Min Cheol Ahn

Spatial and Temporal Variations of a Screening Current Induced Magnetic Field in a Double-Pancake HTS Insert of an LTS/HTS NMR Magnet

This paper presents experimental and simulation results of a screening current induced magnetic field (SCF) in a high temperature superconductor (HTS) insert that constitutes a low-/high-temperature superconductor (LTS/HTS) NMR magnet. In this experiment, the HTS insert, a stack of 50 double-pancake coils, each wound with Bi2223 tape, was operated at 77 K. A screening current was induced in the HTS insert by three magnetic field sources: 1) a self field from the HTS insert; 2) an external field from a 5-T background magnet; and 3) combinations of 1) and 2). For each field excitation, which induced an SCF, its axial field distribution and temporal variations were measured and compared with simulation results based on the critical state model. Agreement on field profile between experiment and simulation is satisfactory but more work is needed to make the simulation useful for designing shim coils that will cancel the SCF.

Analysis of a Joint Method Between Superconducting YBCO Coated Conductors

This paper deals with an analysis on the optimum lap joint method of the YBCO coated conductor (CC) wire. Effective low resistance joint is important for the development of persistent current mode system using the second generation high temperature superconducting (2G HTS) wire, YBCO CC. CC tape is stacked with copper or stainless steel stabilizer, silver layer, YBCO layer, buffer and substrate. The joint between CCs was performed with low temperature InBi solder. YBCO samples with various contact conditions were fabricated and tested. Characteristics of jointed CC tape were evaluated from the V-I curve and contact resistance was derived from experimental results. Characteristics of jointed CC were investigated by electrical analysis and optical micrograph. The BSCCO wires were jointed at different conditions, and the results were compared with the joint between the YBCO CCs.

The effects of co-wound Kapton, stainless steel and copper, in comparison with no insulation, on the time constant and stability of GdBCO pancake coils

Recently, studies of partially insulated, high-temperature superconducting (HTS) coils have shown application in the design and construction of compact, stable and self-protecting HTS coils. This article presents the electrical characteristics of HTS coils based on the turn-to-turn inserted materials and conditions. Three partially co-wound pancake coils were fabricated and tested. Each coil was partially co-wound with Kapton, stainless steel and copper tape at every fourth turn of the winding. Tested coils were co-wound on every turn with Kapton, stainless steel and copper tape, and coils without turn-to-turn insulation were the control group. Charge–discharge, sudden-discharge and over-current tests were performed to evaluate the performance of the fabricated coils. The experimental results show that the properties of materials inserted into the coil can control the time constant (τ). Therefore, HTS coils can be designed for specific purposes according to the time constant control. The experimental results of the study could be useful in designing HTS coil applications.

Operation and performance analyses of 350 and 700 MHz low-/high-temperature superconductor nuclear magnetic resonance magnets: A march toward operating frequencies above 1 GHz

Since 2000, a three-phase program with a final goal to complete a 1 GHz high-resolution low-/high-temperature superconductor (LTS/HTS) nuclear magnetic resonance (NMR) magnet has been conducted at the Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology (MIT). In a LTS/HTS magnet assembly, a HTS insert is placed in the cold bore of a LTS background magnet. To date, two LTS/HTS magnets have been designed, constructed, and tested: a 350 MHz (LH350) in phase 1 and a 700 MHz (LH700) in phase 2. The program’s target has recently been upgraded from the original goal of 1 GHz to a new goal of 1.3 GHz. In this paper, we present extensive performance analyses of the two LTS/HTS NMR magnets. Spatial homogeneity and temporal stability of LH350 and LH700, examined with harmonic analysis, and four key issues that became evident in the operation of these two magnets are discussed: (1) field constant reduction, (2) “large” residual Z1 gradient and its temporal decay, (3) large one-periodic tesseral fi...

Joint Characteristics of YBCO Coated Conductor by Removing a Metallic Stabilizer

This paper presents the development status of joint method between the superconducting YBCO coated conductors (YBCO CC). In general, the YBCO-CC tape consists of an upper stabilizer, substrate, buffer, YBCO and a bottom stabilizer. It is suggested that the stabilizer between the two YBCO-CC tapes need to be removed, in order to improve the characteristics of the joints. The removing process was conducted by using heat and partial etching. The resistance and critical current of the joints were measured from the curve. The optical micrographs were analyzed for microstructures between the joint areas. After several times of over current test, the critical currents of the various types of joint samples were examined for applying to the high temperature superconducting magnet system.

Development of 220 V/300 A Class Non-Inductive Winding Type Fault Current Limiter Using 2G HTS Wire

As a part of the 21st Century Frontier R&D Program in Korea being performed from 2004, a non-inductive winding type superconducting fault current limiter (SFCL) is being developed. The target of the second year in phase II of the program is to develop a 220 V/300 A class non-inductive winding type SFCL as a prototype for a 13.2 kV/630 A class, the final goal of phase II. This SFCL has three solenoid type non-inductively wound coils in series using a 2G high temperature superconducting (HTS) wire and it was tested in sub-cooled nitrogen of 65 K, 1 atm. A coil which is composed of four parallel windings in a bobbin and winding directions are opposite to have non-inductive characteristics. Three coils were connected in series and the total length of 108 m of 2G HTS wire was used. Short-circuit tests were performed at applied voltage of 220 V and the SFCL limited the fault current to a few kA extents at the tests. Recovery time of the SFCL was measured after short-circuit tests.

Normal Zone Propagation in 2-Dimensional YBCO Winding Pack Models

This paper presents the results of a study of 2-dimensional normal zone propagation (NZP) in winding pack models comprised of 12 straight YBCO tapes stacked with wet, i.e., epoxied, insulating spacers. In each winding pack, the YBCO tapes were connected electrically in series in such a way that a transport current through the YBCO tapes zigzagged from one end of the winding pack to the other end of the winding pack. An insulated heater strip placed at the middle of the stack was energized to trigger a quench in YBCO tapes with a combination of heater taps which permits the length of the heated section to be varied for 1- or 2-dimensional NZP. Voltage taps of various lengths were placed within the winding packs to measure the transverse and longitudinal NZP speeds. Measurements with wet-spacers in which a large contact thermal resistance at each YBCO tape and spacer interface is chiefly responsible for very slow transverse NZP velocities, effectively eliminate the interface thermal resistance, resulting in faster transverse NZP speeds. Numerical simulation that neglects interface thermal resistance agrees quite well with experiment.

A Study on the Design of the Stabilizer of Coated Conductor for Applying to SFCL

A superconducting fault current limiter (SFCL) using high temperature superconducting (HTS) wire has been developed in Korea. While the first-generation wire has difficulty in adopting various matrices because it is made by PIT process, the second-generation wire has flexibility in the design of its stabilizer. In general, coated conductor (CC) tape is clad with a thick metal layer as a stabilizer on the superconducting layer such as YBCO. In addition, the metal layer has good electrical conductivity. However, resistive SFCL, when a fault occurs, limits fault current using high resistance of the stabilizer. Therefore, the design of stabilizer is one of the most important parts in the design of SFCL. In this paper, over-current characteristics of CC tapes clad with stabilizers having various specifications were experimentally investigated. It was possible to design the stabilizer of CC based on calculation. An SFCL rated on 13.2 kV/630 A was designed using the optimal design of the stabilizer.

Manufacture and Test of Small-Scale Superconducting Fault Current Limiter by Using the Bifilar Winding of Coated Conductor

Superconducting fault current limiters (SFCLs) have been developed by many research groups. However, there is no standard for current limiting device. Recently, YBCO coated conductor (C.C.) which is named as 2nd-generation wire has been developed rapidly. YBCO C.C. has many advantages for applying to fault current limiting material. In this paper, a bifilar winding type SFCL was manufactured using YBCO C.C. The bifilar coil was wound as pancake type, and the length of C.C. tape used was 8 m. The short-circuit test of the SFCL was performed successfully rated on 30V/80A. The SFCL had a very low impedance in normal operation and limited the fault current effectively when a fault occurred. From the result, it could be confirmed that the bifilar winding type FCL using YBCO C.C. is feasible. Large-scale SFCL using C.C. should be developed in the future

Determination of Maximum Permissible Temperature Rise Considering Repetitive Over-Current Characteristics of YBCO Coated Conductors

Here we suggest stable maximum temperature criteria of YBCO coated conductors (CCs) considering degradation of YBCO by repetitive exposure to excessive temperature rise. Stable maximum permissible temperature was experimentally determined as the maximum temperature at which CC does not suffer from critical current degradation or burnout. By comparing various over-current characteristics with short-circuit characteristics in regards to the pattern of generated resistance and joule heating flux, maximum permissible temperature rise of YBCO CC in superconducting fault current limiter (SFCL) can be determined as 400 K. Our research is expected to provide rudimental basis for the temperature design of various superconducting power applications including SFCL.