Korea Superconducting Tokamak Advanced Research Research Articles

Design and operation results of KSTAR ECH system

The electron cyclotron heating (ECH) system at the Korea Superconducting Tokamak Advanced Research (KSTAR) is a crucial heating devices, which is employed in various research endeavors, including plasma startup, wall cleaning, neoclassical tearing mode control, long pulse operation, and hybrid scenarios development from the first plasma operation. For facilitating advanced scenarios development and magnetohydrodynamics research in KSTAR, it was deemed necessary to install a total of 6 MW ECH systems, consisting of four 105/140 GHz and two 170 GHz systems. To address this, a new ECH room was developed and the existing ECH system was totally rearranged. This study describes the design of the novel KSTAR ECH system and its operation results, and discusses the problems that occur during the ECH system operation. Each ECH system to deliver 1 MW for 300 s was designed to enable stable, safe, and efficient operation in the new ECH room and four 105/140 GHz dual frequency ECH were injected to KSTAR successfully. The polarization angle was experimentally confirmed by using an electron cyclotron wall conditioning, and the beam angle change by ECH antenna was verified by a newly developed IR measurement. However, mixed patterns with higher-order modes were observed in the output beam from matching optics unit of some gyrotrons, which affect the operation efficiency and the output power. In future works, we will analyze the causes for low mode purity and improve the operation efficiency.

Re-evaluation of shielding ability and induced activity of KSTAR with increased neutron yields

To determine the viability of the 2-fold neutron yield increase of the Korea Superconducting Tokamak Advanced Research (KSTAR) owing to the installation of a second neutral beam injection system to increase the plasma density and a divertor material change from graphite to tungsten to reduce the heat load, we performed a radiological evaluation and shielding analysis of an existing building structure and the KSTAR with an increased neutron yield per shot. According to the shielding analysis results, the weekly and annual doses outside the building did not exceed the criteria, and the fluxes and doses in the plasma experimental room were unaffected by the divertor material change. The maximum dose rate in the working area in the tokamak was 2.16E+03 μSv/h at the surface-plasma-facing components. However, the dose rate was reduced to the background level one month after a 1-shot operation of 300 s. Ar-41 was the major nuclide for the air activation calculations; however, the effluent level exceeded the derived air concentration after the 1-shot operation ended. The residual activity concentration of Ar-41 was reduced below the derived air concentration after 243 min. The annual operating condition of the KSTAR is suggested based on the evaluation results.

Calibration of toroidal visible bremsstrahlung diagnostics and reconstruction of effective charge profiles in Korea Superconducting Tokamak Advanced Research (KSTAR).

The investigation of impurity behavior in fusion plasmas is a critical issue in fusion plasma research. The effective charge (Zeff) profile is a widely used measure of the impurity levels in fusion plasmas. In this study, the visible bremsstrahlung emissivity profile is reconstructed using toroidal visible bremsstrahlung (TVB) arrays at Korea Superconducting Tokamak Advanced Research (KSTAR). KSTAR TVB arrays have recently been developed and calibrated using a halogen light source and an integrating sphere. The reconstruction algorithm has been developed using the Phillips-Tikhonov method, and the reconstruction accuracy is assessed with test profiles. Electron density and temperature profiles from Thomson scattering diagnostics are fitted for Zeff calculations. Subsequently, the Zeff profiles in the edge localized mode suppression experiment are reconstructed. In addition, line-averaged Zeff values in the 2020 KSTAR campaign are presented, which are mostly distributed from two to four.

Long plasma duration operation analyses with an international multi-machine (tokamaks and stellarators) database

Combined high-fusion performance and long-pulse operation is one of the key integration challenges for fusion energy development in magnetic devices. Addressing these challenges requires an integrated vision of physics and engineering aspects with the purpose of simultaneously increasing time duration and fusion performance. Significant progress has been made in tokamaks and stellarators, including very recent achievement in duration and/or performance. This progress is reviewed by analyzing the experimental data (109 plasma pulses with a total of 3200 data points, i.e. on average 29 data per pulse) provided by ten tokamaks (in alphabetical order: Axially Symmetric Divertor Experiment Upgrade, DIII-D, Experimental Advanced Superconducting Tokamak, Joint European Torus, JT-60 Upgrade, Korea Superconducting Tokamak Advanced Research, tokamak à configuration variable, Tokamak Fusion Test Reactor, Tore Supra, W Environment in Steady-State Tokamak) and two stellarators (Large Helical Device and Wendelstein 7-X) expanding the pioneering work of Kikuchi (Kikuchi M. and Azumi M. 2015 Frontiers in Fusion Research II: Introduction to Modern Tokamak Physics (Springer)). Data have been gathered up to January 2022 and coordination has been provided by the recently created International Energy Agency-International Atomic Energy Agency international Coordination on International Challenges on Long duration OPeration group. By exploiting the multi-machine international database, recent progress in terms of injected energies (e.g. 1730 MJ in L-mode, 425 MJ in H-mode), durations (1056 s in L-mode, 101 s in H-mode), injected powers, and sustained performance will be reviewed. Progress has been made to sustain long-pulse operation in tokamaks and stellarators with superconducting coils, actively cooled components, and/or with metallic walls. The graph of the fusion triple products as a function of duration shows a dramatic reduction of, at least two orders of magnitude when increasing the plasma duration from less than 1 s to 100 s. Indeed, long-pulse operation is usually reached in dominant electron-heating modes at reduced density (current drive optimization) but with low ion temperatures ranging from 1 to 3 keV for discharges above 100 s. Difficulties in extending the duration may arise from coupling high heating powers over long durations and the evolving plasma-wall interaction towards an unstable operational domain. Possible causes limiting the duration and critical issues to be addressed prior to ITER operation and DEMO design are reported and analyzed.

Effect of coherent edge-localized mode on transition to high-performance hybrid scenarios in KSTAR

This paper deals with one of the origins and trigger mechanisms responsible for the observed performance enhancements in the hybrid scenario experiments conducted in Korea Superconducting Tokamak Advanced Research (KSTAR). The major contribution to the performance improvement comes from a broader and higher pedestal formation. The increase of fast ion pressure due to a plasma density decrease also contributes substantially to the global beta. Although the reduced core plasma volume resulting from the pedestal expansion has a negative effect on the core thermal energy, a considerable confinement improvement observed in the inner core region limits the degradation. The one significant characteristic of high-performance discharges is the presence of Coherent Edge-localized Mode (CEM) activity. CEM is triggered during the pedestal recovery phase between typical ELM crashes and has been found to be related to the increase of particle and heat transport. It appears to underlie two commonly observed phenomena in high-performance hybrid scenario discharges in KSTAR; pedestal broadening and continuous density decrease. Despite the associated transport increase, CEM activities can induce performance enhancement. With the pedestal broadening, ELM crashes become delayed and weakened, which, in turn, allows for a higher pedestal. Moreover, the density decrease directly increases fast ion pressure by extending the beam-slowing-down time. The linear gyrokinetic analysis reveals that the increase of fast ions could initiate positive feedback loops, leading to the stabilization of Ion Temperature Gradient mode in the inner core region.

RF design of helical long-wire traveling wave antenna for helicon current drive in KSTAR

This paper presents the development, fabrication, and implementation of a state-of-the-art helical long-wire traveling wave antenna (LW-TWA) for the Korea Superconducting Tokamak Advanced Research (KSTAR) helicon wave current drive (HCD). A multi-stage design approach has been employed incorporating schematic and simplified radio frequency (RF) modeling, to fulfill critical antenna performance criteria, such as parallel refractive index, while maintaining low electric field properties and load resilience. The advanced design includes a Faraday shield and a structurally rigid support for mechanical stability and minimal RF performance distortion. The sophisticated design and megawatt (MW)-class RF power handling capability of the LW-TWA hold the potential to improve HCD performance in KSTAR plasma.

Energetic passing particle-driven instabilities and their impact on discharge evolution in KSTAR

An experimental study is conducted on the onset and evolution characteristics of energetic particle-driven instabilities in Korea Superconducting Tokamak Advanced Research (KSTAR) with dominant tangential neutral beam injection (NBI). A scan of NBI beam energy shows the evanescence of the sawtooth crash and the concomitant onset of the strong passing particle-driven low-frequency fishbone instability. A quantitative analysis shows that the safety factor (q)-profile in the core region is clamped by a balance between the depletion of energetic passing particles by the fishbone instability and their external replenishment. Two synchronized chirping modes with distinct toroidal mode numbers (n = 1 and n = 5) supersede the fishbone instability after a self-organized q-profile is attained. An analysis shows that the n = 1 mode is likely to be a high-frequency beta-induced Alfvén eigenmode fishbone branch, while the n = 5 mode is an energetic particle mode (EPM). A dynamic system analysis of the synchronized EPM (S-EPM) shows that a stable S-EPM cycle can exist when the coupling between the two modes involved is insignificant. The potential impact of such EPMs on the establishment of a burning plasma scenario with a flat core q-profile is briefly discussed.

Mutual Induction Caused by IVCC Currents on Quench Detection Circuits of the KSTAR TF Coils

The Quench Detection System (QDS) for the Toroidal Field (TF) coils and the Poloidal Field (PF) coils made of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn or NbTi cable-in-conduit conductors is using Wheatstone bridges to compensate induced voltages, which are generated by electrical currents of the superconducting coils and a plasma, on the quench detection circuits of the Korea Superconducting Tokamak Advanced Research (KSTAR) device. On the other hand, the KSTAR device is equipped with In-Vessel Control Coils (IVCCs), which consist of 16 segmented windings made of normal conductors, for fast plasma position control, field error correction, and resistive wall mode stabilization. Electrical currents of the middle Field Error correction Coil (FEC) in a part of the IVCCs have strong mutual induction on the quench detection circuits of the TF coils due to the paths of the FEC's conductors, and the middle FEC currents, which had unusually long and very steep ramps, raised an emergency protection sequence of the TF coils in a KSTAR plasma experiment. Quench detection circuits only using traditional Wheatstone bridges are insufficient to cancel out such a mutual induction; whereas, the mutual induction can be numerically compensated in a theoretically straightforward calculation.

Characteristic test of dual shattered pellet injection systems of KSTAR

Dual shattered pellet injection (SPI) systems are installed in KSTAR (Korea Superconducting Tokamak Advanced Research) to study disruption mitigation in support of the ITER DMS (Disruption Mitigation System) design activities. KSTAR has two identical pellet injectors that can shoot pellets simultaneously from opposite toroidal locations. Each injector can shoot up to of three different diameter pellets. The pellet integrity and velocity after formation have been measured using microwave cavity (MWC) systems since 2019. However, this diagnostic provides only limited information on the pellet integrity, especially for partially broken pellets. Therefore, an optical pellet diagnostic (OPD) system monitoring the pellet has been installed in the first stage viewport of the differential pumping system and is operational since 2021. The KSTAR SPI systems offer a wide parameter range that must be characterized before the experimental campaign. To satisfy the experimental requirements, a proper procedure for pellet formation and acceleration must be commissioned. We carry out the commissioning by varying pellet parameters such as formation duration, temperature, heating, propellant gas pressure, and pulse length. The resulting pellet velocity, integrity, and tilt can be measured with the OPD system. These tests can be performed without plasma operation because no additional information is needed from the plasma response to the injection. This paper describes the results of the characteristic tests of single and multiple injections of dual SPI.

The evolution of the interlocks for KSTAR

Korea Superconducting Tokamak Advanced Research (KSTAR) has three interlock systems. The Supervisory Interlock System (SIS) protects against damage to machines from abnormal events. The Fast Interlock System (FIS) prevents the destruction of the internal vacuum components from the uncontrolled plasma and heating beams. The Personal Safety Interlock (PSI) keeps humans from harmful environments and conditions.This paper presents the evolution of the KSTAR SIS and FIS operations since 2009. The SIS was implemented for KSTAR in 2007 and optimized the critical conditions and actuator actions to improve and avoid issues. The significant changes in the KSTAR interlocks are removing and changing the inappropriate interlock conditions and adding the required conditions to improve efficiency and fit the advanced experimental environments.The first implemented FIS independent of the Central Control System (CCS) in 2009 continues to upgrade its hardware and software to enable the interlocks and actions with high time resolution until 2017. The machine protection logic in the FIS evaluated and developed to reduce operational failures.This paper analyses the modified conditions for interlocks, the actions to protect machines, and their effects.

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.

Evaluation of ITER Real-Time Framework in plasma diagnostics applications

The ITER Real-Time Framework (RTF) is a software suite designed and developed to facilitate the implementation of various real-time applications for ITER plant systems. The main driver to implement RTF was the Plasma Control System (PCS). RTF was designed as a base and development environment for PCS. However, due to its universal architecture, it can also be applied in other systems requiring real-time control or data processing.This paper presents the demonstration system to evaluate RTF in Thomson Scattering (TS) diagnostics. The system was developed to validate the integration of data acquisition hardware with RTF and check it in real-time data processing applications. The presented system covers the whole path of data acquisition, processing and archiving required for a typical Instrumentation and Control (I&C) system for ITER diagnostics.The main components of the RTF-based application developed for the system are device support for the pulsed giga-sample analog-to-digital converter, functional blocks implementing algorithms for analysing pulses from polychromator and calculating plasma electron temperature as well as interfaces for archiving raw data and publishing the measurement results.The overriding goal of the presented work was a detailed functional and performance evaluation of the implemented RTF-based system. The system was tested in laboratory using simulated data and in tokamak conditions at Korea Superconducting Tokamak Advanced Research (KSTAR). During the test campaign at KSTAR, the developed RTF-based application was integrated into the Thomson Scattering system and tested during regular operation of the tokamak with signals from a real polychromator. The results of measurements and performance evaluation are presented and discussed in the paper.

LabVIEW programming for the KSTAR XICS towards real-time ion temperature measurement

The X-ray Imaging Crystal Spectrometer (XICS) for Korea Superconducting Tokamak Advanced Research (KSTAR) provides the ion temperature and toroidal rotation velocity. These parameters are calculated through the Doppler broadening and its shift from the He-like argon spectra. A Matlab-based data analysis program for XICS has been used in KSTAR since 2009. This analysis program utilizes a least-square fit to provide a spatially resolved X-ray image so that it takes a long time for the image file processing procedures including the image correction. A new LabVIEW-based program is recently developed in order to reduce the data processing time with focusing only in the plasma core region where the image correction is not critical. This program integrates the detector control, transferring of the collected image to the analysis server, and image processing for the spectral analysis. It is also shown that a possible solution to acquire the measured image directly from the detector allows quasi-real-time measurements of the ion temperature and toroidal rotation velocity by using the LabVIEW-based program.

Fluctuation effects on the plasma density profiles measured by using a frequency modulated continuous wave reflectometer

Scanning diagnostics requires that for the correct measurement the target should not move during the scan. However, even if a frequency modulated continuous wave reflectometer is operated with a frequency modulation (FM) time of 10 µs, the high-temperature fusion plasma fluctuates even for such a short duration. The fluctuation effect on the reflectometer measurements is investigated by simulating the reflectometer response for various plasma conditions. Because the uncertainty of the zero density position might give an additional distortion to the reconstructed profile, this effect is also investigated. The simulation results show that the density profile reconstruction is not very sensitive to the zero density position but very sensitive to the fluctuations. The FM rate must be 100 times faster than the fluctuation frequency in order to suppress the radial distortion below about 20% of the fluctuation radial amplitude. An algorithm is proposed to reduce the distortion more effectively even at much lower FM rates. The distortion is canceled out by averaging two profiles, one of which is measured as increasing the frequency and the other as decreasing the frequency. The algorithm is applied to the experimental data analysis to confirm the simulation results. Through this study, it is concluded that the FM time of the Korea Superconducting Tokamak Advanced Research reflectometer should be reduced to 2 µs to be able to measure the H-mode plasma density profiles.

A sustained high-temperature fusion plasma regime facilitated by fast ions.

Nuclear fusion is one of the most attractive alternatives to carbon-dependent energy sources1. Harnessing energy from nuclear fusion in a large reactor scale, however, still presents many scientific challenges despite the many years of research and steady advances in magnetic confinement approaches. State-of-the-art magnetic fusion devices cannot yet achieve a sustainable fusion performance, which requires a high temperature above 100 million kelvin and sufficient control of instabilities to ensure steady-state operation on the order of tens of seconds2,3. Here we report experiments at the Korea Superconducting Tokamak Advanced Research4 device producing a plasma fusion regime that satisfies most of the above requirements: thanks to abundant fast ions stabilizing the core plasma turbulence, we generate plasmas at a temperature of 100 million kelvin lasting up to 20 seconds without plasma edge instabilities or impurity accumulation. A low plasma density combinedwith a moderate input powerfor operation is key to establishing this regime by preserving a high fraction of fast ions. This regime is rarely subject to disruption and can be sustained reliably even without a sophisticated control, and thus represents a promising path towards commercial fusion reactors.

Test and Analysis of Laboratory-Scale D-Shaped Co-Wound No-Insulation HTS Single Pancake Coil for TF Coil Application

No-insulation (NI) high temperature superconductor (HTS) toroidal field (TF) coil or its variations have been regarded as potential options for a compact, high magnetic field tokamak due to compactness in size and robustness in operational reliability. However, its real application still faces multiple challenges, e.g., limited experience in the design, construction, and operation of “D-shaped” NI HTS coils for toroid magnets. Here we report, as the first attempt in Korea, fabrication, test, and analysis of a reduced size D-shaped NI HTS single pancake coil to explore the feasibility of NI HTS technology in TF coil. The D-shaped NI HTS coil is designed to be 1/20 of the Korea Superconducting Tokamak Advanced Research (KSTAR) TF coil in height. One of the three test coils is wound with single HTS tape, and the other two test coils are co-wound with double and triple tapes, respectively. Current charging/discharging tests and sudden discharge tests were conducted for each coil, and the results are analyzed by the well-known lumped circuit model of a typical NI HTS coil. The results show possible variations in the contact resistance during charging and sudden discharge within a coil.

Investigation of multiple-ion-source neutral beam operation conditions compatible with motional Stark effect diagnostics.

The motional Stark effect (MSE) diagnostic system at KSTAR (Korea Superconducting Tokamak Advanced Research) often suffers from the drawback of possible systematic uncertainties in measurements due to overlap of the MSE spectra generated from three different ion sources that constitute a single neutral beam injection system. In particular, one ion source injected in the most tangential direction always causes strong spectral overlaps which, therefore, imposes regulations and constraints on the energy combination among the ion sources. A Stokes-vector analysis has been performed to produce operation windows for the energy combination between the ion source used in the MSE measurement and the ion source with the largest tangential injection angle. The analysis includes various practical factors, such as the distortion of the transmission function of bandpass filters and pitch angle profiles collected from a vast amount of KSTAR discharges. The two-dimensional parameter space, or the contour plot, on the expected systematic offsets in the measured pitch angle has been generated from this analysis, which can serve as a quantitative guideline for operating the multiple-ion-source neutral beam heating system.

Experimental results from x-ray imaging crystal spectrometer utilizing double crystal assembly in Korea Superconducting Tokamak Advanced Research (KSTAR)

A double crystal assembly (DCA) for multiple atomic spectra measurements, including helium-like Ar and hydrogen-like Ar, and other impurity lines are applied for the x-ray imaging crystal spectrometer in Korea Superconducting Tokamak Advanced Research. The DCA expands measurable wavelengths much wider so that cross comparisons of the ion temperature and toroidal rotation between two different atomic states are possible. The recent experimental comparison studies for the ion temperature and toroidal rotation of helium-like Ar and hydrogen-like Ar from the DCA are reported.

Characterization of the impurity features deposited on the boronization tungsten tiles exposed in KSTAR tokamak using laser-induced breakdown spectroscopy

• The boron and other impurity elements such as: carbon, silicon, calcium and copper deposition on the surface of tungsten divertor tiles removed from KSTAR has been measured by LIBS. • A subtraction method is proposed to eliminate the W line contribution for B deposition measurement. • The lateral-distribution of the boron and carbon on the three tungsten castellated tiles (Rounded, Chamfered and Double chamfered) were carried out at 10 −5 mbar vacuum condition. Boronization is an important wall conditioning procedure for realizing the steady-state high performance operation of tokamaks such as KSTAR and ITER. The measurement of boron and its distribution on the first wall and divertor has a significant impact on studying the plasma-wall interaction (PWI) such as impurity generation, transportation and deposition behaviors. In this study, the laser-induced breakdown spectroscopic (LIBS) diagnostic method was applied in the lab for investigating the boron impurity deposited on the surface of different shaped tungsten (W) castellated tiles which were removed from the divertor region in the Korea Superconducting Tokamak Advanced Research (KSTAR). Meanwhile, other various impurity elements comprising carbon, silicon, calcium and copper were also observed. A subtraction method is proposed to eliminate the W line contribution for B deposition measurement. The depth profile of the impurity elements in the deposited layer also has been studied by the variation of spectral intensity with consecutive laser pulses. The lateral-distribution of the boron and carbon on the three tungsten castellated tiles (Rounded, Chamfered and Double chamfered) were carried out at 10 −5 mbar vacuum condition. The distribution features of the impurities on the three different shaped tungsten tiles provide valuable information to understand PWI process in KSTAR. This study demonstrates that LIBS is a potential diagnostic approach for monitoring the impurity deposition in tokamak such as KSTAR or ITER.

W-Band Modular Antenna/Detector Array for the Electron Cyclotron Emission Imaging System in KSTAR

A design of a modular antenna/detector array for the electron cyclotron emission (ECE) imaging system at the Korea Superconducting Tokamak Advanced Research (KSTAR) is proposed. The modular antenna/detector array is based on a unit antenna/detector module, which consists of an elliptical mini-lens, a dual-dipole antenna, an antenna balun, a low-noise amplifier, and a metal frame. The proposed modular antenna/detector array resolves the problem in the conventional antenna/detector array where one faulty channel requires the entire array to be removed for the service. With the proposed modular array, each channel module can be easily and independently removed and replaced without interference to the rest of the array, thus minimizing the interrupted service time for maintenance. Moreover, the unit channel modules can be efficiently updated under a variety of the tokamak operation conditions. The antenna/detector modules are optimized to have improved performance, and are tested in a W-band test setup, and consistently provide the gain increase by 10~20 dB as compared with the conventional antenna/detector array. A set of the proposed modular antenna/detector array is currently installed and tested in the KSTAR ECE imaging system, and will consistently produce the improved ECE imaging to monitor MHD instability activities under various plasma operation conditions.