Korea Superconducting Tokamak Advanced Research Plasma Research Articles

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.

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.

Systematic evaluation of the effect of multi-ion-source neutral beam injection on motional Stark effect diagnostic in KSTAR

A newly developed systematic approach to evaluate the effect of multi-ion-source neutral beam injection on the motional Stark effect (MSE) diagnostic in Korea Superconducting Tokamak Advanced Research (KSTAR) is introduced. Based on a set of model equations combined with measured MSE data, this approach attempts to decompose the spectral overlap mixed by the neutral beam injected from multiple ion sources. An optimization algorithm minimizes the error between the measured Stokes parameters and those reconstructed by the separated spectra and thereby, evaluates a time-varying systematic offset caused by multi-ion-source beam injection in MSE measurements, regardless of the energy combination among the ion sources. From its application to a group of KSTAR plasma discharges, this methodology, attempted for the first time in the MSE community, has demonstrated its capability to provide researchers with a guideline on the neutral beam operation along with high-quality MSE measurements.

A study on the feasibility of fast neutron imaging using the D–D fusion neutrons of the KSTAR tokamak

Abstract In the 2015 Annual Plasma Campaign of the Korea Superconducting Tokamak Advanced Research (KSTAR), which mainly used neutral beam injection (NBI) as auxiliary heating, the yield by the Deuterium–Deuterium (D–D) fusion neutrons of 2.45 MeV energy from the KSTAR plasma was estimated to have reached about 1014 n/s. Apart from the primary goal, which is energy production by nuclear fusion, these fusion neutrons may be used in other applications such as aerospace, defense, material, and electronic component research and testing, as well as battery and fuel cell research in the fields of archeology and security. This is because the tokamak is huge and intense as a volumetric neutron source. This work aims to investigate the feasibility of fast neutron imaging using the D–D fusion neutrons emitted from the KSTAR tokamak. For this work, we adopted a fast neutron imaging technique based on a CCD camera in combination with a fiber–optical scintillator (FOS). The first experimental results about the feasibility of fast neutron imaging obtained from the KSTAR Tokamak will be described in this paper.

Effective ion charge (Zeff) measurements and impurity behavior in KSTAR.

A visible bremsstrahlung detector array diagnostic system has been developed on the Korea Superconducting Tokamak Advanced Research (KSTAR) to view the whole minor radius in a narrow region of the continuum free of spectral lines. The interference filters coupled with photomultiplier tubes have been employed to determine the effective charge Zeff by using visible bremsstrahlung data during neutral beam injection in the KSTAR plasma. The Zeff profiles are typically flat for L-mode plasmas and evolve to hollow profiles during the H mode in the KSTAR. A comparison of the visible bremsstrahlung emission based on the calculated Zeff profiles is consistent with measured values of Zeff from a visible spectrometer in the core plasma. The electron temperature is measured by X-ray imaging crystal spectrometry, and electron density needed for the analysis is taken by the assumption of parabolic profiles of these parameters. The line of sight averaged local bremsstrahlung emissivity is determined with low uncertainty, and the radial emissivity is obtained by using the Abel inversion technique. In addition, a dependence of effective charge Zeff on the line-averaged electron density is evaluated, and Zeff is also determined to observe the effect of boronization.

Investigation of instabilities and rotation alteration in high beta KSTAR plasmas

H-mode plasma operation of the Korea Superconducting Tokamak Advanced Research (KSTAR) device has been expanded to significantly surpass the ideal MHD no-wall beta limit. Plasmas with high normalized beta, βN, up to 4.3 have been achieved with reduced plasma internal inductance, li, to near 0.7, exceeding the computed n = 1 ideal no-wall limit by a factor of 1.6. Pulse lengths at maximum βN were extended to longer pulses by new, more rapid control. The stability of the observed m/n = 2/1 tearing mode that limited the achieved high βN is computed by the M3D-C1 code, and the effect of sheared toroidal rotation to tearing stability is examined. As a method to affect the mode stability in high βN plasmas, the non-resonant alteration of the rotation profile by non-axisymmetric magnetic fields has been used, enabling a study of the underlying neoclassical toroidal viscosity (NTV) physics and stability dependence on rotation. Non-axisymmetric field spectra were applied using in-vessel control coils (IVCCs) with varied n = 2 field configurations to alter the plasma toroidal rotation profile in high beta H-mode plasmas and to analyze their effects on the rotation. The rotation profile was significantly altered with rotation reduced by more than 60% without tearing activity or mode locking. To investigate the physical characteristics and scaling of the measured rotation braking by NTV, changes in the rotation profile are analytically examined in steady state. The expected NTV scaling with the square of the normalized applied field perturbation agrees with the measured profile change δB2.1–2.3. The NTV is also found to scale as Ti2.1–2.4, in general agreement with the low collisionality “1/ν” regime scaling of the NTV theory (TNTV-(1/ν) ∝ Ti2.5).

Operation of KSTAR PF Superconducting Magnet Power Supplies Under MG Power System

Poloidal field (PF) power supplies provide 20-25 kA to PF superconducting magnets to generate, maintain, and control the Korea Superconducting Tokamak Advanced Research (KSTAR) plasma. A substantial amount of grid power for PF power supplies is required. Before 2014, available grid power for PF power supplies reached only 50 MVA, which is comparably small to support the rated operation of PF magnets. To achieve high-performance and long pulse operation of KSTAR plasma, the motor generator (MG) system, whose maximum power is 200 MVA, was designed and tested up to 150 MVA with PF magnets in 2014. Frequency fluctuation of line voltage, voltage distortion, and harmonic current increases are found during the operation. The power supply control can be seriously affected by these problems because every PF power supply uses a 12-pulse thyristor converter that is sensitive to the power quality. In this paper, key features of power supply control under grid and MG power system are described, and the test results are presented.

Design of a collective scattering system for small scale turbulence study in Korea Superconducting Tokamak Advanced Research.

The design characteristics of a multi-channel collective (or coherent) scattering system for small scale turbulence study in Korea Superconducting Tokamak Advanced Research (KSTAR), which is planned to be installed in 2017, are given in this paper. A few critical issues are discussed in depth such as the Faraday and Cotton-Mouton effects on the beam polarization, radial spatial resolution, probe beam frequency, polarization, and power. A proper and feasible optics with the 300 GHz probe beam, which was designed based on these issues, provides a simultaneous measurement of electron density fluctuations at four discrete poloidal wavenumbers up to 24 cm(-1). The upper limit corresponds to the normalized wavenumber kθρe of ∼0.15 in nominal KSTAR plasmas. To detect the scattered beam power and extract phase information, a quadrature detection system consisting of four-channel antenna/detector array and electronics will be employed.

Neutron emission in neutral beam heated KSTAR plasmas and its application to neutron radiography

The main mission of Korea Superconducting Tokamak Advanced Research (KSTAR) program is exploring the physics and technologies of high performance steady state Tokamak operation that are essential for ITER and fusion reactor. Since the successful first operation in 2008, the plasma performance is enhanced and duration of H-mode is extended to around 50s which corresponds to a few times of current diffusion time and surpassing the current conventional Tokamak operation. In addition to long-pulse operation, the operational boundary of the H-mode discharge is further extended over MHD no-wall limit(βN∼4) transiently and higher stored energy region is obtained by increased total heating power (∼6MW) and plasma current (Ip up to 1MA for ∼10s). Heating system consists of various mixtures (NB, ECH, LHCD, ICRF) but the major horse heating resource is the neutral beam(NB) of 100keV with 4.5MW and most of experiments are conducted with NB. So there is a lot of production of fast neutrons coming from via D(d,n)3He reaction and it is found that most of neutrons are coming from deuterium beam plasma interaction. Nominal neutron yield and the area of beam port is about 1013–1014/s and 1m2 at the closest access position of the sample respectively and neutron emission could be modulated for application to the neutron radiography by varying NB power. This work reports on the results of quantitative analysis of neutron emission measurements and results are discussed in terms of beam-plasma interaction and plasma confinement. It also includes the feasibility study of neutron radiography using KSTAR.

New development of EPICS based data acquisition system for H-Alpha diagnostic

The H-Alpha diagnostic system has been developed to measure the line integrated intensity in the direction of toroidal and poloidal. The data acquisition (DAQ) system for H-Alpha diagnostics of the Korea Superconducting Tokamak Advanced Research (KSTAR) at the beginning of the first plasma in 2008 was developed with VME form factor digitizer in the Linux OS platform.The VME digitizer module of H-Alpha data acquisition system was modified to measure the low current signal from the photo-multiplier tubes (PMT). The input maximum current values of modified digitizer module are 400nA and low current data is expressed as the value of the voltage between −10V and +10V. At first time, there was no problem to measure H-Alpha signal, but it could not measure the H-Alpha data signal as the KSTAR Plasma density increased. It exceeds digitizer input range, which means the H-Alpha signal is over 400nA, so we should manually change the resistor on the digitizer board to measure the 400nA over current. This is not easy to do and showed instability in the long time operation with high sampling data acquisition.In order to overcome these weak points, a new H-Alpha data acquisition system has been developed with a compact PCI (cPCI) based digitizer and a signal conditioning box for converting the current to voltage in the Linux OS platform. The new data acquisition system was developed based on Experimental Physics and Industrial Control System (EPICS) framework like other KSTAR diagnostics with standard framework (SFW).The main advantages of the new H-Alpha data acquisition system are the capabilities of calculating line integrated intensity during the plasma shot and displaying it in run-time. During the plasma shot, the new data acquisition system performs segment archiving data in MDSplus DB of KSTAR central storage.This paper describes the overview of new H-Alpha data acquisition system with signal condition box, the results of its implementation.

Design and operation results of nitrogen gas baking system for KSTAR plasma facing components

A baking system for the Korea Superconducting Tokamak Advanced Research (KSTAR) plasma facing components (PFCs) is designed and operated to achieve vacuum pressure below 5×10−7mbar in vacuum vessel with removing impurities. The purpose of this research is to prevent the fracture of PFC because of thermal stress during baking the PFC, and to accomplish stable operation of the baking system with the minimum life cycle cost. The uniformity of PFC temperature in each sector was investigated, when the supply gas temperature was varied by 5°C per hour using a heater and the three-way valve at the outlet of a compressor. The alternative of the pipe expansion owing to hot gas and the cage configuration of the three-way valve were also studied. During the fourth campaign of the KSTAR in 2011, nitrogen gas temperature rose up to 300°C, PFC temperature reached at 250°C, the temperature difference among PFCs was maintained at below 8.3°C, and vacuum pressure of up to 7.24×10−8mbar was achieved inside the vacuum vessel.

Development of In-Vessel Vertical Coil Power Supply in KSTAR

The Korea Superconducting Tokamak Advanced Research (KSTAR) device is an advanced superconducting tokamak to establish scientific and technological bases for an attractive fusion reactor [1]. In-vessel vertical coil (IVC) power supply (PS) is developed for vertical stability and shape control of KSTAR plasma. IVC PS in KSTAR is based on a single-phase full-bridge inverter. The output voltage and current are dc ±1 kV and ±10 kA, respectively. Insulated gate bipolar transistor is used as switching power device in IVC PS and maximal 5 kHz of switching frequency is adopted to meet the requirement of fast control by plasma control system. This paper describes the configuration and engineering of the IVC PS system and analyzes both the IVC coil commissioning and the major performances that D-shape plasma achieved in experiments.

Analysis of edge density fluctuation measured by trial KSTAR beam emission spectroscopy system

A beam emission spectroscopy (BES) system based on direct imaging avalanche photodiode (APD) camera has been designed for Korea Superconducting Tokamak Advanced Research (KSTAR) and a trial system has been constructed and installed for evaluating feasibility of the design. The system contains two cameras, one is an APD camera for BES measurement and another is a fast visible camera for position calibration. Two pneumatically actuated mirrors were positioned at front and rear of lens optics. The front mirror can switch the measurement between edge and core region of plasma and the rear mirror can switch between the APD and the visible camera. All systems worked properly and the measured photon flux was reasonable as expected from the simulation. While the measurement data from the trial system were limited, it revealed some interesting characteristics of KSTAR plasma suggesting future research works with fully installed BES system. The analysis result and the development plan will be presented in this paper.

Diagnostic neutron activation system for KSTAR

The Korea Superconducting Tokamak Advanced Research (KSTAR) device has begun deuterium plasma operation and increased neutron generation from D-D fusion reaction in the plasma is expected as the heating power of the plasma increases. A neutron activation system utilizing the pneumatic transfer of encapsulated metal samples has been implemented to monitor the fusion neutron source strength and the total fusion power from the KSTAR plasma. The prototype pneumatic transfer system for the ITER neutron activation system was slightly modified to be used for KSTAR, and a High Purity Germanium (HPGe) detection system was used to count gamma photons from the activated samples. The Monte Carlo code MCNP and the inventory code FISPACT were also used for the calculations to evaluate the total number of neutrons emitted from the D-D fusion reactions in the KSTAR plasma. The analysis of data from an activation measurement obtained from the 2011 KSTAR campaign shows the neutron flux at the irradiation station was 2.2 × 108 cm−2s−1, and the total neutron yield was 4.7 × 1013 n/s for a typical NBI-heated, H-mode KSTAR plasma shot.

Performance of 300 s-beam extraction in the KSTAR neutral beam injector

The first neutral beam injector (NBI-1) has been developed for the Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak. The first long pulse ion source (LPIS-1) has been installed in the NBI-1 for an auxiliary heating and current drive of KSTAR plasmas. The performance of 300 s ion beam extraction in the LPIS-1 was investigated on the KSTAR NBI-1 system, prior to the neutral beam injection for long pulse operation. The ion source consists of a magnetic bucket plasma generator with multi-pole cusp fields and a set of prototype tetrode accelerators with circular-type apertures. The inner volume of the plasma generator and accelerator column in the LPIS-1 is approximately 123 L. The nominal operation requirements for the ion source (IS) were a 100 kV/50 A deuterium beam and a 300 s pulse length. The extraction of ion beams was initiated by the formation of arc plasmas in the LPIS-1, called an arc-beam extraction method. A stable ion beam extraction of the LPIS-1 was achieved with 80 kV/27 A and a beam perveance of 1.19 microperv for a 300 s pulse length. Beam power deposition along the NBI-1 has been measured using water-flow calorimetry (WFC), and the sum of the deposited power on the ion source and beamline components was about 93% of the drained acceleration power (Vacc•Iacc). The beam power deposition was compared to the calculated results of the beam transport with re-ionization (BTR) code.

First neutral beam injection experiments on KSTAR tokamak

The first neutral beam (NB) injection system of the Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak was partially completed in 2010 with only 1∕3 of its full design capability, and NB heating experiments were carried out during the 2010 KSTAR operation campaign. The ion source is composed of a JAEA bucket plasma generator and a KAERI large multi-aperture accelerator assembly, which is designed to deliver a 1.5 MW, NB power of deuterium at 95 keV. Before the beam injection experiments, discharge, and beam extraction characteristics of the ion source were investigated. The ion source has good beam optics in a broad range of beam perveance. The optimum perveance is 1.1-1.3 μP, and the minimum beam divergence angle measured by the Doppler shift spectroscopy is 0.8°. The ion species ratio is D(+):D(2)(+):D(3)(+) = 75:20:5 at beam current density of 85 mA/cm(2). The arc efficiency is more than 1.0 A∕kW. In the 2010 KSTAR campaign, a deuterium NB power of 0.7-1.5 MW was successfully injected into the KSTAR plasma with a beam energy of 70-90 keV. L-H transitions were observed within a wide range of beam powers relative to a threshold value. The edge pedestal formation in the T(i) and T(e) profiles was verified through CES and electron cyclotron emission diagnostics. In every deuterium NB injection, a burst of D-D neutrons was recorded, and increases in the ion temperature and plasma stored energy were found.

Results of Beam Extraction Performance for the KSTAR Neutral Beam Injector

The first neutral beam injector (NBI-1) has been developed for the Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak. The first long pulse ion source (LPIS-1) has been installed on the NBI-1 for an auxiliary heating and current drive of KSTAR plasmas. The performance of ion and neutral beam extractions in the LPIS-1 was investigated initially on the KSTAR NBI-1 system, prior to the neutral beam injection into the main plasmas. The ion source consists of a magnetic bucket plasma generator with multipole cusp fields and a set of prototype tetrode accelerators with circular apertures. The inner volume of the plasma generator and accelerator column in the LPIS-1 is approximately 123 L. Design requirements for the ion source were a 120 kV/65 A deuterium beam and a 300 s pulse length. The extraction of ion beams was initiated by the formation of arc plasmas in the LPIS-1, called the arc-beam extraction method. A stable ion beam extraction of the LPIS-1 was achieved up to 85 kV/32 A for a 5 s pulse length and 80 kV/25 A for a 14 s pulse length. An optimum beam perveance of 1.15 µperv was observed at an acceleration voltage of 60 kV. Neutralization efficiency was measured by a water-flow calorimetry (WFC) method using a calorimeter and the operation of a bending magnet. The full-energy species of ion beams were detected by using the diagnostic method of optical multichannel analyzer (OMA). An arc efficiency of the LPIS was 0.6–1.1 A/kW depending on the operating conditions of arc discharge. A neutral beam power of ∼1.0 MW must be sufficiently injected into the KSTAR plasmas from the LPIS-1 at a beam energy of 80 keV.

Development of two-grating spectrometer for the charge exchange spectroscopy system on KSTAR

The charge exchange spectroscopy (CES) system on Korea Superconducting Tokamak Advanced Research (KSTAR) was installed last year and had been applied to measure the C VI ion temperature and rotation velocity profiles. The ion temperature and rotation velocity profiles had been estimated from the C VI 5290.5 Å (n = 8-7) charge-exchange spectrum signal measured by a Czerny-Turner type spectrometer and a thinned back-illuminated charge coupled device (CCD) camera. However, the Czerny-Turner type spectrometer used for the KSTAR CES system showed so low signal to noise ratio for KSTAR plasmas in the 2010 experimental campaign that the time resolution of the CES system had been limited to 100 ms due to the increased exposure time of the attached CCD camera. Then, new two-grating spectrometer had been developed in order to improve the time resolution of the CES system. The spectrometer consists of two gratings (1200 g/mm and 1800 g/mm each) with additive configuration, concave mirrors (f = 50 cm), and a cylindrical lens (f = 50 cm). The time resolution of the CES system increases by a factor of 2-4 with the two-grating spectrometer. The C VI ion temperature and rotation velocity profiles obtained by the two-grating spectrometer are compared to those by Czerny-Turner type spectrometer in this paper.

Examination of a duo-collection optics design for the Korea superconducting tokamak advanced research (KSTAR) Thomson scattering system

The comparison of collective optic designs is described for the Thomson scattering system of the Korea superconducting tokamak advanced research (KSTAR) device. The optical systems collecting the light emission induced through the interaction between the plasma electrons and a laser beam are the key components for the Thomson scattering system. In the first conceptual design of the collection optics for the KSTAR Thomson scattering system, a duo-lens system covering individually the core and the edge regions of the KSTAR plasma with two optical lens modules was proposed. In optical designs, the number of optical modules is a great concern in the case of limited system space. Here, the duo-lens system is evaluated through a comparison with a uni-lens system covering the whole region of the plasma with a single optical module. The duo-lens system turned out to have 2.0 times and 4.73 times higher light collections of the plasma core and edge compared with the uni-lens system.

Development of high power long-pulse RF transmitter for ICRF heating in fusion researches and cyclotron accelerator

A high power long pulse transmitter whose frequency range is in the range of VHF(Very High Frequency) bands have been widely used for fusion researches and accelerator as well as broadcasting industry. KAERI (Korea Atomic Energy Research Institute) have been developing the transmitters for ICRF heating for KSTAR and the cyclotron accelerator since 1996. The toroidal magnetic field of KSTAR (Korea Superconducting Tokamak Advanced Research) is nominally 3 T so that 25–60 MHz transmitter is required to cover ICRF heating scenarios of the KSTAR. The first one is 2 MW transmitter operating up to 60 MHz and it succeeded in achieving 2 MW for 300 s in 2008 after several failures of tetrode tube at the final amplifier stage. [1] Up to 300 kW RF power was successfully injected to KSTAR plasmas. The second one is the wideband 70 kW/CW transmitter used for the cyclotron accelerator and their frequency range is from 25 to 50 MHz. Its engineering design was finished. The third one is 1 MW/VHF transmitter which was loaned from JAEA. As the operating ICRF frequency of KSTAR is lower than VHF bands, its cavity structure will be modified for KSTAR and the operating frequency would be changed from 110 MHz to 60 MHz. In this presentation, the test results of JAEA transmitter at 120 MHz and lessons from the high power test of 2 MW transmitter will be introduced and the circuit analysis and engineering design work for the second and third amplifiers will be shown.