Hirofumi Yonekawa

Quench Detection Based on Voltage Measurement for the KSTAR Superconducting Coils

To protect the KSTAR (Korea Superconducting Tokamak Advanced Research) superconducting coils against a quench, the quench detection system based on voltage measurement was fabricated. It used a detect/dump scheme which detects the presence of non-recovering normal zones and activates a dump circuit that transfers most of the stored energy into a dump resistor. Even though it is desirable to discharge the coil energy as fast as possible after quench detection, a time delay between the quench detection and the complete dump circuit actuation will be necessary in order that the quench voltage is to be distinguished from various noises. The quench threshold voltage and the delay time should be set before operation for quench voltage detection so that the maximum hot-spot temperature could be limited to 150 K. This paper describes the hardware techniques to prevent the malfunction of the quench detection due to voltage noises arising from the KSTAR operating scenarios. During the first operation of the KSTAR machine, the inductive voltages and other voltage noises were measured and effectively compensated below voltage thresholds. A quench did not occur and the quench detection system was well operated without any false activation.

Influence of current re-distribution on minimum quench energy of superconducting triplex cable against local disturbance

Abstract Stability of multi-strand superconducting cable against local disturbances has been studied experimentally and theoretically. Quench experiments have been made with a superconducting triplex cable made of chrome-plated strands. A heat pulse is applied to a short part of one strand, and the minimum quench energy (MQE) against that local disturbance and the temporal evolution of the strand current during the quench or recovery process are measured. When the transport current divided by the critical current ( I t / I c ) is large, the MQE against a local disturbance almost equals the MQE of the single strand. When the overall I t / I c is less than 0.4, the MQE against a local disturbance is much larger than the MQE of the single strand. In this small I t / I c region, when a heat pulse whose energy is slightly less than the MQE is applied, current re-distribution is observed during the recovery process. Numerical simulations of the quench or recovery process have been made with a computer code named MST. The dependence of the calculated MQE on the I t / I c qualitatively agrees with the experimental result. The numerical and experimental results prove that the stability against local disturbances is improved by the current re-distribution when the I t / I c is less than some threshold value. This threshold appears to be influenced by the contact thermal conductance between strands.

First Commissioning Results of the KSTAR Cryogenic System

The cryogenic system for the KSTAR superconducting (SC) magnets has been commissioned. It consists of a cold box, distribution boxes (DB) and cryogenic transfer lines. The cold box and DB #1 provide 600 g/s of supercritical helium to cool the SC magnets, their SC bus-lines, and the magnet support structures. It also provides 17.5 g/s of liquid helium to the current leads and supplies cold helium flow to the thermal shields. The main duties of the DB #2 are the relative distribution of the cryogenic helium among the cooling channels of each KSTAR cold component and the emergency release of over-pressurized helium during abnormal events such as quenches of the SC magnets. After individual commissioning, the system was integrated and cooled down with the KSTAR device. In this paper, the construction and commissioning results of the KSTAR cryogenic system will be introduced. In addition, we will present the cool-down results of the KSTAR device.

Stability of Superconducting Magnet for KSTAR

The magnet system comprises sixteen toroidal field (TF) coils and fourteen poloidal field (PF) coils for KSTAR (Korea Superconducting Tokamak Advanced Research) Tokamak. All the TF coils and 10 PF coils (PF1 ~ 5 UL) are made of Nb3Sn strands and 4 PF coils (PF6 ~7 UL) are made of NbTi strands. Before operating the superconducting coils, it is critical to verify the operation conditions such as thermo-hydraulic parameters and stability boundaries. In this regards, the TF model coil and CSMC (Central Solenoid Model Coil) have been tested and the operating conditions have been analyzed. Before cool down, parameters such as helium flow through conduits, electrical insulation and coil geometry have been measured at room temperature to confirm the manufacturing soundness. During cool down and current charging, thermo-hydraulic and mechanical parameters by using mechanical sensors have been measured. Also measured was the magnetic field strength in various positions of the coil conductor. Measured data have been analyzed and the results have been compared with the model analysis by simulation code.

AC Loss Characteristics of the KSTAR CSMC Estimated by Pulse Test

The KSTAR Central Solenoid Model Coils (CSMC), which are in the form of split coils with same dimension, have been tested. The CSMC were successfully charged up to 20 kA and down to zero with different ramp rates. Various pulse waveforms were applied to the CSMC to analyse the AC-loss characteristics of the coils. The measurement method was a gas-flow calorimetry. In this work, two types of waveforms, the DC-biased sinusoidal wave (Idc=2, 4 kA; DeltaIac=1 kA; f=0.08~0.67 Hz) and a single triangular pulse (Imax=6~10 kA; dI/dt=0.5~2 kA/s), are selected and analysed. From the measured data at the one outlet where helium channel 3 and 4 are merged, the AC loss parameters, related with the hysteresis loss and the coupling loss, are calculated. The B-field strength differs depending on the position within the cooling channels. The spatial field variation and the ramp rate were 0.048~1.9 T and 8.1~ 71 mT/s for the DC-biased sinusoidal waves, while 0.29~ 4.3 T and 0.024~ 0.86 T/s for the triangular pulses. The measured AC losses are compared with estimated values and the behavior agrees well. The coupling time constant (ntau) varies with the field strength. In case of the triangular pulses, ntau increases by increasing the field amplitude and the maximum value of ntau is 41 ms with Imax=10 kA

Construction and Commissioning of the KSTAR Current Feeder System

The function of the current feeder system (CFS) is for conducting large currents from the power supplies to the KSTAR superconducting (SC) magnets. The CFS consists of SC bus-lines, joints, and current leads. The bus-line conductor is a circular cable-in-conduit conductor (CICC), which consists of a 4.5 mm thick stainless steel 316L seamless pipe containing 324 strands of chrome coated NbTi superconductor and 243 strands of OFHC. The ends of the CICC are assembled with specially designed lap joints. The joining resistance is controlled to less than 2.5 nano-ohm to minimize Joule heating. The outer surfaces of the CICC were electrically insulated up to 15 kV with jackets made of Kapton film and prepreged E-glass tape. Helically wrapped conducting fiber was used to measure the voltages of bus-line quenches. Two pairs of prototype brass leads for poloidal field (PF) and toroidal field (TF) coils have been fabricated and tested up to the currents of 26 kA for the PF leads and 35 kA for the TF leads. The test results satisfied all the requirements so that all 18 leads were manufactured and assembled on site. This paper will describe the detailed manufacturing progress and commissioning results of the KSTAR CFS.

Experimental study on current re-distribution and stability of multi-strand superconducting cables

Stability of multi-strand superconducting cables against local disturbances should be influenced by the current re-distribution among strands in quench or recovery processes. The current re-distribution and its influence on stability are examined experimentally with two types of three-strand cable. One is made of chrome-plated strands, and another is made of strands with bare-copper surface. A heat pulse is applied to short part of a strand, and normal voltage and current in each strand are measured. The large contact resistance between strands in chrome-plated cable affects the current re-distribution. The minimum heater energy required for quench is measured. The experimental results prove that the stability against local disturbances is improved by the current re-distribution.

Quench Simulation and Detection in KSTAR PF Magnet System

To detect quenches in the Poloidal Field (PF) magnet system is more difficult than the Toroidal Field (TF) magnet system due to excessively high inductive voltages generated by PF pulse-currents and plasma currents. According to reference scenarios being considered so far, the maximum voltage across the PF coils is inductively generated up to about 3.5 kV during the start of plasma (SoP) stage in a very short time period. The voltage measured by compensation of the inductive voltage should be below a certain level which is called as the quench voltage threshold. However, the compensated voltage might be higher than the threshold even with the well-designed compensation schemes. Accordingly, the quench voltage threshold and the quench protection delay time should be properly determined for the quench detection not to take a false action which could cause the fast energy discharge. From the quench simulation using the calculation of hot spot temperature and the resistive voltage growth as a function of time, the proper values of the quench detection parameters of the PF magnet system were derived for the maximum hot temperature rise to be limited within 150 K.

KSTAR Magnetic Field Measurement During the Commissioning and Remanent Field Evaluation

An in situ measurement of the magnetic field generated by the assembled superconducting magnet coils was held by using precision Hall sensors during the commissioning of the Korea Superconducting Tokamak Advanced Research (KSTAR) device. This was done in order to investigate the magnetic influence of Incoloy 908, which is the jacket material for the cable-in-conduit conductors (CICCs) of the Nb3Sn coils. After the PF coils were discharged from 1 kA the vertical remanent field at the plasma center was more than 10 G, while the TF coils were not energized. The vertical magnetic field generated by the PF coils had a discrepancy of up to 50 G between measurement and the calculation assuming no magnetic influence of Incoloy 908. Thus, non-negligible ferromagnetism was identified and attributed to Incoloy 908. In contrast, most of the hysteresis observed in the magnetic measurements was eliminated when the Incoloy 908 of the TF windings was saturated by the TF coil charging.

Performance of the Quench Detection System for the KSTAR CS Magnet System

The Korea Superconducting Tokamak Advanced Research (KSTAR) Central Solenoid (CS) magnet system coils are made of Cable-In-Conduit Conductors (CICC) that contains 240 strands and 120 copper strands inside an Incoloy 908 jacket. It consists of 4 symmetric pairs of coils to the equatorial plane. Each coil is wound with no internal joints by the continuous pancake winding method. It operates with pulsing current, which naturally induces inductive voltages across coils. Minimizing the inductive voltages is critical for the quench detection. To suppress the inductive voltages, each coil voltage was measured by using conductive tapes that are wound at the outer surface of the jacket with the same pitch length of the final sub-cable. During the KSTAR campaigns, the voltages were collected and analyzed. In addition, more rejection schemes were applied to enhance the stability and reliability of the quench detection. The paper deals with the up-to-date quench detection method of the KSTAR CS magnet system and its experimental results.