Myunghun Kang

Current Estimation of an HTS BSCCO Magnet Having Multiple Power Sources Based on the Field Dependent

Due to the anisotropy of a tape-shaped HTS wire, the method to determine the operating current of an HTS magnet needs to be different from that of a LTS magnet. This paper presented estimation of the critical current of pancake windings excited by multiple power sources. Critical currents were determined by using field dependent E-J relation. A new method to define the magnetic field of the HTS wire has been proposed in this paper. Calculated critical currents of pancake windings were compared with measured ones of the HTS BSCCO magnet consisting of 10 pancake windings. Effects of multiple power excitation on the increment of the central magnetic field has been also described. Test results showed the estimated critical currents of pancake windings agreed well with that of measured ones and the central magnetic field was increased at multiple power excitation by 28.4% comparing with that of single power excitation.

E{\hbox{–}}J

When the HTS magnet consisting of pancake windings is excited by a single power source, most of the pancake windings operate under their critical currents because currents of the entire pancake windings are limited by the critical current of the pancake winding which has the lowest critical current. By using multiple power sources, all pancake windings are able to conduct their critical currents. This paper describes the HTS magnet with pancake windings which are excited by multiple power sources. Critical current of each pancake winding is determined by using optimization technique. Fabrication of the BSCCO magnet consisting of 10 pancake windings is described and test results of the BSCCO magnet are given. The magnetic field and the perpendicular magnetic field of the magnet excited by multiple power sources are compared with those of the magnet excited by a single power source.

Relation

HTS pancake windings have been commonly used for the insert coil of high field magnets. All pancake windings have been connected in series and excited by a single power source. In that case, currents of the whole pancake windings were limited by the minimum current of the top and the bottom pancake windings where maximum perpendicular magnetic field was applied. This paper proposed the multiple power excitation of the HTS magnet which consists of pancake windings. It was able to increase the magnetic field of the magnet by conducting critical currents to all pancake windings. Results of calculation by using a model BSCCO magnet shows that proposed method was able to generate higher magnetic field comparing with the conventional single power excitation.

Properties of an HTS Magnet With Pancake Windings Excited by Multiple Power Sources

In a high temperature superconducting magnet consisting of pancake windings, the perpendicular magnetic field considerably reduces the value of the critical current of the outer pancake windings due to anisotropy. An air gap was inserted between each pancake winding in this paper to reduce the decrement of the critical current in each pancake winding. In a low temperature superconducting magnet, the central magnetic field decreases when there is an air gap between the pancake windings. On the other hand, the central magnetic field of a HTS magnet increases when an air gap is provided. The properties of the HTS insert/outsert magnet having an air gap between the pancake windings are examined in this paper. YBCO wire and BSCCO wire were used in the insert and the outsert magnets, respectively. An E - J relation and the evolution strategy were adopted to calculate the optimum critical currents of both magnets. The calculation results showed that there was an optimum air gap which maximized the central magnetic field. The optimum air gap was dependent on the specifications of the HTS magnet.

Design Method of a High Field HTS Magnet Consisting of Pancake Windings Having Different Current in Each Pancake Winding

Wide 12-mm high-temperature superconducting (HTS) tapes are being used for HTS windings. The critical current of an HTS winding with a 12-mm HTS tape is calculated in this paper, with consideration of the nonuniform current distribution at the cross-section of the HTS tape. To calculate the nonuniform current distribution, the cross-section of the HTS tape is divided into small elements. The current at each element is calculated by using the Ic - B relation and n-value, which are measured in this paper. A pancake winding wound with a 12-mm YBCO tape is constructed and tested to confirm the validity of the proposed calculation method. The minimum critical current of the 12-mm YBCO tape is 240 A, and the number of turns of the pancake winding is 100 turns. The results of test indicate the need to consider the nonuniform current distribution in the calculation of the critical current of a pancake winding using the 12-mm HTS tape. Unlike the 4-mm HTS tape, the 12-mm HTS tape shows that the current is limited at the innermost turn of the pancake winding.

The Optimal Air Gap Between the Pancake Windings in a HTS Magnet Consisting of Insert and Outsert Magnet

In the HTS magnet consisting of pancake windings, all pancake windings have been connected in series and excited by a single power source. Most of the pancake windings cannot conduct their critical current values at single power excitation. Current of each winding can be controlled separately to its critical current at multiple power excitation. This paper presents the excitation of an HTS YBCO magnet consisting of pancake windings by multiple power sources. To estimate the critical currents of the pancake windings more precisely, the magnetic field variation in the HTS wire was considered. Properties of multiple power excitation were measured by using the HTS YBCO magnet consisting of 8 pancake windings. Test results showed calculated currents agreed very well with measured ones and central magnetic field increased by multiple power excitation.

Critical Current of the HTS Winding Wound With 12 mm YBCO HTS Tape

In a High Temperature Superconducting (HTS) magnet consisting of pancake windings, HTS tape with the same width has been used for all pancake windings. If a wider HTS tape is used for the outer pancake windings, the critical current of the HTS magnet can be increased. This paper shows the properties of BSCCO magnets in which the outer pancake windings are wound with a wider HTS tape. In order to examine the effects of a wider HTS tape, a HTS magnet with an inner radius of 20 mm and a height of 202.4 mm was used. The number of turns of each pancake winding was 150, and the number of pancake windings was 46. The critical current of the pancake winding was determined by using E-J relation. Results of calculation show that the critical current was higher in the HTS magnet with a wider HTS tape at the outer pancake windings than in the HTS magnet with the same width of HTS tape for all pancake windings.

Increment of the Central Magnetic Field of an YBCO Magnet by Using Multiple Power Sources

In large actuators for water gate control of hydraulic dams, precise position measurement of the actuator cylinder is very important. An inductance type sensor can be used to measure the position of the actuator cylinder. This paper presents the effects of the slot shape on the properties of the inductance type sensor consisting of a permanent magnet and pick-up coils. The inductance type sensor is able to measure the position of actuator travelling over 10 m with 0.5 mm resolution. Result of the calculation is able to provide the data for choosing the appropriate shape of the slots to produce the proper magnitude of flux linkage and to minimize the mechanical stress imposed on the cylinder.

Characteristics of Magnets Depending on the Width of High Temperature Superconducting Tapes Applied to the Outer Pancake Windings

This paper examined the properties of an high-temperature superconducting (HTS) magnet, such as the Ic-B relation and critical current, with pancake windings of a 12-mm-wide Zr-doped HTS tape. Considering the width of the Zr-doped HTS tape was 12 mm, which was three times wider than that of a conventional 4 mm tape, the current distribution at the cross-section of the 12-mm tape was assumed to be non-uniform. The HTS magnet consisting of four pancake windings of the 12-mm tape was chosen as the model for calculations and measurements. The inner diameter and the number of turns of the pancake winding were 30 mm and 88 turns, respectively. When the critical current of the HTS magnet was calculated assuming the same critical current for all pancake windings, the critical current of the whole magnet was limited by the middle pancake windings. The limiting of the magnet current by the middle pancake windings was due to properties of the Zr-doped HTS tape. After allocating the pancake windings with higher critical current to the middle part of the HTS magnet, the critical current was limited by the top and bottom pancake windings.

Properties of an inductance type sensor for position control of a long stroke actuator

Due to the magnetic anisotropy of a High Temperature Superconducting (HTS) tape, inserting a gap between pancake windings can increase the central magnetic field and improve the field uniformity of HTS magnets consisting of pancake windings. Although inserting gaps with constant length can increase the central magnetic field, optimizing the length of each gap to maximize the central magnetic field is preferred. This paper shows the test results of HTS magnets with and without optimized gaps. Both magnets consisted of 12 BSCCO-2223 pancake windings. The test results were compared with the calculation results, and confirmed that the central magnetic field of the HTS magnet with optimized gap was increased compared with the HTS magnet without gap. The magnetic field uniformity of the HTS magnet was also improved.