high-T/sub C/ Superconducting Research Articles

<tex>$Q$</tex>-Factor Measurement of Quasi-Optical Dielectric Resonators Under Conditions of the Whispering Gallery Mode Degeneration Removal

An approach that allows one to determine the unloaded quality factor of a quasi-optical dielectric resonator (QDR) under conditions of the resonance line splitting corresponding to twofold degenerative whispering gallery (WG) mode has been proposed. The resonator is represented by an equivalent network as two oscillatory circuits with a certain coupling between them. The approach allows one to determine the Q-factor of a single oscillatory circuit using three measured parameters, namely: 1) ratio between specific amplitudes of the resonator amplitude-frequency response (or its second derivative); 2) resonant frequency; and 3) frequency splitting of the response (or its respective second derivative). The proposed technique is illustrated by the measurements in the millimeter-wave range for sapphire quasi-optical resonators with conducting and high-T/sub c/ superconducting (HTS) film endplates.

Influence of Flux Creep on Dynamic Behavior of Magnetic Levitation Systems With a High-<tex>$T_c$</tex>Superconductor

Significant flux creep may be generated in some high-T/sub c/ superconductors with weak pinning, which could yield an influence on the dynamic behavior of a high-T/sub c/ superconductor-magnet levitation system. To investigate this influence, this article presents a numerical analysis of dynamic features of the levitation generated by an interaction between a high-T/sub c/ superconductor (HTSC) and a permanent magnet (PM) after the flux creep in the superconductor is taken into account in a macro-model of superconductivity. The influence is comprehensively displayed by comparing the predictions of dynamic responses of such systems in which the flux creep in the superconductor is and is not taken into account. The obtained results show that whether or not the flux creep results in a noticeable influence to the levitation of superconductor-magnet systems is mainly dependent upon properties of superconductivity and applied excitation, e.g., critical current density of superconductors, and amplitude and frequency of external excitations. When the critical current density is less than 4.5/spl times/10/sup 8/ A/m/sup 2/, and the system is subjected to a periodic excitation, the influence of flux creep should be taken into account in the theoretical analysis.

Design and Manufacturing of the Large Scale High-<tex>$T_c$</tex>Superconducting DC Magnet for the 2.3 MVA SFCL

To develop a High-T/sub c/ superconducting (HTS) magnet that is applicable to the high voltage electric device applications, it is necessary to consider several important factors such as large critical current, reliable electric insulation system, stable cryogenic status, expected economic benefits, and so on. This paper deals with the skills to design and manufacture the high voltage HTS DC magnet accepting large pulse input current. The multi-stacked HTS solenoid winding was suggested for the HTS magnet having large critical current. To protect the HTS winding, when quenched, the copper layer was suggested as a buffer sharing the large pulse current. The thin grooves on the GFRP bobbins not only guide the HTS winding but also play the role of electrical insulation for the high voltage application. With regard to the thermal stability of the magnet, sub-cooled liquid nitrogen cooling system was used for the cooling system of the HTS DC magnet. To reduce the resistance of the normal conducting parts in the HTS magnet, several HTS tapes were additionally attached on these parts. Based on this study, the HTS DC magnet for the 2.3 MVA class high-T/sub c/ superconducting fault current limiter (HTSFCL) was successfully manufactured and tested. The HTS DC magnet developed by this study endured 2.53 MJ (18.66 MW/sub max/) without any evidence of quench.

Influence of Pitch Length and Winding Direction on Four-Layer HTSC Power Transmission Cable With a Shield Layer

The design for even current sharing in high-T/sub C/ superconducting (HTSC) power transmission cable is required to increase the current transmission capacity and decrease the AC loss. Increasing the contact resistance of each layer is effective method for achieving the uniform current distribution among conducting layers. However, this method increases the power loss and causes the efficiency of HTSC cable system to decrease. Another method to adjust the pitch length and the winding direction of each layer is complicated, which is difficult for the cable designer to find a simple solution for the uniform current distribution. In this paper, we investigated the current distribution of HTSC cable dependent on the pitch length and the winding direction of each layer for two cases: four-layer HTSC cable with a shield layer and four-layer HTSC cable without a shield layer. It could be found through the analysis from the computer simulations that the shield layer of HTSC cable could contribute to the improvement of current distribution of each layer at the specific pitch length and winding direction. The result and discussion for the current distribution calculated for HTSC power transmission cable with a shield layer were presented and compared with the cable without a shield layer.

Shielding Current Density Analysis of Axisymmetric HTS by Element-Free Galerkin Method

The numerical method for shielding current density in axisymmetric high-T/sub c/ superconductors (HTS) applying the element-free Galerkin (EFG) method is presented. In the present study, the axisymmetric simulation code for analysing the time evolution of the shielding current density on HTS by using EFG is developed, and the shielding performances against the AC/DC-hybrid applied magnetic flux are evaluated by the code. The results of computations show that the shielding ability decreases as the applied magnetic flux of DC mode increases.

Fault Current Limiting Characteristics of Resistive Type SFCL Using a Transformer

A transformer is expected to be an useful component of resistive type superconducting fault current limiter (SFCL) using high-T/sub C/ superconducting (HTSC) element with constant critical current for easier adjustment of both the limiting impedance and the initial limiting current level. Therefore, the analysis for the fault current limiting characteristics of the resistive type SFCL using a transformer considering the resistance variance of HTSC element is needed because the resistance of HTSC element during a fault period is affected by the design condition of the transformer. In this paper, we investigated the limiting impedance and the initial limiting current level of the resistive type SFCL using a transformer as a function of the turn numbers' ratio between the primary winding and the secondary winding. The limiting impedance of SFCL was extracted by applying discrete Fourier transform (DFT) equation for the measured voltage and current data of SFCL. It was confirmed from the analysis that the transformer design with the lower turn numbers' ratio of the primary winding for the secondary winding was advantageous for the larger load current capacity of SFCL and that the transformer with the higher ratio design condition, on the other hand, was favorable for the larger fault current limiting capacity of SFCL.

High-<tex>$rm T_rm c$</tex>Superconducting Receiving Coils for Nuclear Magnetic Resonance Imaging

Nuclear magnetic resonance (NMR) microscopy poses high demands on the sensitivity of the receiver coils. We have developed high-T/sub c/ superconducting (HTS) tape receiving coils for nuclear magnetic resonance imaging. The surface receiver coil is constructed from high-T/sub c/ Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ tape coil and cooled in liquid nitrogen temperature. The desired receiver surface coil is numerically simulated and optimized to have high value of the unloaded quality factor. With this HTS receiver coil we have obtained significant improvement in the unloaded Q-value, loaded Q-value and substantial gain in signal-to-noise ratio (SNR). The SNR improvement of 2.4 was achieved in the kiwi imaging and improvement of 2.7 in the braining imaging of rat at 300 K. The MRI microscopy is tested and compared the results with copper receiver.

Temperature Dependence of Intermodulation Distortion in YBCO: Understanding Nonlinearity

Intermodulation distortion (IMD) in high-T/sub c/ superconductor (HTS) devices continues to be important, and a complete understanding is lacking. We have measured the third-order IMD vs power as a function of temperature from 1.7 K to T/sub c/ for YBCO films of several fabrication methods on lanthanum aluminate and sapphire. The measurements used a stripline resonator at either 1.5 GHz or 2.3 GHz depending on the substrate. The results show increasing IMD with decreasing temperature at low power levels. The IMD diverges nearly as 1/T/sup 2/ for temperatures below 5 K. This temperature and power dependence of the IMD is an observation of the nonlinear Meissner effect that has been predicted for superconductors with d-wave symmetry but has not been previously observed. The results agree with a rigorous, many-body calculation that yields directly the lowest-order correction of the nonlinear penetration depth as a function of the current. We discuss the role of intrinsic and extrinsic properties in the IMD. We also discuss the implications for practical devices operated at usual cryocooler temperatures.

Highly Balanced Long-Baseline Axial Gradiometer Based on High-<tex>$rm T_rm c$</tex>Superconducting Tape

The improving quality of high-T/sub c/ superconducting (HTS) tape with critical current densities larger than 1 MA/cm/sup 2/ creates the possibility to construct high quality flexible superconducting electronics operating at liquid nitrogen temperatures. We patterned a symmetric flux transformer into a 700 nm thick YBa/sub 2/Cu/sub 3/O/sub 7-x/ film on a 70 /spl mu/m thick, 85 mm long Hastelloy tape. The center loop of the transformer was coupled to the pickup loop of a SQUID magnetometer in a flip-chip configuration. The two outer pickup loops of the transformer were bent such that they were facing each other perpendicular to the magnetometer plane. The resulting axial gradiometer has a long baseline of 35 mm and a gradient sensitivity of 7.3 nT/(cm/spl Phi//sub 0/). A superconducting shield was used to reduce uniform magnetic fields applied perpendicular to the magnetometer plane. Common-mode rejection ratios less than 10/sup -4/ were achieved in the best case. The noise-limited gradient field resolution was approximately 330 fT/(cm/spl radic/Hz) at a frequency of 10 Hz. This resolution was mainly limited by the flux noise level of our dc-SQUID magnetometer of approximately 45 /spl mu//spl Phi//sub 0///spl radic/Hz. In an unshielded laboratory environment, external noise contributions at 50 Hz were reduced by a factor of approximately 10/sup 3/.

Impedance Variation of a Flux-Lock Type SFCL Dependent on Winding Direction Between Coil 1 and Coil 2

The flux-lock type superconducting fault current limiter (SFCL) uses the magnetic coupling between two coils connected in parallel with each other. This SFCL has the merit that both the resistance of high-T/sub C/ superconducting (HTSC) element and the limiting current capacity can be simultaneously increased through the application of magnetic field into HTSC element. However, unlike other types SFCLs, the different fault current limiting characteristics can appear according to the winding direction between the two coils. In this paper, the relation of the impedance of the flux-lock type SFCL with the resistance of HTSC element was derived from its equivalent circuit using the current and the voltage relationship between the two coils. The limiting impedances of the flux-lock type SFCL due to the winding direction through the discrete fourier transform (DFT) analysis for the voltage and the current data obtained from the fault current limiting experiments were extracted and analyzed from the point of view of the limited line current and the resistance of HTSC element.

Analysis of Operational Characteristics of Flux-Lock Type SFCL Combined With Power Compensator

The flux-lock type superconducting fault current limiter (SFCL) combined with a power compensator was proposed in this paper. This SFCL consists of a flux-lock reactor and a power compensator. The former, which has coil 1 and coil 2 wound in parallel through high-T/sub C/ superconducting (HTSC) element, can perform the fault current limiting operation during a fault period. The latter is composed of a current-controlled inverter and an AC/DC converter between coil 3 and grid, which can be operated as the power compensator for nonlinear load during a normal operation. The specification for a test model was determined and its operational characteristics were analyzed through computer simulation using PSIM program. It was confirmed that the suggested flux-lock type SFCL, combined with the power compensator, could protect the system from over-current by a short circuit accident and compensate the reactive power due to a nonlinear load during a normal operation as well.

Comparison of Fundamental Performance of Different Type of Fault Current Limiters With Two Air Core Coils

This paper theoretically investigates the difference in the impedance, initial current in the limiting operation and required volume of high T/sub c/ superconductor (HTS) between two types of superconducting fault current limiter with two coaxial air core coils. One of them is a "parallel type" which has been proposed by us and another one is so-called transformer (Tr) type. In the case that the HTS generates the same resistance in each fault current limiter, the larger limiting impedance can be obtained using the Tr type than the parallel type. However, the impedance of the Tr type in the normal operation is also larger than that of the parallel one. In the case that the HTS with the same critical current is used, the initial current in the limiting operation for the parallel type is higher than that for the Tr type. In regard to the HTS volume required to satisfy both of the specified impedance and initial current in the limiting operation, there is no significant difference between the parallel and Tr type.

DC Critical Current Estimate of the Short HTS Cable Consisted of Bi-2223 Tapes

The 4-probe method with a voltage tap on terminals has been used for the measurement of the critical current of multi-strand high-T/sub c/ superconducting (HTS) cables. The critical current of cables is obtained as the measured total current divided by the number of conductors when the terminal voltage exceeds the predetermined criterion of critical current. However, because of the nonuniform current share due to the different critical currents, shapes, and other characteristics of each conductor, this method is not applicable to the multi-strand HTS cable. To determine the critical current of multi-strand HTS cable, the critical current of each conductor must be measured with different method. In this paper, the current share and the critical current of each conductor in multi-strand cable were measured with specially made pick-up coils and voltage taps. It is presented that the real critical current of multi-strand is smaller than sum of each critical current of each path. Different resistances appeared in each HTS wire are the main cause of nonuniform current share.

Magnetization Loss in HTS Stacked Tapes by Various Directional External Magnetic Fields

For large power application using high-T/sub c/ superconducting (HTS) tape, parallel face-to-face stacked tape conductor should to be used because single tape is limited in flowing of demanded large current, also, appropriate distance between tapes in stacked tape conductor is needed for high voltage insulation. The stacked tape conductor has different characteristics compared with single tape because of screening effect. In general situation, HTS tapes are used by winding in coil shape, so, magnetic fields generated in the coil are operated as external fields and have various direction to each tape in the coil. In this paper, we research the magnetization loss in stacked tape conductor exposed to external magnetic fields having various direction and compare measured results of stacked tape with those of single tape, and we also present influences of insulation thickness between tapes in stacked tape conductor by comparison to the measured results for noninsulation stacked tape. We solve the magnetization loss in stacked tape conductor by finite element method to examine the experimental results.

Application of Parallelized SOR Method to Electromagnetic Field Analysis of Superconductors

PC cluster systems are becoming popular in the field of high-performance computing. The authors have been studying the electromagnetic field analysis of high T/sub c/ superconductors (HTS) by the finite element method (FEM) for AC loss estimations. Superconductors are highly nonlinear electromagnetic media, and considerable computation time is required to calculate the temporal evolution of the electromagnetic field distribution in superconductors. In this study, parallel computing techniques were applied to the electromagnetic field analysis of HTSs by the finite element method. In one of the FEM codes used by the authors, the successive over-relaxation (SOR) method, was used to solve a system of equations. This part was parallelized using the multicolor SOR method, and it helped in reducing computation time. Data are explicitly passed between the processor elements (PEs) through an MPI. First, the HTS model was analyzed under one operating condition, using the code implemented multicolor SOR method, using four PEs, and the parallelization efficiency was confirmed. Next, the generality of the efficiency of the multicolor SOR method in our numerical simulation was examined under several operating conditions, using different models.

Determination of Heater Triggering Parameters of Superconducting Power Supply With Series-Connected Double-Pancake Load

This paper presents determination of heater triggering parameters of high-T/sub c/ superconducting (HTS) power supply with series-connected double-pancake load, as well as analysis of its characteristics as determined through experiments. A HTS power supply consists of two heaters, an electromagnet, a Bi-2223 solenoid, and a Bi-2223 series-connected double-pancake load of 6.0 mH. The timing sequential control of two heaters and an electromagnet is an important factor to generate pumping-current in the pancake load. The thermal analysis of switching parts of the Bi-2223 solenoid according to the heater input was carried out. Based upon the analysis, the 8 A electromagnet current and the 1.5 A dc heater current were optimally derived, and 8 s and 20 s for the pumping period were selected in this experiment. In the experiment, the maximum pumping current reached about 24 A.

Cryostat for 66 kV/750 A High-<tex>$rm T_rm c$</tex>Superconducting Fault Current Limiter Magnet

This paper describes a design and experimental results of a sub-cooled nitrogen cryostat for 66 kV/750 A high T/sub c/ superconducting (HTS) fault current limiter (FCL) magnet. Six unit coils were designed to be operated in sub-cooled nitrogen at around 65 K. Three sets of Gifford-McMahon (GM) cryocooler were used for cooling liquid nitrogen in the cryostat. Experimental results of cooling down and temperature uniformity in the vessel at current flowing tests of the magnet were reported. A design for high voltage insulation in a liquid nitrogen vessel and an experimental result of overvoltage test of the cryostat were also reported. This work is as a part of the Super-conductive AC Equipment (Super-ACE) project presently being performed, an AC magnet for a FCL is being developed (Yazawa et al., (2001), Yazawa et al., (2002) and Sakai et al. (2000)).

The Shielding Effect of HTS Power Cable Based on<tex>$E$</tex>-<tex>$J$</tex>Power Law

A method for analysing the current distribution in high-T/sub c/ superconducting (HTS) power cable is examined by the aid of the novel use of anisotropic conductivity and 3-D finite element method considering E-J power law characteristic. The detailed current distribution in the cable is illustrated and the shielding effect of HTS shield layer with intervals is also examined. It is shown that AC losses in shield layer with intervals are increased when the interval between wires becomes large.

Design, Fabrication, and Test of High-Tc Superconducting DC Reactor for Inductive Superconducting Fault Current Limiter

A high-T/sub c/ superconducting (HTS) DC reactor has been developed as a part of DC reactor type superconducting fault current limiter (SFCL) rated on 6.6 kV/200 A. The DC reactor was a solenoid coil and was wound with a 4-parallel stacked tape. This coil has 5 layers which are connected in series each other. The inductance of the coil is about 84 mH. This paper deals with the design, fabrication and testing of the DC reactor. For this design of the coil, a prototype solenoid coil had been used. Using the experimental result of a prototype solenoid, the number of stacks and the inductance were designed. The winding machine for the HTS solenoid was manufactured. Using this machine, the large-scale HTS solenoid using a Bi-2223 tape was fabricated successfully. Characteristics of the fabricated coil were observed through a measurement of voltage as current transportation.

Development of 66 kV/750 A High-<tex>$rm T_rm c$</tex>Superconducting Fault Current Limiter Magnet

One of the items included in the Super-conductive AC Equipment (Super-ACE) project, being performed from 2000 to 2004, is a development of a 66 kV/750 A high-T/sub c/superconducting (HTS) fault current limiter (FCL) magnet. This research focuses on fundamental technical items essential for a 66 kV class fault current limiter, that is, high current capacity, high voltage insulation and sub-cooled nitrogen cooling. This paper describes the design of the magnet and the test results obtained so far. The magnet mainly consists of a vacuum vessel, a nitrogen bath, a pair of current leads, cryocoolers, and six sets of unit-coils wound with Bi2223 tapes. The rated current of each coil is about 125 A at 70 K, and so the total current capacity of the magnet is 750 A. The insulation voltage of the magnet is of the 66 kV class. By the end of FY2002, three sets of the unit coils were set connected in the cryostat and some evaluation tests were implemented as a milestone of the program. In the cooling down test, sub-cooled nitrogen of 65 K was obtained with homogenous temperature distribution in the cryogen. In the overvoltage test, no breakdowns were observed in the case of applying both ac voltage of 140 kV for one minute and lightning impulse voltage of 350 kV. Voltage-current characteristics of the coils were measured in sub-cooled nitrogen. The rated current of 375 A was successfully obtained for both direct and alternate current tests. All the targets constituting the milestone of the 66 kV/750 A magnet development were achieved.