate the operation frequency. Good agreement is between our results and the previous published results both confirm the accuracy of the performance of the designed program.

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.

Experimental Demonstration of Gyrotron Frequency Stabilization by Resonant Reflection

This letter presents the results of an experimental and theoretical study of the gyrotron frequency stabilization by a partial reflection of its radiation from an external high-Q resonator. The experiment was carried out with a gyrotron operating in a continuous wave regime at a frequency of 28 GHz at the second cyclotron harmonic. The use of an external quasi-optical two-mirror resonator as a reflector made it possible to reduce the variation of the radiation frequency upon modulation of the guiding magnetic field by order of magnitude to 0.8 GHz/T. The experimental results are in good agreement with the calculations.

Fully redundant auxiliary system for gyrotron-based terahertz sources in long-term operation.

The Wuhan National High Magnetic Field Center is incorporating the flat-top pulsed magnetic field (FTPMF) into pulse gyrotrons. It will be the first chance to make a pulse-magnet gyrotron available for generating a long-pulse radiation of 100ms or above without affecting its high operating frequency and high radiation power. However, unlike continuous wave gyrotrons, pulse gyrotrons in long-term operation have their own challenges, namely, misalignment caused by concussions, much stronger low-frequency electromagnetic interference from the pulse magnet, and inevitable explosion. This article will focus on the difficulties faced by pulse gyrotrons in years of operation, discuss the protection and restoration from failures, and, consequently, propose a fully redundant, explosion-proof, and quickly recoverable auxiliary system for long-term operation of pulse gyrotrons. This system integrates the control unit of traditional pulsed magnets and superconducting magnets so that it can be compatible with any form of gyrotron facilities. Therefore, once the FTPMF or the superconducting magnet is available, the long-pulse radiation will be obtained. Several experimental results, including the most recent explosion, show the reliability of the proposed system.

A New Lumped Approach for the Simulation of the Magnetron Injection Gun for MegaWatt-Class EU Gyrotrons

In the framework of the ITER (International Thermonuclear Experimental Reactor) project, one of the key components of the reactor is the ECRH (Electron Cyclotron Resonance Heating). This system has the duty to heat the plasma inside the tokamak, using high frequency and power radio waves, produced by sets of 1MW gyrotrons. One of the main issues related to the gyrotron operation is the output power drop that happens right after the beginning of a pulse. In this work, we study the underlying phenomena that cause the power drop, focusing on the gyrotron’s MIG (Magnetron Injection Gun) of the 1MW, 170 GHz European Gyrotron prototype for ITER. It is shown how the current emission and the temperature of the emitter are tightly bound, and how their interaction causes the power drop, observed experimentally. Furthermore, a simple yet effective lumped-parameter model to describe the MIG’s cathode thermal dynamics is developed, which is able to predict the power output of the gyrotron by simulating the propagation of the heat inside this component. The model is validated against test results, showing a good capability to reproduce the measured behavior of the system, while still being open to further improvements.

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.