Korea Superconducting Tokamak Advanced Research Magnet Research Articles

Study on quench detection of the KSTAR CS coil with CDA+MIK compensation of inductive voltages

Quench Detection System (QDS) is essential to guarantee the stable operation of the Korea Superconducting Tokamak Advanced Research (KSTAR) Poloidal Field (PF) magnet system because the stored energy in the magnet system is very large. For the fast response, voltage-based QDS has been used. Co-wound voltage sensors and balanced bridge circuits were applied to eliminate the inductive voltages generated during the plasma operation. However, as the inductive voltages are hundreds times higher than the quench detection voltage during the pulse-current operation, Central Difference Averaging (CDA) and MIK, where I and K stand for mutual coupling indexes of different circuits, which is an active cancellation of mutually generated voltages have been suggested and studied. In this paper, the CDA and MIK technique were applied to the KSTAR magnet for PF magnet quench detection. The calculated inductive voltages from the MIK and measured voltages from the CDA circuits were compared to eliminate the inductive voltages at result signals.

Superconducting Magnet Power Supply System for the KSTAR 2ndPlasma Experiment and Operation

The Korea Superconducting Tokamak Advanced Research (KSTAR) device is an advanced superconducting tokamak to establish scientific and technological bases for attractive fusion reactor. This device requires 3.5 Tesla of toroidal field (TF) for plasma confinement, and requires a strong poloidal flux swing to generate an inductive voltage to produce and sustain the tokamak plasma. KSTAR was originally designed to have 16 serially connected TF magnets for which the nominal current rating is 35.2 kA. KSTAR also has 7 pairs of poloidal field (PF) coils that are driven to 1 MA/sec for generation of the tokamak plasma according to the operation scenarios. The KSTAR Magnet Power Supply (MPS) was dedicated to the superconducting (SC) coil commissioning and <TEX>$2^{nd}$</TEX> plasma experiment as a part of the system commissioning. This paper will describe key features of KSTAR MPS for the <TEX>$2^{nd}$</TEX> plasma experiment, and will also report the engineering and commissioning results of the magnet power supplies.

Thermohydraulic Behaviors of KSTAR PF Superconducting Magnet System in Operation

Korea Superconducting Tokamak Advanced Research (KSTAR) is a superconducting tokamak which adopts and NbTi as superconductors and it has been operated since 2008. One of main issues is the performance of superconducting magnet system in the real operation. In view of this issue, the KSTAR magnet system shows the sufficient operational capability through the whole campaigns. For example, we have already achieved the maximum Toroidal Field (TF) current and the bipolar pulsed Poloidal Field (PF) operation up to ± 10 kA. It is very important to operate stably the full superconducting magnet system such as KSTAR and ITER. Each KSTAR magnet consists of Cable-In-Conduit Conductors (CICC) cooled by forced flow of the supercritical helium in cryogenic temperature. The temperature, pressure and mass flow rate of coolant are interacted organically inside of CICC and the behaviors are also complicated during operation. The pressure drop between inlet and outlet of coil, the temperature increasing in accordance with coil current variation, and so on are critical issues related with the flexibility and reliability of superconducting tokamak. Most of all, we agree that the analysis of thermohydraulic behavior is absolutely needed for more stable operation and higher plasma performance. So we are checking the related data and the tendency at every campaign and we are optimizing the operational procedure to fit the plasma scenario. In this paper, we introduce the significant thermohydraulic behaviors of KSTAR PF coils up to now.

Status of the KSTAR superconducting magnet system development

The aim of the Korea superconducting tokamak advanced research (KSTAR) project is to develop a steady-state-capable advanced superconducting tokamak for establishing a scientific and technological basis for an attractive fusion reactor. Since the KSTAR mission includes the achievement of a steady-state-capable operation, the use of superconducting coils is an obvious choice for the magnet system. The KSTAR superconducting magnet system consists of 16 toroidal field (TF) and 14 poloidal field (PF) coils which include 8 central solenoid coils. Both the TF and PF coil systems use internally-cooled cable-in-conduit conductors (CICC). The TF coil system provides a magnetic field of 3.5 T at the plasma centre and the PF coil system provide a flux swing of 17 V s. The major achievement in the KSTAR magnet system development includes the development of CICC, a full size TF model coil, a background magnetic field generation coil system and the construction of a large scale superconducting magnet and the CICC test facility. TF and PF coils are at the stage of fabrication for the KSTAR completion in the year 2007.

Jacketing and Repair of the KSTAR CICC

The KSTAR (Korea Superconducting Tokamak Advanced Research) superconducting magnet system which consists of 16 TF coils and 14 PF coils adopts a superconducting CICC (Cable-In-Conduit Conductor) type conductor. The KSTAR magnet system uses two different types of CICCs-Nb/sub 3/Sn cable with Incoloy 908 conduit and NbTi cable with 316LN stainless-steel conduit. A continuous CICC jacketing system is developed for the KSTAR CICC fabrication and the jacketing system uses the tube-mill process. It consists of forming, welding, sizing and squaring procedures. The welding condition of CICCs and the fabrication process is described. The repair of the CICC is also discussed.

Structural analysis of the KSTAR central solenoid magnet system

The Central Solenoid (CS) magnet system of the Korea Superconducting Tokamak Advanced Research (KSTAR) device consists of four pairs of superconducting coils compressed with the CS coil structure. The CS coil structure is operated as a preload structure to give an axial compression on the CS coils. The required axial compression is applied partly by using the pre-compression components in assembly stage and by the thermal contraction difference between the coil winding pack and the structure during cool down stage as well. In order to investigate the structural integrities and to increase the structural reliabilities of the KSTAR CS magnet, global and local structural analyses have been performed for various operation scenarios by developing three-dimensional finite element models. From the analysis results, it has been found that the CS magnet system can safely withstand the reference operation scenarios and the structural integrity of the CS magnet complies with requirements of the design criteria for KSTAR magnet system.

Electromagnetic loads on the KSTAR magnet system

Several types of Lorenz forces and Joule heating generated in the magnet system of the Korea Superconducting Tokamak Advanced Research (KSTAR) device have been studied numerically and analytically. We have examined 225 plasma equilibrium states to calculate the maximum magnetic forces and to determine which equilibrium state(s) generate such forces. The obtained results are used as input for structural analysis. A precompression should be applied to the eight-segmented central solenoid (CS) coil assembly to prevent free motion of each coil due to attractive and repulsive forces during operation. We have also evaluated the maximum values of the vertical and lateral forces for each of the CS coils and poloidal field coils and also for the entire CS coil stack. The in-plane force due to toroidal field (TF) coil charging and the out-of-plane force due to interaction of the TF coil current with the poloidal field have been computed. The Joule heating on the TF structure due to plasma disruption has also been calculated.

Engineering design status of the KSTAR TF coil structure

The toroidal field (TF) magnet system of Korea Superconducting Tokamak Advanced Research (KSTAR) device consists of 16 superconducting coils enclosed in steel cases. The TF cases are wedged along the inboard straight legs to sustain in-plane centering forces. The inter-coil structures contain adjustable shear keys and conical bolts to provide pre-loading in toroidal direction and to resist in-plane and out-of-plane forces, which are the most critical loads on the TF magnet system. For effective cooling of the TF case, a pad-type cooling channel has been designed to ensure structural and thermal stability. In order to investigate the structural integrity and to increase the structural reliability of the KSTAR magnet system, structural analyses have been conducted. From the analysis results, it has been found that the TF magnet structure can safely withstand the reference scenario operations.

Fabrication of the KSTAR superconducting CICC

A superconducting cable-in-conduit conductor (CICC) is adopted for the Korea Superconducting Tokamak Advanced Research (KSTAR) superconducting magnet system which consists of 16 TF coils and 14 PF coils. The KSTAR Magnet system uses two different types of CICCs-Nb/sub 3/Sn cable with Incoloy 908 conduit and NbTi cable with 316LN stainless-steel conduit. A continuous CICC jacketing system is developed for the KSTAR CICC fabrication and the jacketing system uses the tube-mill process, which consists of forming, welding, sizing and squaring procedures. The design specification of CICCs and the fabrication process is described. The welding of the Incoloy 908 strip joint is also discussed. The fabrication results including the geometrical specification and the void fraction will be discussed.

KSTAR magnet structure design

The Korea Superconducting Tokamak Advanced Research (KSTAR) device is a steady-state-capable experimental fusion device with a fully superconducting magnet system, including toroidal field (TF) coils, central solenoid (CS) coils, and poloidal field (PF) coils. The major design consideration of the magnet system is to meet the KSTAR mission with plasma current of 2 MA and toroidal field of 3.5 T at the major radius 1.8 m and z=0. The preliminary analyses show that the magnet structure design has mechanical, electrical, and thermal stability during operation. The TF magnets have a wedged structure, including coil cases, inter-coil structures, and inter-octant joints. The CS and PF structures are designed to support the electromagnetic forces. To support the coil system against gravity and lateral loads, gravity support and lateral load structures are designed.