Tsuyoshi Yagai

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.

Micro Power Grid System With SMES and Superconducting Cable Modules Cooled by Liquid Hydrogen

For future power system, a micro power grid system, which is mainly composed of several power modules, such as superconducting (SC) cable, superconducting magnetic energy storage (SMES) system, hydrogen system, fuel cell (FC) system, renewable energy modules, and power converter modules, is expected. In the grid system, hydrogen mainly produced by renewable energy is liquefied for cooling down of the SC cable and SMES, and is stored in a tank for generation of the electric power through the FC. Since the SMES has quick response to power fluctuation and the fuel cell with the hydrogen can supply constant electric power for longer time, the combination of the SMES and the FC can generate highly qualified electric power. The cable can simultaneously transfer both electric power and hydrogen fuel with refrigerant energy. We investigate functions of various power modules and simulate the power balance of the micro grid to estimate the energy recovery rate. It is found that the proposed micro grid can reduce the energy transfer loss.

Vibration Characteristics in Magnetic Levitation Type Seismic Isolation Device Composed of Multiple HTS Bulks and Permanent Magnets

An HTS bulk field-cooled by a permanent magnet can levitate stably without any other control systems. The stable levitation can be realized by a specific characteristic of the HTS bulk that the HTS bulk returns to its original position by restoring force against horizontal displacement. We devised a magnetic levitation type seismic isolation device taking advantage of the specific characteristic of the HTS bulk. This device is quite different from conventional one composed of bearing and damper, because this device can completely remove horizontal vibration. We made a model device composed of YBCO bulks and permanent magnets to verify the seismic isolation effect of our device. Levitation force and vibration stability strongly depend on magnetic stiffness between HTS bulk and permanent magnet. Therefore, we prepared an YBCO bulk and a permanent magnet unit arranged on a row and investigated the characteristics of levitation force and horizontal vibration as functions of vibration amplitude, vibration frequency, and initial air gap between the bulk and the permanent magnet unit. Large initial air gap is desirable for levitation force, while small air gap under operation is effective for securing vibration stability.

Study on Three-Phase Superconducting Fault Current Limiter

Superconducting Fault Current Limiters (SCFCLs) have been intensively developed around the world these years, and the commercial SCFCL is expected to be available in near future. The main target of SCFCL include not only negligible small impedance under normal operation, but also fast and effective suppression of large fault current within the first current rise, and moreover repetitive operation with fast and automatic recovery. We designed a new type of three-phase SCFCL which is composed of a three-phase winding reactor type FCL and a magnetic shield type superconducting FCL. The proposed SCFCL is effective for symmetrical fault as well as unsymmetrical faults. In order to verify functions of the proposed SCFCL, we fabricate a small device and carry out the experiments. It is found from the test results that the proposed new type of three-phase SCFCL works to restrict the fault currents in all kinds of fault conditions. Moreover, the simulation results of EMTP have good agreements with the test results.

Analysis of AC Losses in a Tri-axial Superconducting Cable

High temperature superconducting (HTS) cables have been studied because of low loss and compactness, compared with conventional copper cables. Three-phase cables are usually composed of three single-phase concentric cables. Recently, a tri-axial cable, composed of three concentric phases, has been intensively developed, because it has advantages such as reduced amount of HTS tapes, low leakage fields, low heat leak and compactness, compared with the three single-phase cables. We analysed the three-phase current distributions in the tri-axial cable as functions of winding pitches of three concentric phase layers, and showed the balanced three-phase current distributions in the tri-axial cable. The each layer supplies a transport current under external magnetic field with the same frequency and different phase. We formulate the general form of AC loss of the transport current in combination with the external field with different phase, and analyse the AC loss of the tri-axial cable.

Alfven wave excitation and single-pass ion cyclotron heating in a fast-flowing plasma

Alfven wave excitation and ion heating experiments were performed in a fast flowing plasma. When rf waves in the ion cyclotron range of frequency were excited by right- and left-handed helically wound antennas, shear and compressional Alfven waves with azimuthal mode number m=−1 and m=+1, respectively were excited. The dispersion relations of the propagating waves were obtained experimentally and compared with the theoretical ones, including the Doppler effect of the plasma flow. Strong ion heating was observed in the fast-flowing plasma when rf waves were launched by the right-handed helically wound antenna in a magnetic beach configuration. The plasma thermal energy W⊥ and the ion temperature Ti drastically increased during the rf pulse. This large increase was observed under lower-density conditions, where the ratio of the ion cyclotron frequency to the ion-ion collision frequency becomes high. The resonance magnetic field was affected by the Doppler shift due to the fast-flowing plasma.

Analysis of Balanced Three-Phase Current Distributions in a Tri-AxialCable

High Temperature Superconducting (HTS) cables have been studied because of low loss and compactness, compared with conventional copper cables. Three-phase cables are usually composed of three single-phase coaxial cables. Recently, a tri-axial cable, composed of three concentric phases, has been intensively developed, because it has advantages such as reduced amount of HTS tapes, small leakage fields and small heat loss in leak, compared with the three single-phase cables. However, there is an inherent imbalance in the three-phase currents in the tri-axial cable due to the differences in radii of the three-phase current layers. The imbalance of the currents causes additional loss and large leakage field in the cable, and deteriorates the electric power quality. Therefore, we introduce more variables to obtain the solutions of the balanced three-phase currents and homogeneous current distribution in each phase of the tri-axial cable. We propose a tri-axial cable composed of two longitudinal sections with different twist pitches, and hence the number of variables increases. We derive general equations satisfying both the balanced three-phase currents and homogeneous current distribution, as functions of winding pitches. Finally we apply the general equations to the simplest cable and find out reasonable twist pitches

Vibration Transmission Characteristics Against Vertical Vibration in Magnetic Levitation Type HTS Seismic/Vibration Isolation Device

Using a model levitation system composed of an HTS bulk and permanent magnet rows, we investigated the dynamic characteristics of vibration transmission against a vertical vibration as functions of the weight of a levitating object, vibration amplitude, initial and actual gaps between the bulk and the permanent magnet rows. The bulk vibrated in substantially synchronism with the permanent magnet rows and the waveform of relative displacement between the bulk and the permanent magnet rows was sinusoidal. The vibration transmissibility measured in the frequency range below 5 Hz was between 1.00 and 1.08. Using the experimental results of spring and damping constants, we theoretically evaluated the natural frequency and vibration transmissibility of the model system in the frequency range of 0 Hz to 100 Hz. The natural frequency decreased with the weight of the levitating object at a constant actual gap. This means that the vibration removal performance is improved by increasing the initial gap. The larger actual gap at a constant weight of the levitating object was effective for improving the vibration transmissibility in the vibration frequency range above the natural frequency, while the smaller actual gap was effective for improving the damping effect. Therefore, it is important to choose the most suitable field-cooling condition of the bulk by considering the trade-off relationship between the vibration transmissibility and the damping effect according to the weight of the levitating object.

Fault Current Analysis in a Tri-Axial HTS Cable

Tri-axial cable has been rapidly developed recently because of the advantages such as more compact structure, small leakage field, and low heat and AC losses. One the other hand, it causes an inherent imbalanced three-phase currents distribution due to the different radii of concentric layers. In our previous research, it is demonstrated that a proposed tri-axial cable composed of two longitudinal sections with adjusted twist pitches can transmit balanced three-phase currents. However, before the cable can be installed in a real grid, the heat reliability of the cable in steady-state operation and fault condition should be ensured. The heat balance calculation of the tri-axial cable in steady state under the balanced current distribution allows us to estimate thermal parameters of the cable such as inlet and outlet temperatures, pressure drop and operation temperature. When a phase current suddenly changes under fault conditions, the remaining phase currents become large because of mutual inductances between fault and sound phases. Simulation of the typical fault suggests the copper stabilizer amount on the HTS tape for stability and safety. It is found that copper of 2 mm in thickness keeps the cable temperature under 110 K.

Balanced Three-Phase Distributions of Tri-Axial Cable for Transmission Line

High Temperature Superconducting (HTS) cables have been intensively developed because of low loss and compactness, compared with conventional copper cables. A tri-axial cable composed of three concentric phases has been studied, because it has advantages such as reduced amount of HTS tapes and low heat-in-leak, compared with the three single-phase cables. However, there is an inherent imbalance in the three-phase distribution in the tri-axial cable due to differences in radii of the three-phase layers. We proposed a theory to obtain the balanced three-phase distribution for the tri-axial cable by treating two longitudinal cable sections together and adjusting all twist pitches. We derived a generalized formula as functions of winding pitches satisfying the balanced distribution. We designed and fabricated a short HTS tri-axial cable composed of 1 layer/phase to verify the proposed theory. The test results demonstrated that the theory is right for an equivalent impedance circuit model. The theory should be applied to the unbalanced three phase distributions caused by fabrication errors and inherent imbalance of capacitances in the tri-axial cable. We calculate the unbalanced three phase currents and voltages in steady state, and resolve them into symmetrical components to evaluate an imbalance ratio, which is defined as zero-sequence or negative-sequence to positive-sequence component. It is found that the fabrication errors of twist pitch and radius cause the imbalance ratios less than 1%, and the unbalanced capacitances of the cable of 10 km in length cause imbalance ratios of about 1%.