Experimental Results of Simplified Balance Bridges for the KSTAR Quench Detection System

Yonekawa, Hirofumi,Yong Chu,Young-ok Kim,In-sik Woo,Kap-rai Park IEEE 2016 IEEE transactions on applied superconductivity Vol.26 No.4

<P>The primary quench detection system of the Korea Superconducting Tokamak Advanced Research (KSTAR) device has been operated in technical and plasma experiments for eight years. Cowound-type quench detectors have exhibited distinct performance of compensation for both self-induction and mutual induction; on the other hand, some cowound strips tended to break in the long-term operation. In contrast, Wheatstone bridge (WB)-type quench detectors had no wire breakage, and their signal wires are still accessible and repairable. Therefore, WB-type quench detectors are very important to be a backup method of quench detection for the coils. New WBs using a half-bridge at a fixed ratio of resistances of 1: 1 were tested in the KSTAR operation. Upper-lower comparison of the new WBs performed good compensation for induced voltage. The new WBs clearly demonstrated that the bridge circuits may become very simple, and in situ calibration may be unnecessary if the arrangement of voltage taps and quench detectors is optimized with respect to the magnetic field of the tokamak device.</P>

Operational Results of a Thermal-Hydraulic Quench Detection System for the KSTAR TF Superconducting Coils

Yonekawa, Hirofumi,Young-ok Kim,Yong Chu Institute of Electrical and Electronics Engineers 2017 IEEE transactions on applied superconductivity Vol.27 No.4

<P>The Korea Superconducting Tokamak Advanced Research (KSTAR) device is equipped with a prototype of the safety quench detection system (SQD), making use of the variation of the pressure and flow rate in supercritical helium (SHe) in the event of a quench, for the toroidal field (TF) superconducting coils. The SQD prototype measures absolute and differential pressures on orifice flow meters at SHe supply manifolds by using radio-resistant pressure transducers, then variation of the measurements is discriminated by using analog signal interfaces and hard-wired logic solvers of a two-out-of-three (2oo3) voting configuration for validation. The SQD prototype is being operated to obtain technical expertise in KSTAR. TF coil charging and PF coil pulsed operation had a different impact on the temperature variations due to eddy current of the TF coils and structure. Accordingly, the inlet pressure of the TF coils increased by small amount in comparison to the quench detection criterion of the SQD prototype. The plasma current seemed to increase impulse voltage, but only, by ~10 mV in quench detection signals of normal-voltage detection method, and such a noise did not disturbed quench detectors. On the other hand application of the PF currents and plasma current increased outlet temperatures of the TF coils and structure by approximately 0.1 K. The inlet pressure variation due to the temperature variation was too small to achieve the quench detection criterion.</P>

Measurement of the Electric Properties of the KSTAR CS Model Coil

Hirofumi Yonekawa,G. M. Moon,G. S. Lee,J. S. Bak,J. S. Kim,박갑래,K. W. Cho,박수환,W. S. Han,W. W. Park,Y. Chu,Y. J. Lee,Y. K. Oh,Y. M. Park,Y. S. Kim,KSTAR Team 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.52 No.2

The maximum current, the self inductance and the lap joint resistance of a split-pair superconducting pulse magnet were experimentally inspected. The pulse magnet mainly consists of two KSTAR central solenoid (CS) model coils, which are wound, continuous Nb₃Sn cable-in-conduit conductors (CICC) with Incoloy 908 jackets. The pulse magnet initially could not achieve its rated current over several days whereas the temperature of the magnet surfaces and coolant outlets stayed at 5 ~ 8 K. A training effect and a ramp rate limitation were suspected of being the cause at the beginning. However the magnet's supporting structure was found to have been exceedingly warm in that period, and the maximum current clearly corresponded to the temperature of the supporting structure. The self inductance of the pulse magnet was measured with an LCR meter at 0.1 ~ 1 kHz at room temperature, but the inductance was nearly double the design value. The self inductance was also measured by means of a decay time constant method at 5 K and, at values of current above 200 A, was found to be originally the same as the design value, however, it dynamically increased below that current. Supplemental calculation showed that the permeability of Incoloy 908 could have a detrimental impact upon the dynamic variation of the self inductance. The lap joint resistance was successfully obtained by measuring the voltage drops between two CICCs at 5 K. Another trial measuring the voltage drops between the two outer surfaces of the lap joint resulted in a nonphysical result that violated Ohm's law.

Experimental Evaluation of the Influence of External Current on the KSTAR TF Quench Detection System

Yonekawa, Hirofumi,Yong Chu,Young-Ok Kim,Soo-Hwan Park,Kap-Rai Park IEEE 2014 IEEE transactions on applied superconductivity Vol.24 No.3

<P>The quench detection system (QDS) for 16 toroidal field (TF) coils of the Korea Superconducting Tokamak Advanced Research (KSTAR) is using Wheatstone bridges to naturally reduce induced voltage in the quench detection circuits. The TF coils have to be quickly discharged to save the superconductors from thermal runaway if quench occurs; on the other hand, the fast discharge brings about many side effects. Thus, malfunction of the TF QDS should be avoided. According to the results of the TF QDS operation in KSTAR campaigns, the single Wheatstone bridges can sufficiently reduce self-induced voltage even though the induced voltage may remain due to unbalancing the bridges by ferromagnetism of the cable-in-conduit conductor (CICC) jackets at small TF coil current. In contrast, mutually induced voltage by central solenoid (CS) and poloidal field (PF) coils may not be sufficiently reduced by using a single Wheatstone bridge comparing voltages across a number of TF coils due to larger electric circuits in the cryostat. On the other hand, a single Wheatstone bridge comparing single TF coil voltages could not effectively reduce the sharp impulse voltage caused by locally induced current on the vacuum vessel at the end of plasma current with vertical displacement event (VDE).</P>

Inductance Compensation for the Quench Detection of the KSTAR CS Coil

Yong Chu,Yonekawa, Hirofumi,Yeong-ok Kim,In-Sik Woo,Kwang Pyo Kim,Seungje Lee IEEE 2016 IEEE transactions on applied superconductivity Vol.26 No.4

<P>Voltage-based QD is the primary way to guarantee that magnets can be protected against overheating because of its fastest response. However, central solenoid (CS) and poloidal field (PF) coils are operated in pulse currents, so resultant inductive voltages could be hundred times higher than quench detection voltage thresholds. For this reason, some inductive voltage compensation techniques have been adopted to discriminate the quench-induced resistive voltage from the voltage induced by self and mutual inductances between coils. Cowound voltage sensors and bridge circuits are used to eliminate inductive voltages, but achieving additional rejection of remaining inductive voltages is necessary. Simple comparison between bridge voltages or cowound voltages from upper and lower PF coils, which are symmetric to the equatorial plane, has been well operating for KSTAR. Another is mutual inductance compensation, where mutually induced voltages are estimated by using predefined inductances and temporal derivatives of PF coil currents and compared with measured voltages. Rogowski coils to measure the current derivatives of CS/PF coils, precompensated voltages with inductive voltages largely eliminated beforehand, and a data analysis system were prepared for the KSTAR PF 1 coil. Analysis of inductive voltage compensation was presented with the operational result in 2015 KSTAR campaign.</P>

Performance Enhancement of Quench Detection System for the KSTAR Superconducting Pulse Coils

Yong Chu,Jinsub Kim,Yonekawa, Hirofumi,Yeoung-ok Kim,Insik Woo,Seok Chan An,Seungje Lee Institute of Electrical and Electronics Engineers 2017 IEEE transactions on applied superconductivity Vol.27 No.4

<P>Since the first plasma was achieved in 2008, the quench detection system for the Korea Superconducting Advanced Tokamak Research (KSTAR) coils has been successfully operated. Simple bridge circuits and cowound voltage sensors were used for the first quench detectors. In order to enhance the reliability and stability of the quench detection especially for the pulse coils such as the KSTAR CS coil, a couple of schemes to actively reject inductive voltages have been added and tested. They have focused on rejecting as much inductive voltages as possible and increasing the noise rejection ratio (NRR) because the inductive voltage noise is a main factor to disturb the stable quench detection. One scheme is to use the voltage difference between two balanced bridge voltages or two voltages of cowound voltage sensors, where each voltage is the voltage across each of two coils having the same electrical currents and structurally symmetry. The other is to use the voltage difference between the measured voltage and the calculated voltage. With the additional schemes, inductive voltages were reduced so that the NRR could substantially increase up to 20 000. In this paper, the upgraded quench detection system was described for the KSTAR PF1 coil and the performance results were presented together with experimental data in the 2016 KSTAR campaign.</P>

Quench Detection of the KSTAR CS Coil by Considering Plasma Current Effect

Kim, Jinsub,Jeon, Young Mu,Yonekawa, Hirofumi,Kim, Yeong-ok,Kim, Kwang Pyo,Park, K. R.,Chu, Yong Institute of Electrical and Electronics Engineers 2019 IEEE transactions on applied superconductivity Vol.29 No.5

<P>The plasma-driven inductive voltages were not negligible and should be taken into account for the reliable operation of the voltage-based quench detection of the KSTAR CS coil. For the active compensation of such plasma-induced voltage noise, plasma parameters should be measured in real-time and applied to the quench detection system. The selection of plasma parameters by considering their contribution to the noise is of primary importance. A code, which can estimate the induced coil voltage driven by plasma, is developed by using a simplified plasma response modeling. The numerical analysis has showed that there is an apparent influence from the plasma, while the influence from eddy currents induced in passive structures is negligible. The change of plasma current is the primary noise source and the radial and vertical motions of plasma have an effect, although they are small. The performance of the quench detector is developed and tested during the 2017 KSTAR campaign. The noise rejection performance is discussed with various experimental results and numerical estimations.</P>

Characteristic research of pellet injection system in KSTAR

Park, SooHwan,Kim, HongTack,Song, JaeIn,Woo, InSik,Lee, KunSu,Lee, HyunMyung,Kim, YoungOk,Yonekawa, Hirofumi,Kim, KwangPyo,Chu, Yong,Oh, YeongKook Elsevier 2018 Fusion engineering and design Vol.136 No.2

<P><B>Abstract</B></P> <P>20 Hz pellet injection system was installed for improving a fueling system of KSTAR (Korea Superconducting Tokamak Advanced Research) in 2016. Several things of hardware such as a liquid helium transfer line, an extruder nozzle and an extension part of pellet guide tube are changed for 2017 campaign. For plasma experiments, the pellet injection system uses the smallest pellet (1.5 mm of length, 2 mm of diameter). The pellet velocity can be variable from 150 m/s to over 200 m/s by controlling of propellant gas pressure. The clear increase of plasma density observed during KSTAR experiments is also presented in this paper.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The smallest pellet (1.5 mm of length, 2 mm of diameter) injection is successful. </LI> <LI> Slower pellet injection less than 150 m/s is achieved. </LI> <LI> Pellet trajectory is closer to plasma by straight extension part. </LI> </UL> </P>