Korea Superconducting Tokamak Advanced Research Tokamak Research Articles

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.

Progress of x-ray imaging crystal spectrometer utilizing double crystal assembly on KSTAR.

The x-ray imaging crystal spectrometer (XICS) for Korea Superconducting Tokamak Advanced Research (KSTAR) has been upgraded to increase its performance including measurement capabilities and stable operation. A dual crystal assembly for simultaneous measurements of the helium-like and hydrogen-like Ar spectra is successfully installed for improving measurement capabilities. Using a safety viewing port with an illuminator and removing the XICS control system from the harsh KSTAR tokamak hall for a stable operation are newly performed. The experimental results from the improved XICS are investigated.

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.

Detection of slowly rotating n = 1 mode with signal compensation for an externally perturbed field in the KSTAR tokamak

A mode identification method for slowly rotating (or non-rotating) n (toroidal mode number) = 1 plasma instabilities has been newly established with the signal compensation method used for an external time-varying magnetic field. The mode identification method is based on the Fourier decomposition scheme and processes the signal of the magnetic probes (MPs) on the passive stabilizer in the Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak. To exclude the non-plasma magnetic perturbation by the external current coils and their secondary eddy currents on the passive stabilizer that may not be easily characterized, the ARX-SISO (autoregressive with exogenous terms-single input single output) method has been introduced in the signal compensation process. Preliminary off-line analysis presented herein confirms that this method can adequately detect the time evolution of the mode-locking, along with a slowly rotating state, even in the presence of external magnetic perturbations.

Commissioning of vacuum pumping devices of the second KSTAR neutral beam injection system

The Korea Superconducting Tokamak Advanced Research (KSTAR) Neutral Beam Injection (NBI) system is designed to provide additional plasma heating and current drive for high performance and long pulse operation of the KSTAR tokamak using two co-current beam injection systems. Each neutral beam injection system is designed to consist of three ion sources, and each ion source is designed to deliver more than 2.0 MW of deuterium neutral beam power for the 100 keV beam energy. Consequently, the final goal of the KSTAR NBI system is to inject 12 MW of deuterium beam power for the long pulse operation of the KSTAR. The first NBI (NBI-1) has been installed since 2010, which provides a 100 keV deuterium neutral beam of 6.0 MW maximum using three ion sources.The second NBI (NBI-2) also consist with three ion sources which arranged vertically with tangency radius of 1.56 m to provide balanced core heating and off-axis current drive will be constructed until 2019. An ion source will be commissioned 2018 KSTAR campaign, and the other two ion source will be applied to KSTAR from 2019. As the vacuum condition can substantially affect the beam performance, the vacuum pumping system for NBI-2 has been carefully designed. The cryopumps for NBI-2 were designed and the overall vacuum pumping system of NBI-2 was installed including turbo pumps, mechanical booster-pumps, and dry-pumps. In this paper, the detailed engineering design, fabrication, and installation results of large condensation cryopumps and vacuum sub-components were explained. The effective pumping speed and pressure distribution will be evaluated through the commissioning of the vacuum pumping system for NBI-2. Finally, we present the operation procedures of the NBI-2 vacuum pumping system.

Direct measurements of safety factor profiles with motional Stark effect for KSTAR tokamak discharges with internal transport barriers.

The safety factor profile evolutions have been measured from the plasma discharges with the external current drive mechanism such as the multi-ion-source neutral beam injection for the Korea Superconducting Tokamak Advanced Research (KSTAR) for the first time. This measurement has been possible by the newly installed motional Stark effect (MSE) diagnostic system that utilizes the polarized Balmer-alpha emission from the energetic neutral deuterium atoms induced by the Stark effect under the Lorentz electric field. The 25-channel KSTAR MSE diagnostic is based on the conventional photoelastic modulator approach with the spatial and temporal resolutions less than 2 cm (for the most of the channels except 2 to 3 channels inside the magnetic axis) and about 10 ms, respectively. The strong Faraday rotation imposed on the optical elements in the diagnostic system is calibrated out from a separate and well-designed polarization measurement procedure using an in-vessel reference polarizer during the toroidal-field ramp-up phase before the plasma experiment starts. The combination of the non-inductive current drive during the ramp-up and shape control enables the formation of the internal transport barrier where the pitch angle profiles indicate flat or slightly hollow profiles in the safety factor.

Study on the heat flux reconstruction with the infrared thermography for the divertor target plates in the KSTAR tokamak.

An infrared (IR) thermography is the preferred diagnostic that can quantify heat flux by measuring the surface temperature distributions of the divertor plates. The IR thermography is successfully instrumented on Korea Superconducting Tokamak Advanced Research (KSTAR). In this study, finite volume method is considered to solve the heat conduction equations. 1D-, 2D-, and 3D models are developed and compared with various calculation algorithms, such as Duhamel's theorem and THEODOR. These comparisons show good agreement. In order to acquire more efficient and reliable calculation results, we consider two numerical analysis schemes, influence of temperature on thermal properties and image stabilization. Recently, this reconstruction code is successfully applied to the KSTAR IR thermography.

Effects of neoclassical toroidal viscosity induced by the intrinsic error fields and toroidal field ripple on the toroidal rotation in tokamaks

Effects of neoclassical toroidal viscosity (NTV) induced by intrinsic error fields and toroidal field ripple on cocurrent toroidal rotation in H-mode tokamak plasmas are investigated. It is expected that large NTV torque can be localized at the edge region through the 1/ν-regime in the vicinity of Er∼0 in the cocurrent rotating H-mode plasma. Numerical simulation on toroidal rotation demonstrates that the edge localized NTV torque determined by the intrinsic error fields and toroidal field ripples in the level of most tokamaks can damp the toroidal rotation velocity over the whole region while reducing the toroidal rotation pedestal which is clearly observed in Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak. It is found that the NTV torque changes the toroidal rotation gradient in the pedestal region dramatically, but the toroidal rotation profile in the core region responds rigidly without a change in the gradient. On the other hand, it shows that the NTV torque induced by the intrinsic error fields and toroidal field ripple in the level of the KSTAR tokamak, which are expected to be smaller than most tokamaks by at least one order of magnitude, is negligible in determining the toroidal rotation velocity profile. Experimental observation on the toroidal rotation change by the externally applied nonaxisymmetric magnetic fields on KSTAR also suggests that NTV torque arising from nonaxisymmetric magnetic fields can damp the toroidal rotation over the whole region while diminishing the toroidal rotation pedestal.

Radiation damage of the PCO Pixelfly VGA CCD camera of the BES system on KSTAR tokamak

A PCO Pixelfly VGA CCD camera which is part a of the Beam Emission Spectroscopy (BES) diagnostic system of the Korea Superconducting Tokamak Advanced Research (KSTAR) used for spatial calibrations, suffered from serious radiation damage, white pixel defects have been generated in it. The main goal of this work was to identify the origin of the radiation damage and to give solutions to avoid it. Monte Carlo N-Particle eXtended (MCNPX) model was built using Monte Carlo Modeling Interface Program (MCAM) and calculations were carried out to predict the neutron and gamma-ray fields in the camera position. Besides the MCNPX calculations pure gamma-ray irradiations of the CCD camera were carried out in the Training Reactor of BME. Before, during and after the irradiations numerous frames were taken with the camera with 5s long exposure times. The evaluation of these frames showed that with the applied high gamma-ray dose (1.7Gy) and dose rate levels (up to 2Gy/h) the number of the white pixels did not increase. We have found that the origin of the white pixel generation was the neutron-induced thermal hopping of the electrons which means that in the future only neutron shielding is necessary around the CCD camera. Another solution could be to replace the CCD camera with a more radiation tolerant one for example with a suitable CMOS camera or apply both solutions simultaneously.

Design of a Doppler reflectometer for KSTAR.

A Doppler reflectometer has been designed to measure the poloidal propagation velocity on the Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak. It has the operating frequency range of V-band (50-75 GHz) and the monostatic antenna configuration with extraordinary mode (X-mode). The single sideband modulation with an intermediate frequency of 50 MHz is used for the heterodyne measurement with the 200 MHz in-phase and quadrature (I/Q) phase detector. The corrugated conical horn antenna is used to approximate the Gaussian beam propagation and it is installed together with the oversized rectangular waveguides in the vacuum vessel. The first commissioning test of the Doppler reflectometer system on the KSTAR tokamak is planned in the 2014 KSTAR experimental campaign.

On confinement characteristics of the newly developed sawtooth-free plasmas in KSTAR tokamak

Development of advanced scenarios, an important experimental goal of the Korea superconducting tokamak advanced research (KSTAR) project, has just begun. The safety factor (q) profile is a key to achieve these advanced scenarios. Particularly the hybrid scenario, one of the advanced scenarios, can be established generally with low magnetic shear (s) at the center with central q-value above unity so to avoid sawtooth instability. This q-profile was successfully produced using early divertor formation during a plasma current ramp-up phase in KSTAR. Auxiliary heating was also employed during the current ramp-up phase to delay the inductive current diffusion to the center of the plasma. In addition to the early divertor formation method, the target q-profile was attempted to be achieved by modifying the plasma current waveform using the so-called, ‘current-overshoot’ method and the timing of L-mode to H-mode transition. In this work, the confinement characteristics of these sawtooth-free regimes are investigated. The global energy confinement time is calculated and compared with that of conventional H-modes in KSTAR. The confinement enhancement factor reveals that the newly developed discharges are not improved over H-modes contrary to results of other tokamaks. To investigate the reason, transport modeling is performed self-consistently with an integrated simulation package incorporating plasma equilibrium, transport, and heating and current drive. The current ramp-up phase is simulated and impact of early divertor formation, current-overshoot, and early L–H transition on the target q-profile and s/q profile is addressed. The s/q profile is found to be not improved in these discharges compared with hybrid scenarios reported in other tokamaks. Based on these results, future experimental directions are addressed to access the hybrid regimes in KSTAR.

Control and operation of the gas injection systems for KSTAR tokamak

Several gas injection systems such as fueling, massive gas injection (MGI), supersonic molecular beam injection (SMBI) have been installed for the Korea Superconducting Tokamak Advanced Research (KSTAR). Piezoelectric valves with a built-in piezoelectric plate of disc-bender type were used for the precise feedback control of gas flow. The relationship between the displacement and blocking force of the piezoelectric plates is taken into account in designing and fabricating piezoelectric valves. The MGI system was installed for the mitigation of plasma disruption. The MGI valve is composed of two-volume chamber. The valve is equipped with an aluminum piston and an actuator coil of inductance 0.1mH. The power supply unit using a capacitor discharge method has been developed in order to drive the MGI valve. The SMBI system consisting of LN2 cooling line, pulse valve, and its controller was operated for the mitigation of edge localized mode (ELM). The LN2 cooling line is used to increase the density of the injected gas. The status and specification of each gas injection system is presented and the operational characteristics are analyzed.

Experiences on vacuum conditioning in the cryostat of KSTAR tokamak

Korea Superconducting Tokamak Advanced Research (KSTAR) device has been successfully operated for the plasma experiment from KSTAR 1st campaign to 4th campaign. The main pumping system for the cryostat has to maintain the target pressure below 1.0×10−4mbar at room temperature and 1.0×10−5mbar at extremely low temperature for the plasma experiment against the air leak coming from ports of vessel and/or the helium leak from cooling loops in the cryostat.No leak has been detected at room temperature. Unexpectedly, the cold-leak appeared in the cryostat at temperature around 50K during the cool-down in the KSTAR 2nd campaign. We carefully analyzed the characteristics of detected cold leak because it can cause the increase of the base pressure in the cryostat.After the cool-down, the leak detection was performed to locate the position and size of the leak by the pressurizing the loops. As a result, it is found that the cold leak was located at cooling loops for PF/CS coils and CS structure. Nevertheless, the vacuum inside the cryostat was well maintained below 6.0×10−8mbar during the entire operation period. The impact of the He-leak in present status on the plasma operation is negligible. However, we have found that the leak rate increases as a function of time. Therefore careful monitoring on cold-leak is an important technical issue for the operation of superconducting tokamak.

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.

Simulation of edge-divertor plasma transport for high-performance operation in KSTAR

The edge-divertor plasma transport responding to type-I edge localized modes (ELMs) in a hybrid scenario and a conventional H-mode operation of the KSTAR (Korea Superconducting Tokamak Advanced Research) tokamak is simulated using a newly developed two-dimensional fluid code named ‘EPST’, which can be used in time-dependent analyses of the edge–private flux–scrape-off layer region. It is verified through a benchmark test with the B2.5 code for a double-null configuration of the KSTAR tokamak. In this simulation, the KSTAR operation modes are distinguished by their βN values and the threshold pedestal heights for ELM triggering. Type-I ELMs are simulated under the ELM triggering condition by an ad hoc method of time-periodic changes in the transport coefficients in the near-separatrix region. As a result of numerical simulation, the overall distributions of plasma properties responding to the ELM bursts appear to be the same in the edge-divertor region regardless of their βN. However, the increased ballooning limit in the higher βN case makes the ELM frequency become lower, while it makes the maximum heat flux on the divertor plate higher compared with the lower βN case.

Design of neutral beam injection system for KSTAR tokamak

The neutral beam injection (NBI-1) system has been designed for providing a 300 s deuterium beam of 120 kV/65 A as an auxiliary heating and current drive system of the KSTAR (Korea Superconducting Tokamak Advanced Research) tokamak. The deuterium beam is produced from a long pulse ion source composed of a bucket-type plasma generator and a multi-aperture tetrode accelerator with the help of discharge power supplies and high voltage (HV) power supplies. The beamline components (BLCs) include a neutralizer with an optical multi-channel analyzer (OMA) section, a bending magnet (BM), an ion dump assembly, a movable calorimeter, beam scrapers, and a cryo-sorption pump system in a rectangular vacuum tank. A beam duct equipped with bellows and a voltage break is placed between the NBI vacuum tank and the KSTAR vacuum vessel. All data and parameters of the NBI system are controlled by a control and data acquisition (CODAQ) system through the EPICS based Ethernet interface.

Electron cyclotron emission diagnostics on KSTAR tokamak

A new electron cyclotron emission (ECE) diagnostics system was installed for the Second Korea Superconducting Tokamak Advanced Research (KSTAR) campaign. The new ECE system consists of an ECE collecting optics system, an overmode circular corrugated waveguide system, and 48 channel heterodyne radiometer with the frequency range of 110-162 GHz. During the 2 T operation of the KSTAR tokamak, the electron temperatures as well as its radial profiles at the high field side were measured and sawtooth phenomena were also observed. We also discuss the effect of a window on in situ calibration.

Diagnostics for first plasma and development plan on KSTAR

The first plasma with target values of the plasma current and the pulse duration was finally achieved on June 13, 2008 in the Korea Superconducting Tokamak Advanced Research (KSTAR). The diagnostic systems played an important role in achieving successful first plasma operation for the KSTAR tokamak. The employed plasma diagnostic systems for the KSTAR first plasma including the magnetic diagnostics, millimeter-wave interferometer, inspection illuminator, H(alpha), visible spectrometer, filterscope, and electron cyclotron emission (ECE) radiometer have provided the main plasma parameters, which are essential for the plasma generation, control, and physics understanding. Improvements to the first diagnostic systems and additional diagnostics including an x-ray imaging crystal spectrometer, reflectometer, ECE radiometer, resistive bolometer, and soft x-ray array are scheduled to be added for the next KSTAR experimental campaign in 2009.

Analysis of Stress due to the Plasma Disruption Current in the KSTAR ICRF Antenna

The structural strains and stresses of the KSTAR (Korea Superconducting Tokamak Advanced Research) ICRF (Ion Cyclotron Range of Frequency) antenna have been calculated by using a computational code. The stress of the antenna cage is generated by EM forces from the Faraday shield tube. These Lorentz forces originate from the current induced during plasma disruption. For the commissioning of the ICRF antenna in the KSTAR tokamak, an optimized method for its installation is determined by using a structural analysis. The stresses of the antenna cage and the front cavity wall have been analyzed with respect to structural deformation by assuming the main operation scenarios inside the KSTAR port. For the structural parts of the ICRF antenna, deformation or distortion of the structures is dominant in the Faraday shield tube and at the front of the antenna and the stresses are intense in the xed devices near the top of the antenna cage. These stresses are analysed by using the ANSYS code, which is a commercial code for structural and thermal analysis. As a result, the techniques and the mechanism for the installation of the ICRF antenna have been arranged and the commissioning procedure has been performed in the exact position of the KSTAR port.

Software development of the KSTAR Tokamak Monitoring System

The Korea Superconducting Tokamak Advanced Research (KSTAR) project, which is constructing a superconducting Tokamak, was launched in 1996. Much progress in instrumentation and control has been made since then and the construction phase will be finished in August 2007. The Tokamak Monitoring System (TMS) measures the temperatures of the superconducting magnets, bus-lines, and structures and hence monitors the superconducting conditions during the operation of the KSTAR Tokamak. The TMS also measures the strains and displacements on the structures in order to monitor the mechanical safety. There are around 400 temperature sensors, more than 240 strain gauges, 10 displacement gauges and 10 Hall sensors. The TMS utilizes Cernox sensors for low temperature measurement and each sensor has its own characteristic curve. In addition, the TMS needs to perform complex arithmetic operations to convert the measurements into temperatures for each Cernox sensor for this large number of monitoring channels. A special software development effort was required to reduce the temperature conversion time and multi-threading to achieve the higher performance needed to handle the large number of channels. We have developed the TMS with PXI hardware and with EPICS software. We will describe the details of the implementations in this paper.