Hyun Jung Lee

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.

Analysis of the Helium Behavior Due to AC Losses in the KSTAR Superconducting Coils

The KSTAR superconducting magnetic coils, which are made of cable in-conduit conductor (CICC), maintain a superconducting state with forced-flow supercritical helium (4.5 K, 5.5 bar). During current changing of the superconducting magnetic coils, AC losses are generated in the CICC due to dl/dt, and the heat generated from the loss is removed by high heat capacity supercritical helium. At the same time, reversed flow of the helium occurs due to a rapid increase of the helium temperature and momentary changing of the pressure inside the CICC. This phenomenon has been detected in all of the poloidal field (PF) coils, especially in the upper (U) and lower (L) PF1~PF4 coils. The maximum change of the magnetic field in the PF1UL~PF4UL coils is located near the inlet and outlet of the helium cooling channels, and that of the PF5UL~7UL coils is located at the center of the cooling channel. The temperature variation at the helium inlet was always measured to have a time delay after each shot. In the PF1 coil tests, it was measured to have a delay of 26 sec. During the first plasma campaign, this phenomenon was more severe in the case of all PF coils operating together than for a single PF operation. In this paper, we investigated the thermal-hydraulics of this phenomenon.

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.

The AC Loss Measurement of the KSTAR PF1 Coils During the First Commissioning

The AC loss in a large superconducting magnet coil shows a tendency to be changed after assembly. For the ac loss measurement of the Korea Superconducting Tokamak Advanced Research (KSTAR) superconducting coils after assembly, several current scenarios, trapezoidal pulses and a DC offset sinusoidal pulses, were applied to the PF1 Upper (U) and Lower (L) coils during the commissioning. The measurement was done once before and once after the plasma discharge experiments. All coil data were obtained by the tokamak monitoring system and helium distribution system which were designed to measure temperature, pressure, and mass-flow at both inlets and outlets of the coils. The PF1 coils of the cable-in-conduit conductor type were made of Nb3Sn superconducting strands, whose winding scheme is 20 layers with each layer having 9 turns. Each has 10 cooling channels for the heat removal by the supercritical helium at 4.5 K. For the trapezoidal pulse tests, the current was increased up to 4 kA with several different ramp rates and a 2 kA DC offset and 0.5 kA sine waves with different frequencies from 0.1 Hz to 0.3 Hz were applied to the coil. According to the analyses, the AC loss was slightly decreased for the same condition after 700 plasma shots. It was believe that such a result was due to reduced inter-strand resistances which changed the transverse resistance between the inter-strands. The coupling time constant was estimated to be 32 ms in the trapezoidal tests and 13.6 ms for the DC offset sinusoidal pulses. The former is larger than the latter because of the effect of the jacket eddy current loss due to Incoloy 908 which is a ferromagnetic material. Considering the jacket eddy current losses, the coupling time constant was recalculated and the value estimated to be about 13 ms for all current wave forms during first commissioning.

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.

Stability Analysis of the KSTAR PF Busline

The Cable-In-Conduit Conductor (CICC) for the KSTAR buslines is made of NbTi superconducting (SC) strands. A busline consists of several electrical joints, which are the major heat load contributors to the busline cryo-system. In the poloidal field (PF) busline helium circuit, the supercritical helium is fed to the electrical joint of current lead end and comes out to the magnet terminal joint. This helium flow configuration has been verified to maintain the cryogenic stability of the buslines through the KSTAR operation. During the normal operation of the KSTAR PF coil, the heated helium coming out to both the coil and the busline meets at the magnet terminal and exchange heat, but the busline outlet temperature still remained less than magnet outlet temperature. As the buslines for the electrical connection in series of the upper and lower coils for PF1 and PF2 have the helium path through the two terminal joints of magnet, they experience higher temperature than the other buslines mainly due to the larger heat exchange. In this case, the connection buslines are considered to have very low safety margin and have the strong possibility of quenches.

Estimation of Operational Stability for the KSTAR TF Magnet

During the operation of the Korea Superconducting Tokamak Advanced Research (KSTAR), there exist various disturbances that cause the conductors to become unstable. The KSTAR Toroidal Magnet (TF) system was designed to secure the sufficient stability against them. The stability analysis showed that the energy margin of the TF Cable-In-Conduit (CIC) conductor was sufficiently high so that the expected disturbances could not cause the conductor to quench. During the 2nd KSTAR campaign in 2009, the cryogenic stability of the TF magnet system was analysed by energizing it up to 36 kA that is a bit higher than the designed current of 35.2 kA. The temperature increase measured at helium outlets was less than 0.1 K, which was well consistent with the analysis. Even in the reference plasma scenario, it was expected from the analysis that there still exists the sufficient temperature margin more than 4 K. The quench analysis was carried out to validate the design of the present TF CIC conductor from the magnet protections point of view. In this paper, the cryogenic stability of the TF magnet system was estimated and the quench detection parameters were derived for protecting possible damages from quenches.

Analysis of the Reversal Flow Phenomenon of Supercritical Helium Due to AC Losses in the KSTAR PF Magnets at the Low Current

Superconducting magnets of the Korea Superconducting Tokamak Advanced Research (KSTAR) are cooled by supercritical helium with 4.5 K, which was supplied and recovered by the 9 kW of the Helium Refrigerator System (HRS). While current is being charged, the supercritical helium expands to both side of the helium inlet and the outlet of the magnets due to the generated AC losses. To maintain the pressure gradient, both the supply and the return pressures of the HRS are increased at the same time and the differential pressure of the HRS was reduced after the event. However, the pressure rising in the magnets may block the helium flow or create reversal flow of the helium. During unipolar experiment of PF1 magnet up to 2 kA with 1 kA/s of ramp-up rate, the mass flow rate was decreased at the PF1 cooling tube (manifold) in the helium distribution system (HDS), whereas the pressure is increased and the temperature is to be increased or decreased according to compression and expansion of the heated helium in the magnets. For the bipolar experiment of PF1 up to ±2 kA with 1 kA/s ramp-up rate and 2 kA/s ramp-down rate, the conditions in the helium flow were drastically changed, especially the mass flow rate was measured to be maintained at zero for a few second (more than 4 s). This behavior could decisively affect the cryogenic stabilities in the magnet, and may impose a major limit on the long pulse operation of KSTAR. In this paper, we investigated this behavior and analysed by using 1-dimentional thermo-hydraulic code, GANDALF.