Korea Superconducting Tokamak Advanced Research Device Research Articles

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.

KSTAR Status and Upgrade Plan Toward Fusion Reactor

Korea Superconducting Tokamak Advanced Research (KSTAR) program has made significant advances in the development of long pulse and high-performance plasma scenarios, utilizing the advantages of the fully superconducting tokamak. Ten years of KSTAR operation have produced outstanding progress in plasma control, extending the operating window of the plasma discharges, and achieving the H-mode up to 1.2 MA in plasma current, up to 90 s at top duration, and up to 2.16 in elongation. The unique features of the KSTAR device also produced a record 30 s long pulse in H-mode operation without an edge-localized-mode (ELM) crash, and progress in the fundamental transport physics through systematic study, using unique diagnostic capabilities. Therefore, the next near-term research focus will be on how to extend the high performance plasmas up to 300 s by overcoming scientific and engineering obstacles to steady state operation. For fusion reactors, the tokamak device needs to sustain a permanent plasma current. It is also known that the bootstrap current should play an important role in the plasma current for an efficient fusion reactor, and its contribution should be over as high as possible. An internal transport barrier discharge based on the reverse shear profile of the safety factor is one of the promising candidates for enhancing the bootstrap current. However, even in reverse shear mode, an additional external off axis current drive (CD) should be provided to control the current profile, as well as to supplement the plasma current. KSTAR has pursued steady state to operation, aiming for a fully non inductive state, based on real time (RT) plasma control by maximizing the long pulse capabilities of the superconducting machine. In order to deposit current at an arbitrary position, the first requirement is to have many highly efficient CD actuators capable of profile diversity. KSTAR has prepared several RF CD portfolios in addition to a conventional on and off axis CD neutral beam injection (NBI) system. The final target of an off axis current drive will be a 4 MW off-axis NB, 8 MW on-axis NB, 6 MW electron cyclotron heating (ECH), and 4 MW radio frequency heating and current drive (RFHCD) (ion cyclotron (IC), lower hybrid (LH), Helicon CD). In addition, as a long-term goal, demonstrating a long pulse core edge compatible plasma in a reactor relevant condition is important in present medium size tokamak, since it is expected to sustain a very high pressure pedestal height plasma using super H-mode, with parallel upgrades of the superconducting toroidal field (TF) coils.

Progress of the KSTAR Research Program Exploring the Advanced High Performance and Steady-State Plasma Operations

Korea Superconducting Tokamak Advanced Research (KSTAR) program is strongly focused on solving the scientific and technological issues in steady-state high performance plasma operation in preparation for ITER operation as well as the design basis for DEMO. In this regards, KSTAR has made significant advances in developing long pulse and high performance plasma scenarios utilizing the advantage of the fully superconducting tokamak. Ten-year of KSTAR operation showed the outstanding progress in the plasma control extending the operation window of the plasma discharges achieving the H-mode up to 1 MA in plasma current, up to 72 s in flat top duration, and up to 2.16 in elongation. In addition to the long pulse discharge, high performance discharges with high betas (βN ~ 3) could be achieved in the broad range of edge safety factor (q95) without external error field correction. The unique features of the KSTAR device (magnetic accuracy with extremely low error fields, steady-state capable heating systems, in-vessel control coils, and advanced imaging and profile diagnostics) has been fully exploited to explore the unveiled physics as well as to exploring the systematic solution for suppression of edge localized mode (ELM) crash. Achieved examples are the record long pulse of H-mode operation without an ELM crash (~ 30 s up to date), and progress in the fundamental transport physics through systematic study using these unique capabilities. Based on the previous research results, intensive research will be followed to explore the advanced high beta operation (βN ~ 4) with fully suppressed harmful MHD instabilities aiming the integrated solution for DEMO. In this regards, an additional current drive systems and in-vessel structures will be upgraded.

Development and Experimental Evaluation of a Prototype of the TF Secondary Quench Detection System for KSTAR Device

A prototype of the toroidal field (TF) secondary quench detection (SQD) system was developed and implemented with the Korea Superconducting Tokamak Advanced Research (KSTAR) device to carry out the design verification of the SQD. The SQD can detect a quench based on the change of absolute pressures (APs) and mass flow rates of helium in the cooling lines of the TF coils. If the primary quench detection system of the TF coils cannot detect a quench or the fast discharge of the TF coils cannot be carried out as planned, SQD should work to prevent the TF coils and the peripheral structures from severe damages. In addition, SQD should operate in the reliable and stable condition against the disturbances caused by the poloidal field coil discharge, plasma perturbation, and any faults of subsystems of the KSTAR device. The 2-out-of-3 (2oo3) voting configuration was applied to the SQD to enhance the reliability and stability of quench detection. The prototype SQD consists of AP and differential pressure transducers, signal interfaces, logic solvers, and interlock systems. All the transducers were selected from metallic types with no electronic circuit in order to reduce the failure rates caused by strong electromagnetic field and radiation around the tokamak. The transducers were installed in the manifolds of the helium inlet lines of the KSTAR TF coils. Their signals were amplified and compared with the reference voltage for quench decision in the signal interface unit. The quench signal generated by the signal interface unit was transmitted to the 2oo3 voting modules of the logic solvers. The design requirements of SQD were verified through testing the prototype SQD in the 2014 KSTAR campaign.

Simulation study of the disruption load in KSTAR device with a modified passive stabilizer

The structure of the passive stabilizer system in KSTAR (Korea Superconducting Tokamak Advanced Research) device was recently modified mainly in relation to the upgrade of the vertical position control system. Corresponding to this design change in one of the plasma facing components, a simulation study was performed to see how the disruption load and the characteristics varied. During the vertical drift phase, the vertical growth rate is shown to be increased by about 8 times while the vertical force on the passive stabilizer is reduced by about 5 times for the new passive stabilizer compared with the old one. Meanwhile, during the thermal and current quench phase, relatively small differences were observed in the disruption load and characteristics between the two models.

The maintenance record of the KSTAR helium refrigeration system

Korea Superconducting Tokamak Advanced Research (KSTAR) has a helium refrigeration system (HRS) with the cooling capacity of 9 kW at 4.5 K. Main cold components are composed of 300 tons of superconducting (SC) magnets, main cryostat thermal shields, and SC current feeder system. The HRS comprises six gas storage tanks, a liquid nitrogen tank, the room temperature compression sector, the cold box (C/B), the 1st stage helium distribution box (DB#1), the PLC base local control system interconnected to central control tower and so on. Between HRS and cold components, there's another distribution box (DB#2) nearby the KSTAR device. The entire KSTAR device was constructed in 2007 and has been operated since 2008. This paper will present the maintenance result of the KSTAR HRS during the campaign and discuss the operation record and maintenance history of the KSTAR HRS.

Predictive modeling of pedestal structure in KSTAR using EPED model

A predictive calculation is given for the structure of edge pedestal in the H-mode plasma of the KSTAR (Korea Superconducting Tokamak Advanced Research) device using the EPED model. Particularly, the dependence of pedestal width and height on various plasma parameters is studied in detail. The two codes, ELITE and HELENA, are utilized for the stability analysis of the peeling-ballooning and kinetic ballooning modes, respectively. Summarizing the main results, the pedestal slope and height have a strong dependence on plasma current, rapidly increasing with it, while the pedestal width is almost independent of it. The plasma density or collisionality gives initially a mild stabilization, increasing the pedestal slope and height, but above some threshold value its effect turns to a destabilization, reducing the pedestal width and height. Among several plasma shape parameters, the triangularity gives the most dominant effect, rapidly increasing the pedestal width and height, while the effect of elongation and squareness appears to be relatively weak. Implication of these edge results, particularly in relation to the global plasma performance, is discussed.

Stable plasma start-up in the KSTAR device under various discharge conditions

A time series of static nonlinear ferromagnetic calculations was performed to mimic the time-dependent modelling of plasma start-up by assessing the effects of the ferromagnetic Incoloy 908 used in the superconducting coil jackets of the Korea Superconducting Tokamak Advanced Research (KSTAR) device. Time-series calculations of a two-dimensional axisymmetric circuit model with nonlinear ferromagnetic effects enabled us to find appropriate waveforms for the KSTAR poloidal field coil currents that satisfied various start-up requirements, such as the formation and sustainment of field nulls, a sufficient amount of magnetic flux for further plasma current ramp-up, sufficiently large Et ·Bt/B⊥ > 1 kV m−1 contours for successful breakdown, plasma current toroidal equilibria, etc. In addition to the aforementioned requirements, the results introduced in this report also provided the positional stability of the plasma current channel against radial as well as vertical perturbations by compensating the field deformation originating from the ferromagnetic effects. With the improved positional stability, robust plasma start-up was achieved during the 2010 KSTAR campaign under various discharge conditions such as the recovery process from plasma disruptions.

MHD stability analysis for advanced tokamak modes in the KSTAR device

The Korea Superconducting Tokamak Advanced Research (KSTAR) device aims to demonstrating the steady-state operation of high-performance advanced tokamak (AT) modes. In order to meet this research goal it is critical to have a good magnetohydrodynamic (MHD) stability, so that KSTAR adopted a strong plasma shape and a conducting wall close to plasma for such stability. An early calculation during the KSTAR design phase had shown that a target AT mode stable up to β N above 5 can be then obtained. A recent work by Katsuro-Hopkins et al. [O. Katsuro-Hopkins, S.A. Sabbagh, J.M. Bialek, H.K. Park, J.G. Bak, J. Chung, et al., Equilibrium and global MHD stability study of KSTAR high beta plasmas under passive and active mode control, Nucl. Fusion 50(2010) 025019] showed, however, that the maximum β N value can be substantially lower than 5, unlike the earlier result. In this work, we present a more detailed study on the MHD stability limit of the KSTAR target AT mode and try to clarify the discrepancy observed in the previous two works. It is shown that in the reverse-shear plasma the target mode with β N above 5 can be obtained if the pressure profile is relatively peaked, but the maximum β N value is substantially reduced below 5 if the pressure profile becomes broader. This result suggests the importance of a proper control of the pressure profile to get the high-beta AT mode in KSTAR.

Qualification Test Results of the KSTAR Superconducting Coils From the Construction to the Commissioning Steps

To achieve the first plasma of the Korea superconducting tokamak advanced research (KSTAR), the KSTAR superconducting coils were tested in advance. As they should operate in excessively low temperature of 4.5 K and high magnetic field environment of 7.5 T, it is crucial to monitor the cryogenic and the structural behaviors of KSTAR device during the commissioning period including a cool-down. The temperatures of the KSTAR toroidal field (TF) coil and the poloidal field (PF) coils were measured during the entire operating period. The mechanical stresses on the TF and PF structures were continuously monitored to check if they go beyond the limiting value calculated through the simulation. The alignment of the KSTAR device was checked by using displacement sensors. The TF coils were successfully supplied with 15 kA DC current for 8 hours, and the maximum 5 kA/s current variation of the PF coils were tested. For the main experiment, the interlock test of the quench detection system for the KSTAR coils was carried out at reduced currents of 1 kA. From these results the quench protection circuit, and the current-flow of the KSTAR superconducting coils proved to be well performed for the first plasma operation.

Overview of the KSTAR vacuum pumping system

The installation of the Korea Superconducting Tokamak Advanced Research (KSTAR) device vacuum pumping system for both the vacuum vessel and the cryostat was finished in the middle of 2007. After the successful self-commissioning of the main vacuum pumping system by August 2007, the system successfully operated in the integrated system commissioning and 1st plasma experiments of the KSTAR device. The vacuum pumping system has successfully operated and achieved the target vacuum pressure for the vacuum vessel and the cryostat during the machine commissioning and 1st plasma experiments with no severe troubles and problems, which is one of the main factors that led the KSTAR project to achieve successful 1st plasma. Furthermore, the vacuum pumping system has been satisfactorily operated by remote control and synchronization in gas puffing experiments. This paper describes the final configuration of the KSTAR vacuum pumping system, the results of vacuum commissioning, and the other results of the vacuum conditions that relate to wall conditioning.

Installation of a fast framing visible camera on KSTAR

Visible camera technologies have made remarkable progress in recent years, and the fast camera has proven itself to be a capable imaging diagnostic in studies of specific fusion plasma issues such as the start-up physics, plasma wall interactions, edge-localized modes, and disruptions. For the purpose of favorable visible imaging, a fast framing camera has recently been installed on the Korea Superconducting Tokamak Advanced Research (KSTAR) device. The camera uses a complementary metal-oxide semiconductor detector with a maximum resolution of 1280x1024 at 1000 frames/s (fps) and a minimum resolution of 1280x16 at 64 kfps. A 2-m-long viewport having a novel optical rail system was installed on a tangential port to view the tokamak interior. The system is fully controlled from the main control room and protected by a shutter from deposits. To verify that the camera electronics are safe from the high magnetic field and its rapid time variation, possible influences are considered theoretically and experimentally. In this work, we present the design and installation of the fast camera system on the KSTAR device with discussions on the field variation effect issues.

Completion of the KSTAR construction and its role as ITER pilot device

The construction of the Korea Superconducting Tokamak Advanced Research (KSTAR) device has been completed by August 2007. Key achievements in the KSTAR construction are (i) fabrication process establishment in the Nb 3Sn superconducting coils including interface components, (ii) development of magnet structures to resist high magnetic force and thermal loads of TF and PF coils, (iii) accurate assembling of all components using special assembly tools, and (iv) development of ancillary systems including a control system, magnet power supplies, diagnostic systems, heating systems, and cryogenic facility. Device commissioning has been started and the milestone for first plasma discharge is by the end of 2008. The KSTAR operation plan is targeted on the high performance plasma confinement research and long pulse operation achievement using the fully superconducting tokamak device. There is a lot of engineering similarity between KSTAR and ITER, such as using Nb 3Sn superconductor for TF and CS magnets. The experience of the KSTAR device could be an important technical basis for the design, procurement, and initial operation of the ITER device, as a role of a ITER pilot device.

Quench Protection System for the Superconducting Coil of the KSTAR Tokamak

This paper presents two-stage high-power mechanical-thyristor switches developed as 40-kA direct current (dc) circuit breakers for the superconducting coil energy dump system of Korea superconducting Tokamak advanced research (KSTAR). An advanced switching system used in the quench protection should allow multishot mode of operation without maintenance and long lifetime with arcless dc current commutation. The system is designed to combine the advantages of the both mechanical and solid-state power switches. The mechanical stage is to conduct dc current for normal superconducting coil operation and the thyristor stage is to provide fast arcless dc current interruption for quench protection (QP). Compared with the all solid-state switches and vacuum interrupters of equal capabilities the proposed switch is much cheaper than the dc breakers and has lower power dissipation in current conducting state Kuchinski in Proc. Power Ele., 899-903, 1995; Benfatto IEEE Power Del., Vol. 3, No. 4, pp. 1372-1376, Oct. 1998. Also, it is much faster than the vacuum interrupters. The system is designed to break 40-kA dc current within 200 ms. Also, KSTAR device, a configuration of the energy dump system and coil power supplies are presented

Preliminary design of the soft x-ray array tomographic diagnostic system for Korea Superconducting Tokamak Advanced Research (KSTAR) plasmas

Analyses of x-ray (SXR) emission from tokamak plasmas provide valuable information about magnetohydrodynamics activities and transport phenomena. In this study, design and optimization of the SXR detector arrays were performed for the KSTAR (Korea superconducting tokamak advanced research) device. Geometrical arrangement of the array system was made by coverage mapping and tomographic reconstruction tests. The space-time tomography algorithm was developed for KSTAR-like plasmas using the fast maximum entropy method combined with singular value decomposition. Spatial setup of the components in each detector array was determined and optimized by geometrical calculation. A curved beryllium window and a developed cooling system will be mounted on the detector arrays. Due to the existence of the complex structures between outer vessel and passive stabilizers, each array is designed to be miniaturized. Hardware-optimization of the array system was cross-checked with the tomographic test results.

Operating Characteristics of the KSTAR Superconducting TF Coil

The Korea Superconducting Tokamak Advanced Research (KSTAR) device, a steady-state-capable advanced Tokamak, is to be built in Korea. The KSTAR device is made up of 16 toroidal field (TF) and 14 poloidal field (PF) superconducting coils. A simulation code for the transient operation of the KSTAR TF and PF coils is developed. For stable Tokamak operation, the operating characteristics of the KSTAR superconducting magnet system are studied for operation scenario. We calculated the temperature margin of superconducting cables, cryogenic flow parameters and the heat deposition in TF and PF coils. The effect of the 3D heat conduction and the eddy current loss in the supporting structure and vacuum chamber of KSTAR is also included.

Overview of Korea Superconducting Tokamak Advanced Research diagnostics

The Korea Superconducting Tokamak Advanced Research (KSTAR) project is the Korean national fusion program for constructing a superconducting tokamak, that is capable of a steady-state operation. Currently, the KSTAR diagnostic group is concentrating on conceptual design activities to provide measurements of the plasma behavior that is necessary to satisfy the KSTAR missions. A diagnostic overview for the KSTAR device is presented.