J.H. Joo

The Characteristics of the Normal-Zone Propagation of the HTS Coils With Inserted Cu Tape Instead of Electrical Insulation

In the case of motors and generators, the benefits of using superconductors could be represented by the reduction of 50% in both losses and sizes compared to the conventional machines of the same rating. The high temperature superconducting (HTS) coils wound with HTS conductors have very low normal zone propagation velocity (NZPV) and complicated quench behaviors because the conductors have a low thermal conductivity and large specific heat. According to these reasons, in case of HTS coils and magnets, it is difficult to expect the self-protection like as low temperature superconducting (LTS) coils. In this paper, we suggest a method which inserts the Cu tape between turn-turn winding in the HTS coil instead of electrical insulation to improve the self-protection ability of HTS coils. Because the thermal and electrical contacts along the transverse direction are enhanced by inserted Cu tape instead of insulator and the whole thermal stability of HTS coils would be increased. To clarify the quench behavior of this suggested coil, we investigate the normal-zone propagation properties of the fabricated HTS pancake coils wound with 2G conductor and Cu tape under the cryocooled operation using GM cryocooler. The effects of inserted Cu tape and the improved the self-protection ability for the suggested HTS coil will be discussed.

Study on Optimized Configuration of Stacked HTS Bulk Annuli for Compact NMR Application

In design and manufacture of a compact NMR (nuclear magnetic resonance) magnet which consists of a stack of high temperature superconducting (HTS) bulk annuli, spatial homogeneity and temporal stability of trapped magnetic fields are key issues. This paper presents a study on optimized configuration of stacked HTS bulk annuli for the compact NMR application. A three-stack bulk magnet was designed and tested at two different temperatures of 20 and 77 K, and two different background fields of 3 and 5 T. The test results were compared with analytic results to verify the validity of the calculation. Then, various optimized patterns were investigated numerically to find out an optimized stack pattern of HTS bulk annuli in a given condition. The results prove that with the proposed optimized method trapped field strength as well as its homogeneity of a bulk-annulus stack can be improved.

The Dynamic Characteristics of the HTS Bulk Superconducting Actuator Driven With AC Electromagnets

Electric device applications of a high temperature superconducting (HTS) bulk magnet, having stable levitation and suspension properties due to their strong flux pinning force, has been proposed and developed. We have been investigating a three-dimensional (3-D) superconducting actuator using HTS bulks to develop a non-contract transportation device that moves freely in space. It is expected that our proposed 3-D superconducting actuator will be useful as a transporter used in a clean room where silicon wafers which dislike mechanical contact and dust are manufactured. The proposed actuator consists of an HTS bulk (trapped magnetic field by field cooling method) magnet for the mover and two-dimensional arranged multi-pole electromagnets as the stator. Up to now, the electromagnets consisted with ironed cores and copper coils were used as a stator, and each electromagnet was individually controlled using DC supplies. The movement of HTS bulk (mover) was not smooth because of the DC control. In this paper, the new electromagnets driven with AC control were proposed to improve the moving characteristics of the bulks. The characteristics of horizontal movement by newly proposed system will be presented.

The Characteristics of the Normal Transition in the Longitudinal and Transverse Directions in Cryocooled YBCO Coils

Recently, the superconducting devices using cryogenic and superconducting technologies were widespread in the field of electric power. The 2G wires with YBCO superconducting layer, metal substrate and stabilizer layer are expected to be used in future superconducting applications, because it has high Jc properties in high temperature and high magnetic fields. In order to operate safely at high transport currents, the thermal stability of the wounding wire should be addressed in terms of detection and protection against the quench. On the other hand, the understanding of the 3-dimensional normal zone expansion during a quench is very important to design the superconducting devices such as magnets and cables. This paper presents the results of an experimental study for normal zone propagation properties in the longitudinal and transverse directions using the pancake coil wound with 2G wires which have Cu and SUS stabilizers under the cryocooler cooling. And the minimum quench energy (MQE) of the pancake coils wound with two different kinds of stabilizers as functions of operating temperature and transport current will be described.

Characteristics of a 3-D HTS Actuator With Various Shaped Electromagnets

We have been investigating a three-dimensional (3-D) superconducting actuator using HTS bulk to develop the transportation device with non-contact and move in free space. Proposed superconducting actuator consists of the stator constructed by two-dimensional arranged electromagnets and HTS bulk superconductor as a mover. The 2-D arranged electromagnets were individually controlled to move the HTS bulk. It is expected that our proposed 3-D superconducting actuator will be useful as a transporter used in clean rooms where silicon wafers are manufactured. Wafers must be protected from contact with dust and other contamination. To realize the superconducting actuator with HTS bulks, the electromagnetic characteristics of HTS bulk, such as trapped field strength and patterns, should be clarified and the optimized controlling system is also required. Therefore, in this study, to construct the optimal 3-D actuator system, the fundamental characteristics of the levitation for HTS bulks with individually controlled four and eight-polarities electromagnets were investigated experimentally. And two kinds of cap cores attached the top of the core to improve the control ability of proposed actuator were fabricated and tested.

Transport current loss and I/sub c/ degradation of HTS tapes under mechanical load

Performance of an HTS (high temperature superconductor) tape is evaluated by measuring the critical current I/sub c/, AC loss and mechanical properties of the short samples. However, in coils or magnets for applications, the HTS tapes are under mechanical load caused by winding tension, bending and sometimes twisting. Also thermal contraction by the cooling changes the tension. The mechanical load reduces the critical current of the superconducting wire, and the I/sub c/ degradation affects the AC loss of the wire. We measured the I/sub c/ degradations and AC losses (transport current loss) of commercial Bi-2223 tapes processed by a PIT (powder in tube) method under tension. The I/sub c/ and AC loss measurements are done in the same time with the same voltage taps on the tape conductors. The measurement results and discussions on the relationship between I/sub c/ degradation and AC loss are presented.

Study on Quench Protection Method With Regards to Normal Transition Behavior for HTS Coil

This paper presents the quench detection method with using normal transition behavior for HTS coil. If the HTS coil starts to propagate a normal zone by an initial quench, it will be recovered the superconducting state or runaway thermally. These behaviors were mainly determined by variety parameters like the magnitude of a disturbance, HTS coil size, the structure of winding wire and cooling condition. However, the first step of detection or protection for coils should be started from detecting the presence of a non-recovering normal zone. This paper classifies the initial quench behaviors of HTS wires or coils as a model of 3-type tendencies, and suggests suitable detection techniques for each tendency with using the end-to-end voltage of coil. In additional, this method is able to detect a quench without a decisive criterion used at other quench detection methods. In order to confirm the validity of the method, we used the HTS sample coil wound with YBCO tapes. This paper presents these results and the quench protection properties with fabricated detection method.

Development of Quench Detection Method Based on Normal Transition Behaviors for HTS Coils

Recently, the superconducting devices using cryogenic and superconducting technologies were widespread in the field of electric powers. The quench protection is one of the important issues for safety operating of the superconducting devices, because the excessive joule heating brings about damage to the superconducting magnets when the quench occurs. However, if the HTS coil starts to transit normally by an initial quench, it will be recovered into the superconducting state or runaway thermally. These behaviors were mainly determined by the magnitude of a disturbance, HTS coil size, the structure of winding wire and cooling condition. Therefore, in order to detect the transition into the normal state at HTS coil more accurately, it needs to investigate these effects as parameters. This paper describes the electromagnetic and thermal behaviors of HTS during normal transition by the initial quench. The aim is the development of the quench detection method about HTS coil for 5 MW HTS motor after an initial quench. It adopted the hardware and the software that were able to estimate the quench by monitoring the voltage and temperature for the behavior between the thermal runaway and the recovery. This paper will be presented these results and the quench protection properties with fabricated detection method.

Effect of winding direction on four-layer HTS power transmission cable

Abstract A high temperature superconductor (HTS) power transmission cable is usually made of several layers of helically wound superconductor tapes. The current distribution among the conductor tapes is controlled mainly by pitches and winding directions of the layers, because the inductance of the layer is determined by the pitch and the winding direction. Two winding directions––right and left––are possible for each layer, and we can list up many possible combinations of winding directions. If we choose a good combination we can find the optimal pitches and make an even current distribution. However, with a bad combination of the winding direction, it is almost impossible to find a solution. We developed a software code to calculate the currents of the layers. With this software we studied the effect of the winding direction on a four-layer cable by a statistical way. A brief description of the software, calculation results and discussions will be presented.

Study on the Electrical Contact Resistance Properties With Various Winding Torques for Noninsulated HTS Coils

In the case of motors and generators, the benefits of using high-temperature superconducting (HTS) coils can be represented by the reduction of 50% in both losses and sizes compared to conventional machines. However, it is hard to establish quench detection and protection devices for the HTS coils applied to the rotors of motors and generators. Therefore, the stability of the coils is lower than for the quiescent coils applied to NMR, MRI, and so on. Therefore, it is important to improve the self-protection ability of HTS coils. We have studied the methods to improve the self-protection ability of HTS coils by removing the turn-to-turn insulation and inserting metal tape instead of the electrical insulation. When the coils are energized, there are voltages generated by sweep rate of the current (di/dt) and inductance of the coil. Because noninsulated HTS coils have electrical contacts along the transverse direction, a bypass current is generated along the transverse direction and noninsulated HTS coils have a risk of Joule heating in the stabilizer and metal substrate. Furthermore, the electrical contact resistance of noninsulated HTS coils was changed by a winding torque. In this paper, we measured the electrical contact resistance along the transverse direction with various winding torques for noninsulated HTS coils.