Electron Cyclotron Resonance Heating System Research Articles

Preliminary design and thermal analyses of the steerable mirror cooling channel of the DTT ECRH

Abstract Divertor Tokamak Test (DTT) facility, the Italian fusion machine, foresees the Electron Cyclotron Resonance Heating (ECRH) system to provide additional heating power to plasma ohmic-heating. Such system will be composed of an initial set of 8 mm-wave 1-MW sources at 170 GHz, which will be raised to 16 in a second stage of installation. The power of each source will be launched into the plasma by a pair of mirrors in the port plug, while a series of mirrors is used for the transmission of microwave beams. They are subjected to microwave heating load and the mirrors closest to the plasma also to radiative and neutronic loads. In order to avoid a performance loss of the beam transmission, due to the deformations, caused by the increment of temperature on the mirrors, it is necessary to design a cooling circuit inside them. This paper aims to make a preliminary design, using FEA tools, of the cooling circuit in the steerable mirror (M2) of the ECRH Launcher. A thermal analysis was performed to study how geometrical and thermal choices can influence the dissipation of the heat and the maximum temperatures of the structure. First of all the best geometric configuration was individuated: a cooling circuit dug in the mirror with a shape of an elongated spiral with constant pitch on y-axis and the inlet not centred in the origin. Starting from this geometry, parametric analyses were carried out to establish the correct ranges of applicability and study the influence of the inlet temperature and the volume flow rate of the refrigerant fluid on the temperature of the mirror. The simulations confirmed the theoretical expected thermal trends, but also that the preliminary tested values of the input parameters could be used to realize an efficient cooling for the structure. The analyses are set up within ANSYS environment, on the basis of the provided data and constraints.

Operation and shutdown dose rate analysis of CFETR ECRH system

Electron Cyclotron Resonance Heating (ECRH) is one of auxiliary heating systems to be installed in CFETR. For heating purpose of ECRH, a penetration space has been designed to transmit the wave to plasma. This will significantly increase the neutrons streaming through the wave channel and result in heavy irradiation of ECRH port structures. For nuclear safety analysis, the irradiation dose during operation of CFETR should be limited according to the regulation of radiological zone. In case of failure, the ECRH is planned to be maintained by workers after shutdown and waiting time of one month until the dose rate decreases to low level of 100 μSv/h. The shielding capabilities of ECRH port has been evaluated in view of dose effect both during operation and after shutdown of CFETR. For that purpose, a 3-D neutronics model has been built to launch the nuclear analysis. The operation dose rate was directly transferred from the neutron and photon flux by using of ICRP-74 flux to dose conversion factors. The shutdown dose rate was obtained with NASCA code system by coupling the Monte Carlo particle transport and FISPACT activation calculation. The obtained operation and shutdown dose data can be provided for the shielding optimization of ECRH wave system.

A Compact Two-Frequency Notch Filter for Millimeter Wave Plasma Diagnostics

Sensitive millimeter wave diagnostics in magnetic confinement plasma fusion experiments need protection from gyrotron stray radiation in the plasma vessel. Modern electron cyclotron resonance heating (ECRH) systems take advantage of multi-frequency gyrotrons. This means that the frequency band of some millimeter wave diagnostics contains more than one narrow-band gyrotron-frequency line, which needs to be effectively suppressed. A compact standard waveguide notch filter based on coupled waveguide resonators with rectangular cross-section is presented which can provide very high suppression of several gyrotron frequencies and has low insertion loss of the passband.

Study of Dual-Frequency Polarizer for Electron Cyclotron Resonance Heating Systems of 105 and 140 GHz

In electron cyclotron resonance heating (ECRH) systems, the desired polarization is achieved by using two polarizers, a linear polarizer and an elliptical polarizer. These two polarizers serve as an important part of a 63.5-mm corrugated metallic waveguide transmission line for the ECRH. The dual-frequency polarizers (DPs) for the 105- and 140-GHz systems were designed in the present article by using the optimization of the phase difference method. The linear polarizer of 105-GHz shared an optimized polarizer with an elliptical polarizer of 140 GHz. The elliptical polarizer of 105-GHz shared another optimized polarizer with a linear polarizer of 140 GHz. The polarization characteristics of the single-frequency and DPs at the frequencies of 105 and 140 GHz were analyzed and compared. The polarization purity of the combination of the two polarizers was also calculated. The results indicated that arbitrary polarization state could be successfully achieved with the use of the designed DPs.

Development of the body power supply for ECRH on EAST

Electron cyclotron resonance heating (ECRH) plays a major role of plasma heating and it is also an important way of inhibiting plasma instability. The high frequency modulation operation of ECRH system is proposed, which is helpful for the suppression of non-classical tearing modes (NTMs). A 30kV/100mA high voltage body power supply (BPS) with 5 kHz modulation ability for 1MW/1000s Gyrotron has been developed. The BPS adopts linear power supply technology, switching power supply technology and pulse step modulation (PSM) technology. A modulation from 0 up to 100 % is possible, and the rising time is less than 20us. The output voltage ripple factor of the BPS is less than 0.5%. The test results show that the BPS can meet the requirements of ECRH system, and it has strong generality, which can provide reference for the design of other body power supply.

Design and testing of the 40 kV/20A solid-state switch based on SiC MOSFETs for ECRH on J-TEXT Tokamak

Cathode power supply provides high voltage and large energy for electron gyrotron on J-TEXT tokamak. When faults (e.g. internal arc fault) happen on electron gyrotron, the power supply should interrupt within few tens of microseconds. Although the cathode power supply could switch off quickly by itself, frequent failures could decrease the reliability of the power supply without additional protection measures. For the safety of electron cyclotron resonance heating (ECRH) system, solid-state switch is necessary in ECRH protection. A design of a 40 kV/20A solid-state switch was proposed in this paper. To improve the switching performance of the switch, the power devices connecting in series was silicon carbide (SiC) metal-oxide-semiconductor field-effect transistor (MOSFET), which replaced traditional solution such as silicon (Si) insulated gate bipolar transistor (IGBT). For the purpose of evaluating and comparing the characteristic of SiC MOSFET and Si IGBT, a dual-pulse test was completed and SiC MOSFET showed the significant advantages. Furthermore, a voltage-clamped snubber circuit was adopted to ensure the voltage balance for each SiC MOSFETs during static and dynamic process. Then, the analysis and validation of the circuit was provided. Finally, the solid-state switch was built and tested by experiment. The results show that the switch works adequately, the switching-off time is 4.7us with 38 kV voltage and 20A current capacity.

A High-Voltage Solid-State Switch Based on Submodule Topology of SiC MOSFETs for J-TEXT Tokamak

Due to safety and reliability concerns, high-voltage solid-state switch is required for electron cyclotron resonance heating system on J-TEXT Tokamak. It cuts off the power supply within a little time usually in microseconds, when a fault occurs. At present, wide-bandgap semiconductors based on silicon carbide (SiC) show superior material properties enabling potential power device operation at higher voltage, current, and switching speeds than current silicon (Si) semiconductors. SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) are considered to be a better solution for the basic component of the switch than Si insulated gate bipolar transistors (IGBTs). To solve the problem of voltage balancing in series-connected MOSFETs, a submodule with four, 1.7 kV/40 A rated MOSFETs in series has been developed and experimentally tested. A voltage-clamped snubber circuit is designed to control the overvoltage of each MOSFET. The turn-off performance of the proposed circuit and traditional circuit has been compared by simulation, and the submodule has also been evaluated in a dual-pulse test. Finally, the solid-state switch with 16 submodules in series has been tested by a dummy load. The experimental results show that switch can operate successfully, and the switching performance satisfies the anticipated demand.

Exploring fusion-reactor physics with high-power electron cyclotron resonance heating on ASDEX Upgrade

The electron cyclotron resonance heating (ECRH) system of the ASDEX Upgrade tokomak has been upgraded over the last 15 years from a 2 MW, 2 s, 140 GHz system to an 8 MW, 10 s, dual frequency system (105/140 GHz). The power exceeds the L/H power threshold by at least a factor of two, even for high densities, and roughly equals the installed ion cyclotron range of frequencies power. The power of both wave heating systems together (>10 MW in the plasma) is about half of the available neutral beam injection (NBI) power, allowing significant variations of torque input, of the shape of the heating profile and of Qe/Qi, even at high heating power. For applications at a low magnetic field an X3-heating scheme is routinely in use. Such a scenario is now also forseen for ITER to study the first H-modes at one third of the full field. This versatile system allows one to address important issues fundamental to a fusion reactor: H-mode operation with dominant electron heating, accessing low collisionalities in full metal devices (also related to suppression of edge localized modes with resonant magnetic perturbations), influence of Te/Ti and rotational shear on transport, and dependence of impurity accumulation on heating profiles. Experiments on all these subjects have been carried out over the last few years and will be presented in this contribution. The adjustable localized current drive capability of ECRH allows dedicated variations of the shape of the q-profile and the study of their influence on non-inductive tokamak operation (so far at q95 > 5.3). The ultimate goal of these experiments is to use the experimental findings to refine theoretical models such that they allow a reliable design of operational schemes for reactor size devices. In this respect, recent studies comparing a quasi-linear approach (TGLF) with fully non-linear modeling (GENE) of non-inductive high-beta plasmas will be reported.

Experimental Study on Collimation Characteristics of an ECRH System with a 28GHz/50kW Gyrotron

In this paper, the collimation characteristics of an electron cyclotron resonance heating (ECRH) system with a 28GHz/50kW gyrotron in EXL-50 are studied experimentally. Through using laser trackers and designing matching tooling, test errors of horizontal distances and vertical distances between the transmission line and the flange center are all within 1mm, which are less than 0.4% and 0.07% respectively. In addition, online beam spot test with a 150 ms short pulse from a 28 GHz gyrotron also proves good collimation characteristics of the ECRH system. Finally, spherical plasma is formed through injecting the microwave to the vacuum chamber.

Collective Thomson scattering with 77, 154, and 300 GHz sources in LHD

Collective Thomson scattering (CTS) is one of attractive diagnostics for measuring locally and directly the fuel temperature and the velocity distribution of fast ions in fusion plasmas. A mega-watt class source of millimeter or sub-millimeter waves is required to detect a weak scattered radiation superimposed on background radiation owing to electron cyclotron emissions (ECEs) from plasmas. Based on electron cyclotron resonance heating (ECRH) system with the frequencies of 77 GHz and 154 GHz in the Large Helical Device (LHD), the CTS diagnostic system has been developed to measure bulk ion temperatures from a few keV to ∼10 keV and fast ions originated from 180 keV-neutral beam injection in the LHD. The measured CTS spectra and their time evolutions are analyzed with the electrostatic scattering theory. The bulk ion temperatures obtained from CTS spectra increase with the neutral beam injections and decrease with the heating terminated. The velocity map of simulated fast ions explains that the bumps on tail of measured CTS spectra are caused by the co- and counter- fast ions. A new prescription for anisotropic velocity distribution function is proposed. As for 154 GHz bands, the CTS spectrum broadenings for D and H plasmas are distinguished reasonably at the same temperature, and its ion temperatures are comparable to those of the charge exchange recombination spectroscopy. As reactor-relevant diagnostics, a 300 GHz gyrotron and a corresponding receiver system have been implemented in LHD to access high density plasmas with low background ECEs. The recent progress for CTS diagnostics and their spectrum analysis with the probe frequencies of 77 GHz, 154 GHz, and 300 GHz in the LHD experiments is described.

Design of a Launcher for the 105-GHz ECRH System on J-TEXT

To enhance parameters of the plasma and improve the magnetic confinement performance, a 105-GHz electron cyclotron resonance heating (ECRH) system is being built on the J-TEXT. We designed a quasi-optical launcher which mainly consists of a focusing mirror and a steering mirror. The focusing mirror is optimized by considering the localized deposition, and the steering mirror is close to the plasma boundary for flexible adjustment of the toroidal and poloidal injection angles of the wave. The toroidal and poloidal rotation angles of the steering mirror are independently controlled by the mechanical structure. For our design, the beam radius at the plasma center for perpendicular incidence is about 20 mm; the adjustable range of the toroidal and poloidal injection angles is ±14° and ±31°, respectively. These can meet the requirements for physics experiments on J-TEXT.

Equivalent Model of Transient Gyrotron Cathode Response

The gyrotron is the high power millimeter wave source in the electron cyclotron resonance heating system for the tokamak. The cathode power supply is one of the most important ancillary devices for gyrotron. Some interesting transient phenomena about the cathode voltage and the cathode current was found in the gyrotron operation in the electron cyclotron resonance heating system for the Experimental Advanced Superconducting Tokamak. The cathode voltage drops to about 10% of the original value by about 90 {\mu}s in the overcurrent case, which is much longer than the 25 {\mu}s in the normal case. In order to explain these phenomena, an equivalent circuit model of the magnetron injection gun was proposed. The equivalent circuit is composed of parallel resistors and capacitors, and it can explain the test results very well. Using the equivalent circuit model to analyze gyrotrons may become an effective means of developing and operating an electron cyclotron heating system.

Design and development of the W‐band corrugated waveguide mode converter for fusion plasma experiments

In this paper, a W-band mode converter is designed using a circular corrugated horn and its detailed numerical study is presented. The proposed mode converter was fabricated, and the measured results are validated. The device converts fundamental mode (TE11) into a Hybrid mode (HE11) mode in a frequency range of 75 GHz to 105 GHz. The reflection coefficient is less than −15 dB and a peak directivity of 24.5 dB is achieved. The corrugated horn also shows good mode purity. The measured gain of the device was found to be 18.05 dB. The designed mode converter will be useful in Electron Cyclotron Resonance Heating system for fusion plasma experiments.

Design of high-power wideband polarizer for electron cyclotron resonance heating systems on CFETR

The desired polarization for electron cyclotron resonance heating (ECRH) systems is generated by a pair of polarizers integrated into the transmission lines.This paper presents the design of a high-power wideband polarizer for 170-GHz and 230-GHz ECRH systems on China fusion engineering test reactor (CFETR). A low-loss, high-stability, and high-efficiency polarizer is required for multifrequency, high-power, and long-pulse ECRH systems to realize the arbitrary polarization state at different operating frequencies. Hence, two sinusoidal-grooved wideband polarizers with shallow grooves and identical parameters are designed herein. The polarization characteristics of wideband polarizer are presented for wideband polarizers at the frequency of 170-GHz and 230-GHz, in particular for the case using two identical polarizers consecutively. The results indicate that the two consecutive identical polarizers can achieve the arbitrary polarization state at frequencies of 170-GHz and 230-GHz.

Modelling one-third field operation in the ITER pre-fusion power operation phase

In the four-stage approach of the new ITER Research Plan, the first pre-fusion power operation (PFPO) phase will only have limited power available from external heating and current drive (H&CD) systems: 20–30 MW provided by the electron cyclotron resonance heating (ECRH) system. Accessing the H-mode confinement regime at such low auxiliary power requires operating at low magnetic field, plasma current and density, i.e. 1.8 T and 5 MA for a density between 40% and 50% of the Greenwald density. H-mode plasmas at 5 MA/1.8 T will also be investigated in the second PFPO phase when ITER will have its full complement of H&CD capabilities installed, i.e. 20–30 MW of ECRH, 20 MW of ion cyclotron resonance heating and 33 MW of neutral beam injection. This paper describes the operational constraints and the H&CD capabilities for such scenarios in hydrogen and helium plasmas, to assess their viability and the issues it will be possible to address with them. The modelling results show that 5 MA/1.8 T scenarios are viable and will allow the exploration of the H-mode physics and control issues foreseen in the ITER Research Programme in the PFPO phases.

Design of the Voltage Measurement and Feedback Regulation System of 100-kV High-Voltage Power Supply on J-TEXT Tokamak

A 100-kV/60-A high-voltage power supply (HVPS) based on pulse step modulation (PSM) technology has been established on J-TEXT Tokamak to feed the electron cyclotron resonance heating (ECRH) system for plasma heating. To fulfill the waveform monitoring, feedback regulation, and protection requirements, a high speed and precision voltage measurement and transmission system with high voltage and ground isolation is designed and applicated to the HVPS. A feedback regulation method is proposed to not only suppress the output voltage overshoot during startup of the HVPS operation but also ensure the precision of the output voltage on flat stage under the change of load conditions. Experiment results on the J-TEXT HVPS show that the voltage measurement system behaves well both in precision (measurement error $ ). The feedback regulation method can reduce the overshoot voltage to less than 1%.

Electromagnetic Compatibility in Electron Cyclotron Resonance Heating System

The electron cyclotron resonance heating (ECRH) is a very important plasma heating method in experimental advanced superconducting tokamak (EAST). There are four gyrotrons in the EAST ECRH system that can generate 4-MW/100-s/0.14-THz wave. The ECRH system is a complex terahertz system that involves ultralow temperature, ultrahigh vacuum, strong magnetic field, high-power electromagnetic wave, and weak diagnostic signal. There are many electromagnetic compatibility problems in the ECRH system. This is the first report about the electromagnetic compatibility for the ECRH system. In the operation of the ECRH system, many electromagnetic compatibility problems have been found and resolved. We can suppress the interference and improve the accuracy of power monitoring and the effectiveness of radio frequency protection by using the magnetic rings. The influence of the magnetic field on the current transformers was analyzed. Some electromagnetic compatibility issues in the data acquisition system were discussed. After solving the electromagnetic compatibility problems, the EAST ECRH system can stably output high-power terahertz wave.

The anode power supply based on the pulse step modulation technology for the ECRH system on J-TEXT tokamak

A 105 GHz/500 kW electron cyclotron resonance heating system on J-TEXT tokamak is under development using a diode type GYCOM gyrotron with a single-stage depressed collector. To modulate the output power of the gyrotron, we designed a 33 kV/1 A anode power supply based on the pulse step modulation technology. The power supply consists of 40 modules with a fixed output voltage of 800 V and 10 modules with a fixed output voltage of 100 V, and these modules are connected in series. In this way, the output voltage of the anode power supply can be adjusted accurately without pulse width modulation. Results of the bench test with dummy load indicates that the output capability and the protection time in fault condition of the designed power supply can meet requirements of the ECRH system on J-TEXT.

Microsecond Overshoot Characteristics of PSM High-Voltage Power Supply

The gyrotron is a critical component of electron cyclotron resonance heating systems and is highly sensitive to the rising edge of the cathode’s high-voltage power supply (HVPS). HVPS is based on pulse-step modulation technology, which requires a rapidly rising edge and low-voltage overshoot. An equivalent mathematical model is thus established to analyze the transient characteristics of HVPS. An optimized control strategy is obtained from detailed theoretical calculations verified via simulations. A time interval of $1 ~\mu \text{s}$ is used to ensure that the voltage overshoot is controlled to within 1% over the rated output voltage range. Finally, the theoretical analysis is verified through experimental tests.

Commissioning of Electron Cyclotron Resonance Heating (ECRH) system on tokamak Aditya-U

The Aditya tokamak has been upgraded to Aditya-U by changing it’s vacuum vessel from rectangular to circular to accommodate the diverter coil for shaped plasma. The 42 GHz ECRH (Electron Cyclotron Resonance Heating) system has been integrated with the tokamak. The system is capable to deliver 500 kW power for 500 ms duration. The ECRH system with modified layout has been integrated successfully on Aditya-U; the burn pattern close to tokamak port ensures good Gaussian beam. Earlier the Aditya Tokamak used to operate at lower magnetic field (Max. 1.0 T) and the ECRH assisted low-loop voltage (˜6 V) plasma start-up experiments were carried out at second harmonic. Now the upgraded Aditya is operated at higher magnetic field up to 1.5 T. Initial ECRH experiments have been carried out at fundamental harmonic at magnetic field from 1.2 T to 1.35 T. Approximately 150 kW to 250 kW power in fundamental O-mode is launched from low field side. The ECH pulse of ˜40 ms is launched at the flat-top of plasma current. The effect of ECR heating is seen as some increase in soft X-ray signal. In some small plasma shots, the EC power is launched after the shot and EC breakdown effect has been observed.