Electron Cyclotron System Research Articles

Flat-top plasma operational space of the STEP power plant

Abstract STEP is a spherical tokamak prototype power plant that is being designed to demonstrate net electric power. The design phase involves the exploitation of plasma models to optimise fusion performance subject to satisfying various physics and engineering constraints. A modelling workflow, including integrated core plasma modelling, MHD stability analysis, SOL and pedestal modelling, coil set and free boundary equilibrium solvers, and whole plant design, has been developed to specify the design parameters and to develop viable scenarios. The integrated core plasma model JETTO is used to develop individual flat-top operating points that satisfy imposed criteria for fusion power performance within operational constraints. Key plasma parameters such as normalised beta, Greenwald density fraction, auxiliary power and radiated power have been scanned to scope the operational space and to derive a collection of candidate non-inductive flat-top points. The assumed auxiliary heating and current drive is either from electron cyclotron systems only or a combination of electron cyclotron and electron Bernstein waves. At present stages of transport modelling, there is a large uncertainty in overall confinement for relevant parameter regimes. For each of the two auxiliary heating and current drive systems scenarios, two candidate flat-top points have been developed based on different confinement assumptions, totalling to four operating points. A lower confinement assumption generally suggests operating points in high-density, high auxiliary power regimes, whereas higher confinement would allow access to a broader parameter regime in density and power while maintaining target fusion power performance.

Overview of recent results from the ST40 compact high-field spherical tokamak

Abstract ST40 is a compact, high-field (BT0 ≤ 2.1T) spherical tokamak (ST) with a mission to expand the physics and technology basis for the ST route to commercial fusion. The ST40 research programme covers confinement and stability; solenoid-free start-up; high-performance operating scenarios; and plasma exhaust. In 2022, ST40 obtained a central deuterium ion temperatures of 9.6 ± 0.4 keV, demonstrating for the first time that pilot plant relevant ion temperatures can be reached in a compact, high-field ST. Analysis of these high-ion temperature plasmas is presented, including a summary of confinement, transport and microstability characteristics and energetic particle instabilities. Recent scenario development activities have focused on establishing diverted H-mode plasma discharges across a range of toroidal fields and plasma currents, along with scenarios with high non-inductive current fractions. In future operations, beginning in 2025, a 1MW dual frequency (104/137 GHz) electron cyclotron (EC) system will be installed to enable the study of EC and electron Bernstein wave plasma start-up and current drive. Predictive modelling of the potential performance of these systems is presented.

IMAS modelling of First Wall EC heat loads for the ITER rebaseline

This paper describes the modelling work carried out within the ITER Integrated Modelling and Analysis Suite (IMAS) [1] using the TORBEAM wave code [2] to estimate heat loads on the first wall (FW) from operating the Electron Cyclotron (EC) system in various scenarios of the ITER new baseline currently under preparation. These results provide guidance on how to best operate the EC system in terms of polarisation and operating launchers for the various phases of the Research Plan and of a pulse, to limit FW heat loads. Strategies to best protect the FW from EC power losses are also discussed.

Investigation of electron cyclotron wave absorption and current drive in CFETR hybrid scenario plasmas

The investigation of electron cyclotron (EC) wave absorption and current drive has been performed for the China Fusion Engineering Test Reactor (CFETR) hybrid scenarios using the TORAY code. To achieve the physics goal of the EC system in CFETR, a total of four wave frequency values and nine locations of launching antennas have been considered, and the injection poloidal and toroidal angles have been scanned systematically. The electron cyclotron current drive (ECCD) efficiency of the 170 GHz EC system is quite low due to the wave-particle interactions being located at the low-field side. To optimize the ECCD efficiency, the wave frequency is increased up to 221–250 GHz, which leads to the power being deposited at the high-field side. The off-axis ECCD efficiency can be significantly enhanced by launching EC waves from the top window and injecting them towards the high-field side. The optimized ECCD efficiency at ρ = 0.32 and at ρ = 0.4 is 2.9 and 2.2 times that of 170 GHz, respectively.

Development status of the HVPS and control system for KSTAR ECH system upgrade

The Korea Superconducting Tokamak Advanced Research (KSTAR) is planned to be installed with an electron cyclotron (EC) system to achieve a 6 MW radiofrequency (RF) output by 2024, and this installation is currently in progress. At present, five gyrotrons are installed for the EC system of the KSTAR. Among these, four EC systems are installed with 105/140 GHz dual-frequency gyrotrons, each generating 950 kW of RF power at 140 GHz and 800 kW of RF power at 105 GHz. Moreover, an EC system additionally installed in 2022 is a 170 GHz single-frequency gyrotron, and it generates 1 MW of RF power. The high-voltage power supply (HVPS) and the EC integrated control system (ECICS) were developed in 2019 to obtain a more stable and efficient EC system. Several useful functions were added to the newly developed ECICS for efficient EC system operation. Real-time control operations and shot retrigger are now possible with the application of this integrated control system. In addition, a function to automatically save and recall shot information was added and is essential for reducing operator errors. The new HVPS was designed in the pulse step modulation (PSM) topology, and a crowbar system was applied as a double protection device to protect the gyrotron. Instead of the heavy and bulky oil-cooling type high-voltage switch, a light and low-maintenance natural air-cooling type commercial switch was applied. The newly developed HVPS and ECICS were applied in the 2020 and 2021 KSTAR campaigns. Unfortunately, serious problems such as noise and timing issues were discovered during the two-year KSTAR campaign. Therefore, the second upgrade of the HVPS and ECICS was conducted in 2022 to realize a more stable and accurate EC system. The core part of the ECICS was developed anew, and the structure of the control system was significantly upgraded. In addition, the control method of the HVPS PSM module was newly developed. Moreover, the communication method between the ECICS and the controller of the HVPS was upgraded. In this paper, the upgraded HVPS and ECICS are explained in detail, and a discussion on the experimental results of the upgraded system for the next KSTAR campaign is also presented.

Plasma heating by electron cyclotron wave and the temperature effects on lower hybrid current drive on EAST

This paper presents the progress in the long—pulse operation of the electron cyclotron (EC) system and the achievements in high—electron temperature plasmas by the combined EC and lower hybrid (LH) waves heating since the EC system was built in 2015. An electron temperature of up to 12 keV with a duration over 100 s was realized by the simultaneous heating of EC and LH waves at the line-averaged density ∼ 1.8 × 1019 m−3. The plasma heating effect strongly depends on the location of EC power deposition. H-mode plasmas with solely EC wave auxiliary heating have been obtained on EAST for the first time. These H-mode discharges show an enhanced confinement factor H 98(y,2) around 1.0, which is higher than the previous H-mode using LH power alone (Xu et al 2011 Nucl. Fusion 51 072001). The total heating power is very close to the threshold value for L–H transition according to the international tokamak scaling. In addition to the temperature effects inside the separatrix, higher electron temperature produced by EC wave is found to reduce the LH power loss in the scrape-off layer due to collisional absorption, which is beneficial to further increase the LH current drive efficiency. Ray-tracing/Fokker–Planck modeling results indicate that higher electron temperature can shorten the LH wave propagation on EAST in a multiple-pass regime, thus decreasing the collisional dissipation.

Quantifying electron cyclotron power deposition broadening in DIII-D and the potential consequences for the ITER EC system

The injection of electron cyclotron (EC) waves fulfills a number of important tasks in nuclear fusion devices for which detailed knowledge of the spatial power deposition profile is critical. This deposition profile is commonly determined using forward models such as beam or ray tracing. Recent numerical and experimental studies have shown that small-angle scattering of the EC beam as it passes through the turbulent plasma edge can cause significant broadening of the effective deposition profile, leading to considerable underestimation of the deposition width by forward methods. However, traditional inverse methods to determine the deposition profile from measurements overestimate the deposition profile width due to transport broadening. In this work, we implement three novel methods to resolve the EC power deposition profile from measurements that counteract transport broadening by simultaneously resolving transport and power deposition. We validate their assumptions and compare the results from these methods to the traditional break-in-slope method as well as to the TORAY ray-tracing code in a set of DIII-D discharges spanning five different confinement modes. We show that the four different inverse methods, novel and established, paint a consistent picture of deposition broadening. Specifically, we show that the measured power deposition profile is between 1.6 and 3.6 times wider than the TORAY profiles. Moreover, we show the considerable consequences that this level of broadening can have for ITER.

ECH and ECCD modelling studies for DTT

In this work the Electron Cyclotron (EC) physics performances of the EC system foreseen for the new Divertor Tokamak Test facility (DTT) are investigated using the beam tracing code GRAY on the flat top phase of the most recent DTT full power scenario. The whole core plasma region can be reached by EC beams with complete absorption, assuring bulk heating and core current drive (CD) for profile tailoring, and NTM mitigation in correspondence of the rational surfaces. A detailed analysis regarding modifications of the EC propagation, absorption and CD location due to density fluctuations caused by pellet injection is performed. The compatibility between the EC system and the pellet injection system is verified: the density variations due to pellet injection are foreseen to negligibly influence the EC performances, allowing the EC beams to reach the plasma central region for bulk heating and to drive current on the rational surfaces for NTM mitigation. Finally, the polarization variations originated by the angle steering foreseen for the operational and physics tasks accomplishment during the flat top phase of the discharge are assessed. Negligible power losses have been found keeping fixed polarization during the needed steering.

Plasma heating and improvement of lower hybrid current drive efficiency by electron cyclotron waves on EAST

The electron cyclotron (EC) system on EAST consists of four gyrotrons with a frequency of 140 GHz (second harmonic of the extraordinary mode), each of which is expected to deliver a maximum power of 1.0 MW and be operated at 100-1000 s pulse length. Significant progress in long-pulse operation has been achieved recently, including the pulse duration up to 1056 s with EC power injected into plasma of 0.55 MW and the pulse duration of 310 s with EC power of 1.6 MW (output by 3 gyrotrons). High electron temperature (Te >12 keV) plasma measured by Thomson scattering was produced with the combination of EC and lower hybrid (LH) waves. It is found that the plasma heating effect depends on the EC power location greatly. By adjusting the EC power location, the plasma current profile can be modified. As a consequence of the increment of electron temperature by electron cyclotron resonance heating (ECRH), the lower hybrid current drive (LHCD) efficiency is improved, benefiting for the long-pulse operation. In addition, a synergy effect between EC and LH current drive was observed in steady-state operation on EAST.

A new 3MW ECRH system at 105 GHz for WEST

The aim of the WEST experiments is to master long plasma pulses (1000s) and expose ITER-like tungsten wall to deposited heat fluxes up to 10 MW/m2. To increase the margin to reach the H-Mode and to control W-impurities in the plasma, the installation of an upgraded ECRH heating system, with a gyrotron performance of 1MW/1000s per unit, is planned in 2023. With the modifications of Tore Supra to WEST, simulations at a magnetic field B0∼3.7T and a central density ne0∼6 × 1019 m−3 show that the optimal frequency for central absorption is 105 GHz. For this purpose, a 105 GHz/1MW gyrotron (TH1511) has been designed at KIT in 2021, based on the technological design of the 140 GHz/1.5 MW (TH1507U) gyrotron for W7-X. Currently, three units are under fabrication at THALES. In the first phase of the project, some of the previous Tore Supra Electron Cyclotron (EC) system components will be re-installed and re-used whenever possible. This paper describes the studies performed to adapt the new ECRH system to 105 GHz and the status of the modifications necessary to re-start the system with a challenging schedule.

The Double-Disk Diamond Window as Backup Broadband Window Solution for the DEMO Electron Cyclotron System

The second variant of the electron cyclotron heating and current drive system in DEMO considers the deployment of 2 MW power Gaussian microwave beams to the plasma by frequency steering. Broadband optical grade chemical vapor deposition diamond windows are thus required. The Brewster-angle window represents the primary choice. However, in the case of showstoppers, the double-disk window is the backup solution. This window concept was used at ASDEX Upgrade for injection of up to 1 MW at four frequencies between 105 and 140 GHz. This paper shows computational fluid dynamics conjugated heat transfer and structural analyses of such a circumferentially water-cooled window design aiming to check whether it might be used for DEMO microwave beam scenarios. This design was then characterized with respect to different parameters. Temperature and thermal stress results showed that it is a feasible window solution for DEMO, but safety margins against limits shall be increased by introducing design features able to make the fluid more turbulent. A first design change is proposed, showing that, in combination with a higher inlet flow rate, the maximum temperature in the disks can be reduced from 238 to 186 °C, leading, therefore, to lower thermal gradients and stresses in the window.

Preconceptual Design of the Port Cell Section for the EU DEMO Equatorial EC System

The EU demonstration power plant (DEMO) Tokamak will be equipped with an electron cyclotron (EC) system for plasma heating and magnetohydrodynamic (MHD) control. Up to six launchers will be installed into equatorial ports with the aim to inject maximum 130-MW millimeter-wave (mm-wave) power at dedicated positions into the plasma. The mm-waves will be generated in gyrotrons placed in a distinct building at distant location. From this gyrotron hall, a combined transmission line (TL) system of quasi-optical multibeam mirrors and individual waveguides (WGs) will propagate the mm-waves into the tokamak building. That followed, an optical system, composed of miter bends, diamond windows, valves, and microwave bellows, connects the TL through the gallery and the port cell with the EC launchers. This article presents the preconceptual computer-aided design (CAD) of this latter section of the EC system. Based on its general scheme and the given WG trajectories, the model takes into account the available space in the port cell with respect to required clearance for supply systems, assembly, and maintenance. At the preconceptual state, the design includes CAD models of all relevant mm-wave components in realistic dimensions at a level of details, appropriate to demonstrate the feasibility of the concept.

Development and status of high-voltage power supply and integrated control system for KSTAR ECH system

Korea Superconducting Tokamak Advanced Research (KSTAR) is planning to install an auxiliary heating device comprising six electron cyclotron (EC) systems with a total of 6 MW of radiofrequency (RF) output. Two EC systems were operated in the 2019 KSTAR campaign. To compensate the shortcomings identified during the operation of the previous two systems, the power supply was upgraded and a new integrated control system was developed. A power supply is a device that affects the RF beam quality generated by the gyrotron of an EC system and ensures efficient operation. Features such as real-time operation, a crowbar system, and a redundancy module were added to the upgraded power supply. An integrated control system is an important device that protects a gyrotron and supports stable operation. The overall structure of the integrated control system was changed to enable real-time operation, and functions such as retrigger and shot-information were added. In the 2020 KSTAR campaign, a new EC system including the upgraded power supply and the new integrated control system was added, and a total of three EC systems were operated. The new EC system was successfully employed at KSTAR. In this paper, we explain in detail the upgraded power supply and the new integrated control system and show and discuss the results of their application to the 2020 KSTAR campaign.

Implications of parasitic absorption of electron cyclotron waves on ITER operation around half-field

The ITER Research Plan envision operation around half of the nominal magnetic field (i.e. around B = 2.65 T) as a path to baseline operation. This work discusses constraints on the optimal range of magnetic field, which is bounded in the lower limit by the presence of the third-harmonic electron cyclotron resonance at half field, and on the upper limit by the loss of core heating and current drive. It will be shown that increasing the magnetic field by only 3%, i.e. to 2.75 T, eliminates the third harmonic parasitic absorption without compromising demonstration of access to H-mode, while operating at a magnetic field of 3.0 T—previously proposed for optimal use of the ion cyclotron system—would impair the use of the electron cyclotron system for core-heating and current drive. Operation at 2.65 T would still be possible if the polarization of the equatorial launcher is changed from X-mode to O-mode in the current flattop phase.

Large Area Diamond Disk Growth Experiments and Thermomechanical Investigations for the Broadband Brewster Window in DEMO

An electron cyclotron system (ECS) is required in the European DEMO fusion reactor for plasma heating, magnetohydrodynamic control, especially the suppression of neoclassical tearing modes (NTMs), and thermal instability control. Microwave beams of 2 MW, generated by gyrotrons, shall propagate through corrugated HE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> waveguides (WGs) of 63.5-mm inner diameter and finally reach specific locations in the plasma by a dedicated set of mirrors. The deployment of the beam may be done either by angular steering, i.e., tilting moveable mirrors, or by frequency steering, i.e., tuning the gyrotron frequency. In the last option, for long pulse operations, broadband optical grade diamond windows are required as confinement and/or vacuum barriers both at the torus and the gyrotron side. The Brewster-angle window represents a promising solution due to its intrinsic simplicity. However, the WG aperture imposes a minimum Ø180 mm for the disk at the Brewster-angle of 67.2° for diamond. This is an ambitious target, considering 140 mm as the state of the art. First, this article shows the very promising results of diamond growth experiments carried out by Diamond Materials GmbH with the first worldwide free-standing optical grade Ø180-mm disk having an average unpolished thickness of 1.3 mm. For the first time, one side of this large disk was polished and loss tangent measurements were performed at the center and over a large area. Then, the performance of the window featuring these large disks was characterized by a consistent set of thermomechanical analyses in view of the forthcoming window prototyping activity.

Analysis and optimal design of LCC resonant converter for anode power supply in ECRH

The high-voltage anode power supply for the electron cyclotron system is proposed based on the LCC resonance circuit. The working process of the LCC resonant converter in the capacitive region is studied, and an analytical model of the capacitive LCC circuit is proposed. The analysis points out the boundary between the capacitive area and the inductive area of the converter. Aiming at the voltage drop between on-load and no-load in the open-loop of the LCC circuit, the characteristics of the capacitive LCC resonant circuit are analyzed to optimize the resonance parameters. Experimental results show that the proposed analytical model has high accuracy, and the voltage drop in the open-loop is 11.7%, which verifies the correctness of the optimized design.

Integration concept of an Electron Cyclotron System in DEMO

The pre-conceptual layout for an electron cyclotron system (ECS) in DEMO is described. The present DEMO ECS considers only equatorial ports for both plasma heating and neoclassical tearing mode (NTM) control. This differs from ITER, where four launchers in upper oblique ports are dedicated to NTM control and one equatorial EC port for heating and current drive (H&CD) purposes as basic configuration. Rather than upper oblique ports, DEMO has upper vertical ports to allow the vertical removal of the large breeding blanket segments. While ITER is using front steering antennas for NTM control, in DEMO the antennas are recessed behind the breeding blanket and called mid-steering antennas, referred to the radially recessed position to the breeding blanket.In the DEMO pre-conceptual design phase two variants are studied to integrate the ECS in equatorial ports. The first option integrates waveguide bundles at four vertical levels inside EC port plugs with antennas with fixed and movable mid-steering mirrors that are powered by gyrotrons, operating at minimum two different multiples of the fundamental resonance frequency of the microwave output window. Alternatively, the second option integrates fixed antenna launchers connected to frequency step-tunable gyrotrons. The first variant is described in this paper, introducing the design and functional requirements, presenting the equatorial port allocation, the port plug design including its maintenance concept, the basic port cell layout, the transmission line system with diamond windows from the tokamak up to the RF building and the gyrotron sources.The ECS design studies are supported by neutronic and tokamak integration studies, quasi-optical and plasma physics studies, which will be summarized. Physics and technological gaps will be discussed and an outlook to future work will be given.

Design and Development of Fast Protection and Fast Monitoring Module Compatible With NI Compact RIO System for ITER-India Gyrotron Test Facility

The ITER electron cyclotron (EC) system will have gyrotron sources used for plasma heating and current drive applications. During gyrotron operation, different faults may be encountered and in the case of critical faults, the High Voltage Power Supplies (HVPS) should be switched off in $ time scale, which otherwise could damage the internal components of a gyrotron tube. Also during the operation, the main operating parameters need to be acquired on fast time scales (~5 Msps) to know the gyrotron output characteristics. A solution is required catering to both the requirements, which should be cost-effective, and also comply with recommended hardware and software framework. In this context, a custom prototype module compatible with NI-CompactRIO (NI-cRIO) system is under development for ITER-India Gyrotron Test Facility (IIGTF). The module has protection and monitoring functions both in parallel in a single unit. It has a transmitter (TX) and receiver (RX) card connected optically to complete the link. The TX has fast sampling analog-to-digital converter (ADC), fast comparator, field-programmable gate array (FPGA), and an optical transmitter. The TX sends monitoring and trip information optically to the RX. The RX card has an optical receiver, FPGA, and digital-to-analog converter (DAC), which generates back the original signals. The heart of the module is FPGA logic, which performs encoding and decoding (as per 8b10b standard) of ADC serial data, thereby eliminating the use of any serial converter ICs. The module can be plugged into the cRIO chassis and also can be used as a standalone module. This article discusses the concept, design, development, and test results of fast protection and fast signal monitoring module

EU DEMO EC equatorial launcher pre-conceptual performance studies

The preliminary conceptual design for the Electron Cyclotron (EC) system of the future European DEMOnstration fusion power plant is ongoing in the EUROfusion Consortium. This represents one of the key aspects in order to assess the performances and the integration capabilities of such a system in EU DEMO as well as in the alternative reactor configuration Flexi-DEMO. Different options for the antenna, namely remote steering antenna (RSA), open ended waveguides (OEWG) and the front steering antenna (FSA) later on renamed in mid steering antenna (MSA) to notify that the MSA is protected behind the breeding blanket (BB) in DEMO, are investigated, analyzing their performance for several injection angles and launch points. This activity considers the constraints given by physics and engineering requirements, as for example the maximum power per port and the necessary local current drive to stabilize neo-classical tearing modes (NTMs) with a proper deposition width. The beam tracing calculations have been performed on different scenarios, providing information on plasma accessibility and deposited power. The microwave design and initial ideas about the ex-vessel EC transmission lines routing will be shown.

Initial port integration concept for EC and NB systems in EU DEMO tokamak

The integration of the heating and current drive (HCD) systems in the EU DEMO tokamak must address a number of issues, namely space constraints in the tokamak building, remote handling requirements, breeding blanket penetration, neutron and photon radiation shielding, compliance of penetrations of the primary vacuum with safety and vacuum criteria, and a large number of loading conditions, in particular heat, electromagnetic (EM), and pressure loads in normal and off-normal conditions. A number of pre-conceptual design options for the vacuum vessel (VV) port and the port-plug are under assessment and need to be verified against all requirements and related criteria. The identification of the functional (or physics) requirements of the HCD systems remains an on-going process during the pre-conceptual design phase, hence some initial assumptions had to be made as a basis for development of the design of the VV ports and the HCD port plugs.The paper will provide an overview of present margins in the functional/physics requirements and the rationale behind the assumptions made in order to facilitate development of the pre-conceptual design options. Furthermore it will introduce the initial design concepts of the electron cyclotron (EC) Launchers and the neutral beam (NB) injectors integrated in equatorial ports. The NB duct design in DEMO and related issues such as transmission and re-ionization losses will be also addressed.