Y.S. Kim

OVERVIEW OF KSTAR INTEGRATED CONTROL SYSTEM

After more than 10 years construction, KSTAR (Korea Superconducting Tokamak Advanced Research) had finally completed its assembly in June 2007, and then achieved the goal of first-plasma in July 2008 through the four months commissioning. KSTAR was constructed with fully superconducting magnets with material of and NbTi, and their operation temperatures are maintained below 4.5K by the help of Helium Refrigerator System. During the first-plasma operation, plasmas of maximum current of 133kA and maximum pulse width of 865ms were obtained. The KSTAR Integrated Control System (KICS) has successfully fulfilled its missions of surveillance, device operation, machine protection interlock, and data acquisition and management. These and more were all KSTAR commissioning requirements. For reliable and safe operation of KSTAR, 17 local control systems were developed. Those systems must be integrated into the logically single control system, and operate regardless of their platforms and location installed. In order to meet these requirements, KICS was developed as a network-based distributed system and adopted a new framework, named as EPICS (Experimental Physics and Industrial Control System). Also, KICS has some features in KSTAR operation. It performs not only 24 hour continuous plant operation, but the shot-based real-time feedback control by exchanging the initiatives of operation between a central controller and a plasma control system in accordance with the operation sequence. For the diagnosis and analysis of plasma, 11 types of diagnostic system were implemented in KSTAR, and the acquired data from them were archived using MDSpius (Model Driven System), which is widely used in data management of fusion control systems. This paper will cover the design and implementation of the KSTAR integrated control system and the data management and visualization systems. Commissioning results will be introduced in brief.

Experimental Evaluation of a Mutually Induced Voltage Correction Method to Improve the KSTAR Quench Detection System

The superconducting state of the coils and bus lines of the Korea Superconducting Tokamak Advanced Research is monitored by a quench detection system. The system utilizes co-wound conductive strips and Wheatstone bridges, and in addition compares two quench detection signals for upper and lower coils to reduce induced voltage. The co-wound conductive strips and Wheatstone bridges, however, are very sensitive to mutual induction by neighboring coil currents, and on the other hand the comparison of the quench detection signals is not assured in a vertically asymmetric magnetic field. Thus, a numerical correction method of the inductances and induced voltages on quench detection circuits for the CS and PF coils was evaluated using actual measurements. The inductance on each quench detection circuit had a peculiar profile probably caused by magnetism of the conductors. The amplitudes of the induced voltages certainly reflected the magnitudes of the inductances; however, the correction of the quench detection signals was difficult because the inductances available for the correction were obtained by alternately applying each coil current.

Experimental Evaluation of Inductance and Its Impact on Quench Detection of the KSTAR Coils

Windings of Nb3Sn cable-in-conduit conductors of the toroidal field (TF) and the 1st-5th poloidal field (PF) coils of the Korea Superconducting Tokamak Advanced Research (KSTAR) have Incoloy-908 jackets, which exhibit weak ferromagnetism. The inductance of the PF coils was measured by monitoring the induced voltages on the coils while TF coils were steadily energized at 20 kA, and one of each pair of PF coils was charged up to 4 kA at 2 kA/s. At a large PF current, the measured self inductance was very close to the design value. In contrast, the measured self inductances of the PF1-PF5 coils increased 90-135% by decreasing their current below 2 kA; whereas those of the PF6-PF7 coils almost stayed at the same levels. On the other hand, the measured self inductances were much smaller than the other measurements obtained at no TF current. The self induced voltage was almost cancelled out by using a single Wheatstone bridge of the quench detectors; whereas, the mutually induced voltage was cancelled out by numerically subtracting outputs of two Wheatstone bridges for coils, which were symmetrically located in the assembled coils.

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

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).

Mechanical Behavior of the KSTAR Central Solenoid Coil Structure During Thermal Cycles

The central solenoid (CS) coil system of the KSTAR device consists of four pairs of coils with up-down symmetry and a set of support structure. The CS coil system should endure various loads including thermal and electromagnetic load during operation. One of the most important roles of support structure is to preserve the structural stability of coil system from vertical forces by applying the pre-compression on the coil stack. The main assembly procedure of the coil stack is as follows: 1) coil subassembly and stacking, 2) structure assembly, 3) preloading process by heating shells in the support structure, and 4) measurement on the final dimension. The special jigs and heating system for the preloading were designed to apply pre-compression on the coil stack during the assembly at room temperature. Moreover, a few tens of thermocouples and strain gauges were installed for measuring temperature and thermal expansion of preloading structures. Since the final assembly of the CS coil was successfully completed with about 7.3 MN of preloading in 2006, the CS coil system has been experienced four thermal cycles from 300 K to 5 K until 2011. This paper describes the main results of applying the pre-compression on the coil stack and mechanical behaviors of preloading structures during thermal cycles.

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

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

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

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.

Electric Resistance Measurement of the KSTAR Superconducting Coils With Small Resistive Joints

An electric resistance of lap joints assembled with Korea Superconducting Tokamak Advanced Research (KSTAR) poloidal field 1 (PF1) superconducting coils and bus lines was measured by 4-wire method with some improvements to reduce induced noise in measurements at cryogenic temperature. Generally, a large current has to flow through the joints in order to measure its resistance by electrical methods; however, an induced voltage generated by a DC power supply is much higher than a voltage drop on the joints. The improved method used a bypass resistor installed in parallel to the DC power supply and the coils in order to naturally reduce ripples of the coil current. Transitional waveforms of the coil current and tap voltages were approximated by non-linear curve fitting with constraints based on an electric circuit equation. The resistance and inductance were evaluated by solving simultaneous equations of an applied voltage on the conductors. The induced noise reduction by this method was helpful to evaluate the joint resistance.