Electron Cyclotron Resonance Heating System 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.

Thermo-mechanical and quasi-optical analysis of the ECRH equatorial launcher steerable mirror towards the CFETR

This study reports a prototype design of the front steerable mirror (SM) in the Electron Cyclotron Resonance Heating (ECRH) system, which is used to validate the manufacturing technology of the large-scale quasi-optical mirror in the China Fusion Engineering Test Reactor (CFETR). As a plasma-oriented component, the SM will experience more neutron heat and plasma radiation heat than the remote focusing mirrors, and the simulation results indicate that the peak temperature of the SM would then reach 292.5 °C, the maximum deformation and the maximum equivalent stress would be 1.16 mm and 446 MPa, respectively. The beam exhibits little distortion because the thermal deformation gradient around the wave center is small, and the quasi-optical analysis shows that the position of the wave heating plasma should be shifted by -6 mm along the local Z-axis, while the amplitude of the central electrical field would increase by 0.3%, from 294.7 kV/m to 295.5 kV/m.

Steady-State Operation Control of Gyrotrons in Support of Long Pulse Plasma Operation in EAST

In order to realize the 1000 s long pulse operation of the ECRH (Electron Cyclotron Resonance Heating) system on EAST (Experimental Advanced Superconducting Tokamak), we developed an ECRH steady-state operation control system. Four working modes were designed, namely manual restart mode, automatic restart mode, timed alternate operation mode, and protection trigger alternate operation mode. The FPGA-based NI cRIO was used as the control device, and the LabVIEW was used to write the program. The system has a quick response and accurate timing sequence, and can realize the long-pulse steady-state operation of the gyrotrons in EAST ECRH system. The steady-state operation control system has been verified in nearly two rounds of EAST experiments, helping EAST achieve the 1056 s long pulse discharge.

Development of the electron cyclotron resonance heating system for Divertor Tokamak Test

The Divertor Tokamak Test (DTT) facility, whose construction has started in Frascati (Italy), will be equipped with an ECRH (electron cyclotron resonance heating) system including 32 gyrotrons as microwave power sources. The procurement of the first batch of sources with 16 MW total power, based on 170 GHz/≥ 1 MW/100 s vacuum tubes, is in progress and will be available for the first DTT plasma. The system is organized into four clusters of 8 gyrotrons each. The power is transmitted from the Gyrotron Hall to the Torus Hall Building (THB) by a quasioptical transmission line (TL), mainly composed of large mirrors shared by eight beams coming from eight different gyrotrons and designed for up to 1.5 MW power per single beam, similar to the TL installed at the stellarator W7-X. One of novelties introduced in the DTT system is that the mirrors of the TLs are embodied in a vacuum enclosure, using large metal seals, mainly to avoid air absorption and risk of arcs. The main reason is to reduce the risk of air breakdown, maintaining a pressure of 10−5 mbar far away from the Paschen minimum. The TL estimated volume is between ∼70 and ∼85 m3. The direct connection of the TL to the tokamak vacuum vessel has been evaluated, and different solutions have been proposed in order to prevent a possible impact on DTT operations. The microwave power is injected into the tokamak using independent single-beam front-steering launchers, real-time controlled and located in the equatorial and upper ports of four DTT sectors. In-vessel piezoelectric walking drives are the most promising candidates for the launcher mirror movement considering their compactness and capability to operate in an environment with strong magnetic field under ultra-high vacuum. The DTT ECRH system design, presented here, is based mainly on existing and assessed solutions, although the challenging adaptations to the DTT case are considered.

Progress on the conceptual design of the antennas for the DTT ECRH system

One of the main goals of the Divertor Tokamak Test (DTT) facility is to reach a ratio of power crossing the separatrix over the major radius of about 15 MW m − 1, as the one expected in DEMO. For this purpose, up to 45 MW of external additional heating power shall be coupled to the plasma, provided by Electron Cyclotron Resonance Heating (ECRH), Ion Cyclotron Resonance Heating and Neutral Beam Injection. The foreseen total ECRH power installed at full power shall be 32 MW, generated using 1 MW/170 GHz gyrotrons, for 100 s. The present ECRH system foresees two launching antennas per DTT sector, based on the front-steering concept. The equatorial antenna is dedicated to plasma heating and current drive and the upper antenna to the stabilization of MHD instabilities. This paper focuses on the latest design concept for these two antennas and on the definition of the ex-vessel matching optics unit of the last section of the evacuated Transmission Line (TL). The design has been updated to be compatible with the insertion of CVD diamond windows, to separate the vacuum environment of DTT from the one of the TL. This choice requires adding corrugated waveguide sections between the last mirror of the TL and the first mirror inside the port, requiring some adaptation of the in-vessel optics and of the supporting structure. The possibility to modify the steering range for the launching mirror has been also investigated to be compatible with the new design of the first wall and for the upper antenna, to reach the q = 2 surface in the new plasma scenario.

Conceptual design of the DTT ECRH quasi-optical transmission line

The Divertor Tokamak Test (DTT) facility is being realised in Frascati, Italy for the study of the power exhaust issues in view of DEMO tokamak. A multi-MW Electron Cyclotron Resonance Heating (ECRH) system is foreseen with a first set of 16 gyrotrons at 1 MW and 170 GHz available for the first stage of operation; additional 16 gyrotrons with power from 1 MW to 1.2 MW are foreseen in a later stage for a total of up to 33.6 MW of ECRH at plasma. ECRH system itself is part of the 45 MW of external additional heating coupled to the plasma, provided also by Ion Cyclotron Resonance Heating and Neutral Beam Injection. The Transmission Line (TL) is fully quasi-optical, between 84 m and 138 m long and will transfer power up to 1.5 MW per beam from the building hosting the ECRH sources to the Torus Hall. The main section runs in a straight elevated corridor at a few meters above the ground level. In order to have low power losses and a simpler and manageable system, the long TL run is made by four quasi-optical Multi-Beam (MB) lines each transmitting 8 beams via shared mirrors, similar to the W7-X Stellarator TL. The system is thus organised in “clusters” each one made of the 8 gyrotron sources and the respective transmission line and launchers. Avoidance of losses in air and microwave leaks is assured by a vacuum enclosure of the whole line with the use of metallic gaskets. Single-Beam (SB) transmission lines are foreseen at the beginning and end of the MB line, to cover the distance from the gyrotron to a beam-combining mirror and from a splitting mirror to the launcher. The optical design has to cope with space constraints in the building and inherent conversion losses at the reflection on metallic mirrors, whose number was minimised. First evaluations with the electromagnetic modeling tool GRASP and first concepts for the MB mirror cooling are reported.

Development of the control system for electron cyclotron resonance heating on J-TEXT tokamak

A 105 GHz/500 kW/1 s electron cyclotron resonance heating (ECRH) system has been developed on J-TEXT tokamak since 2017. The core component of the ECRH system is a gyrotron manufactured by Gyrotron Complexes Ltd. (GYCOM Ltd.), which generates microwaves of a certain frequency and power. To guarantee safe and stable operation, it is necessary to design a specialized control system. The control system is expected to perform time sequence trigger, protection, signal monitoring, communication, and data acquisition. The hardware is built with real-time processors and data acquisition modules from National Instruments. The control program is realized by LabVIEW. Test results indicate that the control system can commit stable and safe operation of the gyrotron, which guarantees the integrated commissioning tests of the whole ECRH system and ECRH related physics experiments. Under the operation of this control system, the gyrotron can generate microwaves as expected, and the ECRH system is well protected when a fault takes place.

Electron Cyclotron stray radiation detector studies for JT-60SA

Most of the present and future magnetic fusion devices rely on high power mm-waves injection for a variety of applications like plasma breakdown, heating and control. Since under specific operating conditions the mm-waves might not be fully absorbed, potentially damaging in-vessel components, the detection of the non-absorbed radiation is recommended. Among the options for stray radiation detection, we consider here the possibility to exploit the solid body bolometer and its use for JT-60SA. Analysis of the residual power fraction expected for low absorption plasma conditions is presented, considering the dependence on the Electron Cyclotron Resonance Heating (ECRH) system and plasma parameters of JT-60SA. The residual power hitting the wall is evaluated looking at direct illumination and cross-polarization effects. The possibility to integrate a sensor for EC stray radiation, adapting the solution of the detector developed for ITER, is the main outcome of this study. In the ITER bolometer the differential thermocouple principle is considered to single out the nuclear heating and the microwave loads. A key component of the detector is the metal body with a ceramic coating layer, optimized for 170 GHz and 60 GHz. Mm-waves characterization of different coating samples has been performed to identify the optimal solution for JT-60SA considering its multi-frequency ECRH system (138 GHz, 110 GHz, 82 GHz), and will be here presented. The experience from JT-60SA equipped with these sensors will be also useful for their future use in ITER.

Study of magnetic effects on DTT ECRH front-steering mirror

In the final configuration of the Divertor Tokamak Test (DTT) facility, 32 front-steering launchers of the Electron Cyclotron Resonance Heating (ECRH) system will be distributed in 4 sectors of the machine. Two different antennas are hosted in the equatorial and in the upper port of each ECRH sector, with 6 and 2 single launchers respectively. This setup will deliver up to 35.2 MW installed power to the plasma, making such ECRH system the most powerful ever implemented at the time of its completion. To comply with compactness and performance requirements, a fully in-vessel driving system has been proposed for the steering mirrors. The system relies on UHV-compliant piezoelectric walking drives to provide biaxial steering capability. The main drawback of piezoelectric drives is their low driving force, in an environment where magnetic torques can be very high because of variable magnetic fields, as those induced by the non-axisymmetric in-vessel coils for plasma control, and especially in case of disruption events. Therefore, the materials of the water-cooled steering mirrors must be chosen in such a way as to minimize magnetic torques while guaranteeing adequate heat conduction. Numerical and analytical models of the magnetic torques acting on the steering mirror have been developed. The models have been applied to the steering mirror to quantify magnetic torques due to various sources and dynamic regimes. The currently proposed Copper–Chromium–Zirconium mirror was proven to be critical. Therefore, compliance requirements for the mirror have been computed, and different solutions have been numerically evaluated for magnetic torque mitigation and heat conduction capability, including combination of different materials and implementation of a tungsten-coated dielectric mirror.

Experiments with reduced single pass absorption at ASDEX Upgrade – instrumentation and applications

Reflecting gratings have been installed in the vacuum vessel of ASDEX Upgrade for all beamlines of the electron cyclotron resonance heating system. Potentially unabsorbed millimetre wave power after the first pass through the plasma is redirected towards the plasma centre. This increases the efficiency of heating schemes with reduced single pass absorption like O-2 or X-3. In order to monitor beam position and power, thermocouples were installed into the gratings. A numerical model was developed to evaluate the beam intensity during short pulses from the thermocouple measurement in a non-stationary environment. An experiment was carried out, where only the X-3 resonance is present in the plasma, and the millimetre wave beam shine-through was measured successfully as a function of the central plasma electron temperature. This allows to deduce the X-3 absorption experimentally. Scanning the launching angles, it seems possible to measure the 2D beam cross section after the first pass through the plasma.

The development of 1 MW ECRH system on J-TEXT

A 1 MW electron cyclotron resonance heating system has been designed and developed for J-TEXT tokamak not only to enhance the total heating power but also to provide another possible way to control the plasma parameters. The system is composed of two gyrotrons, both with an output power of 500 kW electron cyclotron wave power. The development progress of the system is elaborated and discussed. The commissioning tests for the gyrotrons were implemented, and each gyrotron realized its standard output successfully. With the assistance of this system, a variety of experiments have been carried out on J-TEXT, mainly related to plasma heating, plasma disruption, plasma start-up, current drive and so on.

MISTRAL campaign in support of W7-X long pulse operation

Following two initial campaigns [1], Stellarator Wendelstein 7-X (W7-X) has now completed the construction phase by installation of active cooling of all plasma facing components. The machine is presently commissioned for the next campaign (OP2) aiming at 1 GJ per pulse, e.g. 100 s at 10 MW, eventually aiming at 18 GJ, e.g. 1800 s at 10 MW. The key heating system is the Electron Cyclotron Resonance Heating (ECRH) system, consisting of 10 gyrotrons with power per gyrotron ranging from 0.6 MW up to 1.0 MW at 140 GHz. A phased upgrade of the installation is in progress with the addition of 2 gyrotrons and the development of 1.5 MW and 2.0 MW gyrotrons, such that at the end of the upgrade 4 gyrotrons will be available in each power class of 1.0, 1.5 and 2.0 MW [2]. The increased ECRH power, combined with O2 and X3 heating schemes at high densities, will lead to increased microwave stray radiation. This is non-absorbed microwave power that diffuses inside the vessel and is incident on all in-vessel components including vacuum windows and attached diagnostic systems. A fraction of the stray radiation is absorbed by resistive or dielectric losses of these components, leading to thermal loads that scale with stray radiation levels and pulse length. At W7-X a high power microwave stray radiation launch facility ’MISTRAL’ is available that is used to qualify invessel components for use at specified microwave surface power densities [Wm−2]. This paper reports on MISTRAL campaigns in 2020 2021 for testing of stray radiation loads during OP2 in W7-X, as well as on an EUROfusion program assessing stray radiation loads on ITER components. A dedicated, absolutely calibrated, caloric load was developed for the campaign to obtain measurement of stray radiation power levels as well as to conveniently expose samples. Amongst other we report on shielding concepts using metal enclosures combined with microwave absorbing coatings and dielectric heating of vacuum windows.

Recent progress of the ECRH system and initial experimental results on the J-TEXT tokamak

In order to broaden the range of the plasma parameters and provide experimental conditions for physical research into high-performance plasma, the development of the electron cyclotron resonance heating (ECRH) system for the J-TEXT tokamak was initiated in 2017. For the first stage, the ECRH system operated successfully with one 105 GHz/500 kW/1 s gyrotron in 2019. More than 400 kW electron cyclotron (EC) wave power has been injected into the plasma successfully, raising the core electron temperature to 1.5 keV. In 2022, another 105 GHz/500 kW/1 s gyrotron completed commissioning tests which signifies that the ECRH system could generate an EC wave power of 1 MW in total. Under the support of the ECRH system, various physical experiments have been carried out on J-TEXT. The electron thermal transport in ECRH plasmas has been investigated. When ECRH is turned on, the electron thermal diffusivity significantly increases. The runaway current is elevated when a disruption occurs during ECRH heating. When the injected EC wave power is 400 kW, the conversion efficiency of runaway current increases from 35% to 75%. Fast electron behavior is observed in electron cyclotron current drive (ECCD) plasma by the fast electron bremsstrahlung diagnostic (FEB). The increase in the FEB intensity implies that ECCD could generate fast electrons. A successful startup with a 200 kW ECW is achieved. With the upgrade of the ECRH system, the J-TEXT operational range could be expanded and further relevant research could be conducted.

Inhibition of parametric decay in heating microwave beams during fluctuations of the density profile in the edge island of Wendelstein 7-X

Experimental evidence of parametric decay instability (PDI) is observed in the Wendelstein 7-X stellarator, when high-power microwave beams cross a stationary magnetic island at the plasma edge. Here, trapping and build-up of upper hybrid waves within a density bump (measured within the island by alkali beam emission spectroscopy) is responsible for the reduction of the instability power threshold below the maximum gyrotron power. In this paper, we provide the first experimental evidence of the connection between the trapping mechanism in the island density bump and excitation of PDI-related signals. We show correlations of periodic crashes in the PDI-related signals with quasi-continuous fluctuations at the plasma edge, which, additionally, cause a flattening of the density profile in the island. We demonstrate that flattening of the experimental density profiles can suppress the trapping mechanism and inhibit the low-threshold PDI. PDI on the edge island could alter the power deposition profile and reduce the efficiency of the electron cyclotron resonance heating system, simultaneously posing a serious threat to the optimal operation of microwave-based diagnostics and plasma-facing components.

Recent EAST Experimental Results and Systems Upgrade in Support of Long-Pulse Steady-State Plasma Operation

In recent years, EAST has achieved a 60-s scale steady-state plasma operation (SSO), which is electron dominant heated by the electron cyclotron wave and low hybrid wave. Substantial progresses have been made in the development of long-pulse operation, such as active control of radiative divertor, edge localized mode (ELM) suppression, and He plasma experiment. To further develop the steady-state operation scenario and study the related critical physical issues, the upgrade of EAST has been completed in 2020. The auxiliary heating systems were rearranged and upgraded with a total heating power capability of 33 MW. The 2.45-GHz lower hybrid current drive (LHCD) launcher has been updated from full active multijunction (FAM) to passive active multijunction (PAM) to avoid damage caused by plasma–material interaction. The injection power capability of electron cyclotron resonance heating (ECRH) system is upgraded to 1.4 MW. The upgraded ion cyclotron resonance frequency (ICRF) antenna injects low parallel wavenumber ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k_{\vert \vert }$ </tex-math></inline-formula> ) spectra with good single pass absorption. The neutral beam injection (NBI) system is moved from F-port to D-port and is reoriented from counterclockwise to clockwise injection. The armor material of lower divertor has been upgraded from graphite to tungsten improving the steady-state heat handling capability from 2 to 10 MW/m2. Based on the upgraded pumping system and improved conductance, the effective pumping speed for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D_{2}$ </tex-math></inline-formula> of the EAST lower divertor increased twice. With all these upgrades, 100-s scale steady-state operation ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{e0} &gt;10$ </tex-math></inline-formula> keV) and 1056-s high-performance operation with 1.73-GJ injection energy were achieved in 2021 campaigns. The experience in high-performance long-pulse operation obtained on EAST will lay a foundation for the ITER and Chinese Fusion Engineering Testing Reactor (CFETR) operation.

Small-Signal Analysis and Controller Design of Anode Power Supply for ECRH

The anode power supply is very important to the stability of the output wave power of the electron cyclotron resonance heating (ECRH) system. In order to improve the stability and anti-interference ability of the anode power supply, the small-signal analysis and controller design of the anode power supply are carried out in this article. First, the anode power supply is simplified and equivalent. Then, the LCC resonant circuit and the linear amplification circuit are established, respectively, for detailed analysis and research, and the small-signal models of these two parts are established. The transfer function of the whole system is obtained, the stability and rapidity of the anode power supply are analyzed, and the controller parameters are designed. Finally, the simulation model and prototype are built. The experimental results show that the system has better dynamic response ability and anti-interference ability after adding the controller.

Study of a 105 GHz short-pulse dummy load for the electron cyclotron resonance heating system with the quasi-optical method

The accurate power measurement is important for an ECRH system in tokamak. The dummy load is designed and developed for the measurement of the millimeter wave power. This work analyzes the dummy load based on the quasi-optical method and the ray tracing method. The reflectivity and thermal deposition of the dummy load have been considered to ensure the safety of the entire system. High-power tests have been carried out at a 105 GHz/500 kW ECRH system. The results of the tests indicate that the designed dummy load is stable and valid.

Design status of the ECRH system for CFETR

The China Fusion Engineering Test Reactor (CFETR) is the proposed next-generation fusion facility in China. During its two operational phases, a large fraction of externally driven current is foreseen to meet the scientific goals of the two operating scenarios for CFETR. As one of the key design activities of auxiliary heating mix for CFETR, the design of Electron Cyclotron Resonance Heating (ECRH) system is in progress considering the engineering and physical requirements. The conceptual design of the EC system, especially the launcher, will be presented in this paper. Based on the physical requirements and the technical availability, the frequency is chosen to be 170 GHz in order to draw on the technologies development with International Thermonuclear Experimental Reactor (ITER), and the Front Steering Antenna (FSA) is selected as an example to investigate the feasibility of the plant integration design. The beam tracing calculations have been performed to analyze the performance of power deposition and absorption from several launching points. The microwave and optical design of launcher has been carried out to evaluate the maximum power per port and minimize Breeding Blanket (BB) openings.

Study of non-convergence problem of the integral method for polarizer design on the ECRH system

On the electron cyclotron resonance heating system, the integral method represents a class of classical mathematical methods for polarizer design. However, the non-convergence problem caused by the Fredholm integral equation of the integral method usually leads to an incorrect result. This sometimes may confuse the beginners. The simplified kernel function based on the infinitesimal approximation is presented in this paper, which effectively solves the non-convergence problem, making it easier to use and simplifying the code of calculation for beginners. A code is developed based on this method, then the calculation results are compared with the calculation results of the coordinate transformation method, and the experimental results of the low power test platform. The comparison results and low power test results indicate that this method can be used for polarizer design.

Development of fast steerable launcher for ECRH system

Electron cyclotron resonance heating (ECRH) launcher is an electro-mechanical system mainly consisting of mirrors (flat and profiled), a steering mechanism, a vacuum barrier window and a UHV (Ultra High Vacuum) compatible gate valve which focuses and steers the ECRH beam into plasma. Presently, Steady-state Superconducting Tokamak (SST)-1 ECRH system has a 42 GHz Gyrotron capable of delivering 500 kW of RF power for the maximum duration of 500 ms. A new steerable ECRH launcher has been developed which can steer the EC beam along both poloidal and toroidal directions. The span of mirror movement in poloidal and toroidal directions is ±20° and ±50°, respectively. The poloidal motion of the mirror can be done in real-time during plasma operation and the mirror can be rotated by 20° in 100 ms. Two AC servo motors independently control the mirror movements through an assembly of motion feedthroughs, spur gears and mechanical linkages. The accuracy, repeatability and speed of both the motions have been measured under UHV in a specially designed vacuum chamber.