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Locating of normal transitions in high temperature superconducting (HTS) coils is important for protection and safety design of HTS apparatus. A general method to locate the normal transitions is to measure resistive voltages along HTS windings by many voltage taps directly soldered to the HTS coils. However, electrical insulation characteristics of the HTS coils are deteriorated because it is necessary to remove electrical insulations of the HTS wires for the soldering. It is a serious problem especially for AC HTS coils to which high voltages are applied. Therefore the authors have presented a non-contact voltage measurement method that can detect the resistive voltages without removing the insulations by voltage dividing capacitors. So far the authors have verified the principle of the non-contact method. In this paper, a method to locate the normal transitions in a Bi2223 HTS coil based on the non-contact method is proposed. The proposed method can not only detect the normal transitions but also locate their positions. It is experimentally confirmed that the proposed method is useful for locating the normal transitions.

AC high temperature superconducting (HTS) coils have been developed for transformers, motors and so on. Quench detection and protection system are essential for safety operations of the AC HTS facilities. The balance voltage method is universally used for the quench detection and protection, however especially for AC operations, the method has risks in terms of high voltage sparks. Because the method needs a voltage tap soldered to a midpoint of the coil winding and the AC HTS facilities generally operate at high voltages and therefore high voltage sparks may occur at the midpoint with no insulation. We have proposed the active power method for the quench detection and protection. The method requires no voltage tap on the midpoint of the coil winding and therefore it has in-built effectiveness for the AC HTS facilities. In this paper, we show that the method can detect the quench in an HTS transformer and moreover our proposed quench protection circuits which consist of thyristors are simple and useful for the AC HTS facilities.

AE (Acoustic Emission) measurements are well known methods to detect mechanical signals from superconducting coil The mechanical signals could be generated by micro cracks of epoxy resins, the motion of superconductors and the thermal expansion of superconductors, which were generated before and/or after a quench. We have presented a time-frequency visualization of AE signals as a method to detect the quench. We can detect very small AE signals regardless of lectromagnetic noises and can find the time of the AE occurrence and the frequency bands of AE signals by using this method. Recently it has been presented that YBCO superconductors are delaminated and degraded by a transverse tensile stress. The delamination is accompanied with AE signals. Also, it is known that amplitudes and frequency bands of AE signals vary with causes of AE occurrence. In this paper, we present time-frequency analyses of AE signa s caused by the delamination of a YBCO superconductor and the micro of epoxy resins.

High temperature superconducting (HTS) coils require reliable quench monitoring systems for safety operations. Also, locating of normal transition areas is important for developing reliable HTS coils. Resistive voltage measurement method is universally used for the quench detection and the location. The method needs many voltage taps soldered to superconducting windings and therefore insulations of superconducting wires have to be removed for the soldering and then it has high risks in terms of high voltage sparks. In this paper, the authors present a new detection method of normal transitions by using capacitor taps which can detect normal transitions regardless of the insulations (by no destruction) and has no risks about the high voltage sparks. In this method, two electro-conductive sheets are attached on a surface of a superconducting coil and are connected with an outside capacitor. Then two capacitors are created on the surface of the superconducting coil by sand iching the insulation between each electro-conductive sheet and the superconducting wire. The normal transitions can be detected as a voltage of the outside capacitor by voltage dividing with the three capacitors. A demonstration test was carried out for a Bi2223 coil and the quench detection and the location were successfully achieved.

The authors have been developing an AC power source with a high temperature superconducting (HTS) transformer which is small in size and can supply large AC current. The HTS transformer needs a detection system of normal transitions for high reliability. The authors have reported the active power method as the detection system. However, the conventional method could not detect early the normal transition in a secondary coil of the transformer which was wound with a bundle of some HTS tapes since resistive voltage in the normal area is too small due to current sharing among the HTS tapes. In this paper, the authors propose a method to detect the normal transitions in the bundle conductor by improving the active power method. The early and correct detection can be achieved by measuring each active power of each HTS tape. In order to measure the active power, current leads are attached independently to all the HTS tapes. The current leads enable to measure the resistive voltage in each HTS tape, and each active power can be measured. The proposed method works well when even one of secondary tapes transits to the normal conducting state. Experimental results show the usefulness of the proposed method.

The authors have been developing a large AC current source with a single-phase Bi2223 superconducting transformer. The authors aim to develop the source of supplying current with the maximum frequency of 1 kHz and the maximum amplitude of 1 kA. However, the conventional transformer has some leakage inductance resulting in large voltage drop, and therefore it was unable to transport the current with high frequency and large amplitude. In this paper, the authors propose a configuration of the transformer with less leakage inductance than inductance of the conventional one. Improved current source has output current with the maximum frequency of 1 kHz and the maximum amplitude of 900 A. In addition, a protection system for normal transition in the transformer is proposed. The authors have proposed the active power method to detect the normal transition. For high frequency and large current, however, the normal transition may be incorrectly detected by using the method due to large AC loss. The authors show a new method to avoid the incorrect detection by an empirical formula based on measurement results of the AC loss. From the experimental results of the normal transition, the authors show that the proposed method is more correct to detect the normal transition than the conventional method.

When a quench occurs in a superconducting magnet, excessive joule heating in normal region may damage the magnet. It is necessary to detect the quench as soon as possible and discharge magnetic energy stored in the magnet. The authors have presented a quench detection/protection system based on an active power method which detects the quench regardless of a self-inductive and mutual-inductive voltages and electromagnetic noise. In the conventional active power method, the inductive voltages are removed by cancel coils. In this paper, the authors propose a method to cancel an inductive voltage using a capacitor circuit. The quench detection/protection system becomes more precise and smaller than the conventional system through the capacitor circuit.

When a quench occurs in a superconducting magnet, excessive joule heating may damage the magnet. We have presented the quench protection system based on an active power method. Our previous quench protection tests have been carried out for small superconducting magnets whose self inductances are less than several hundred mH to verify principles of our proposed system. In this paper, we present experimental results of quench protection tests of a cryocooler cooled 6 T NbTi superconducting magnet (self inductance 15.5 H), which is a commercial size magnet made by Tamakawa Co., Ltd. We confirmed that our proposed system could inhibit the maximum temperature of the superconducting magnet (initial temperature 4.3 K) after the quench to less than about 44 K at operation magnetic field 5.5 T. Experimental results suggest that our proposed system is useful for practical used superconducting magnets.

When a quench occurs in a superconducting magnet, excessive joule heating may damage the magnet. The authors have presented a quench detection/protection system based on an activ e power method, which can detect the quench quickly and precisely by measuring an active power dissipated within the magnet after the quench. In this paper, the authors show usefulness of a softwa re quench detection/protection system for a cr yocooled NbTi superconducting magnet which is a commercial size magnet made by Tamakawa Co., Ltd.. Experimental results show that the proposed system is useful for the practical used superconducting magnet.

In high temperature superconductors, local and excessive temperature rise occurs after normal transition because normal zone propagation velocities are slow at high operation temperature and then the superconductors may be degraded or burned. Therefore, it is essential to detect the temperature rise in high temperature superconducting coils and protect them as soon as possible. The authors have presented a detection method based on time-frequency visualization of AE signals. In this paper, the authors discuss detection and frequency analysis of AE signals accompanying with epoxy cracks caused by thermal expansion in an HTS coil by using the method.

The normal zone propagation velocities in high T c superconductors are slow at high operation temperature and therefore local and excessive temperature rise generates at quench occurrence, and then the superconductors are degraded or burned. Therefore it is essential to detect the temperature rise in high T c superconducting (HTS) coils as soon as possible and protect them. The authors have presented a quench detection method for HTS coils by time–frequency visualization of AE signals and have shown its usefulness for a HTS coil with height and outer diameter of 40–50 mm. In this paper, the authors present a quench detection/protection system based on superior method in quench detection time to the previous method and show its usefulness for a larger HTS coil (height and outer diameter: 160–190 mm) than the previous coil.

When the quench occurs in the superconducting coil, the excessive joule heating in the normal region may damage the coil. It is necessary to detect the quench in the coil as soon as possible and discharge the magnetic energy stored in the coil. We have presented a quench protection system based on the active power method which detects a quench regardless of the self-inductive and mutual-inductive voltage for multi-layer superconducting coils. In this paper, the usefulness of the developed system is confirmed for two-layer superconducting coils wound with Bi2Sr2Ca2Cu3Ox HTS tapes.

The authors have been developing a small and light power source with a hybrid single-phase Bi2223 high temperature superconducting (HTS) transformer. The transformer consists of a primary copper coil and a secondary Bi2223 superconducting coil and can output current of 500 A. For safe operation of the transformer, it is important to detect normal transitions in the transformer and to protect it from excessive heating in the normal area. The authors have presented a protection system based on the active power method and magnetic flux detection coil. However, in the conventional method, the active power does not become zero in a superconducting state due to AC loss and therefore incorrect recognition may be caused. In this paper, the authors propose a method to reduce the loss signals by configuration and mounting position of the magnetic flux detection coil. Through experimental results for a hybrid single-phase Bi2223 superconducting transformer, the authors show that the proposed method can detect the normal transitions more accurately than the conventional one.

It is important to locate positions of normal transitions in a high temperature superconducting coil for identifying its design and fabrication weakness. The authors have proposed capacitor type voltage terminals as a contactless method to measure voltages of the coil windings through insulation of the windings. This method was useful for locating the positions of the normal transitions. Higher resolution of the location is achieved by attaching many terminals to the coil. Then shorter electro-conductive sheets of capacitor type voltage terminals are needed. However, the shorter sheets make the measured voltage signals small and the signals are supposed to be incorrectly measured. In this paper, the authors propose a method to achieve high resolution locating by long electro-conductive sheets of capacitor type voltage terminals. Numerical values of the measured voltage by the terminals depend on the positions of the normal transitions and therefore analysis of the values can achieve high resolution locating without increasing the number of the terminals. Through experimental results for a Bi2223 HTS coil, it was confirmed that the proposed method could achieve the high resolution locating.

To understand the effects of the hot pressing on the pinning behavior for the SiC doped MgB2 tape wire, magnetic field and temperature dependences on a critical current density Jc and the pinning force have been studied. The MgB2 tapes were fabricated using an in situ powder-in-tube (PIT) method, the hot pressing process with nano-SiC powders and the carbon steel sheath. Jc of the hot pressing processed wire is enhanced especially at low field compared with that of the no hot-pressed wire. It is suggested that the enhancement of Jc is caused by the improvement of the MgB2 grain connectivity came from the hot pressing. The pinning analysis using the pinning models of a surface pinning and a normal point pinning lead to that the scaled curve of the pinning force for the SiC-doped MgB2 tape with the hot pressing process showed anisotropy in a lower field. This result suggests that the pinning center of the hot pressing processed MgB2 wire forms into inequi-axial shape, which is different from that in the tape without the hot pressing having the equi-axial pinning center in our previous report. It was also found that the scaled pinning force of the SiC-doped MgB2 tape with the hot pressing is larger than the surface pinning model in a higher field. This suggests that the pinning force was enhanced by the hot pressing in a higher field.

The minimum quench energy (MQE) and normal zone propagation velocity (NZPV) of three kinds of Nb3Al superconductors fabricated by the rapid heating, quenching and transformation (RHQT) process were measured under various conditions of applied magnetic field (10-14 T), temperature (7-11 K), and transport current (80-95% of the critical current), while cooled by a cryocooler for developing the over 20-T class cryogen-free magnet. As a result, MQE values were related to the critical current density (J c); high MQE was obtained for low J c. It is assumed that J c has a stronger influence on the MQE than specific heat, thermal conductivity, resistivity, and other parameters of the composite superconductor including the matrix and the stabilizer. NZPV was mainly proportional to the transport current density varying with applied field and temperature. The second contribution to NZPV is assumed to be heat capacity depending on the wire configuration.

In an attempt to understand the effects of SiC addition on the pinning behavior for the in-situ PIT MgB2 tape wire, field and temperature dependencies on critical current density, Jc and the pinning force were studied by considering applied magnetic field direction for the rectangular-shaped wire. As a result, it was clarified that the SiC-doped wire has the equiaxial pinning center and the un-doped wire has the inequi-axial pining center. Taking the pinning scaling law into account, it is found that the SiC-doped MgB2 wire has two kinds of the pinning center and the dominant pinning center changes with temperature, that is, the pinning centers are the grain boundary and the precipitates at high and low temperatures, respectively. The un-doped MgB2 wire is assumed to have anisotropy of the pinning mechanism.

When the quenching occurs in a superconducting coil, excessive joule heating in normal area may damage the coil. It is necessary to detect quenching in the coil as soon as possible and discharge the magnetic energy stored in the coil. Therefore, we propose a superconducting coil protection system based on an active power method. The system is highly resistant to the noise and does not require cancel voltage taps, so it is useful for both AC and DC coils. We have presented the effectiveness of the system using some test coils cooled in LN 2 or LHe. However, we have not discussed the effectiveness of the proposed system for helium-free cryocooled magnets, in which a larger temperature rise occurs after quenching than in liquid-cooled magnets. In this paper, we verify the effectiveness and practicality of the proposed system through coil protection tests for a cryocooled Nb 3 Al LTS coil.

Recently, HTS coils have been developed for electric power apparatuses. In superconducting coils, local and excessive joule heating may give damage to the superconducting windings when a quench occurs and therefore it is essential that the quench is detected quickly and precisely so that the coils can be safely discharged. Resistive voltage measurement method is universally used for the quench detection, however, it is vulnerable to an electromagnetic noise which causes insufficient quench detection and at least needs a central voltage tap in windings. In a large superconducting coil, a lead-wire from the central voltage tap may cause a short-circuit when high voltage will be applied. In this paper, we present a quench protection system based on the active power method which detects a quench by measuring the instantaneous active power generated in a superconducting coil. The protection system based on this method is very strong against to the noise and no more needs a central voltage tap. The performance of system developed by us is confirmed by using test coil wound with Bi-2223 HTS tapes.

The new in situ MgB 2 tapes with/without SiC addition and the hot pressing process have been fabricated using the carbon steel sheath. The critical temperature ( T c ), critical current density ( J c ) and the AC transport current loss of those tapes have been studied and discussed in various conditions. The MgB 2 tapes with the carbon steel sheath that improved the core hardness and homogeneity, resulting in high J c , and the SiC addition, and the hot pressing brought narrow T c transition width. As the carbon steel has ferromagnetism, it affected the loss of the MgB 2 tape. The large losses that were two to three orders of magnitude higher than Norris’ theoretical curve were obtained. It was clarified that the main loss of the MgB 2 tape was the hysteresis loss of the carbon steel sheath.

Pre-bending effects of Nb3Sn composite wires, enhancement in Jc at the as-cooled condition and Jc peak, Jcm, against applied tensile strain, are well known in especially Cu-Nb reinforced Cu stabilized Nb3Sn wire. In an attempt to understand the effects, three directional strain analyses using FEM were studied by considering thermally-induced residual strain due to temperature difference between the reaction temperature and the cryogenic temperatures including the pre-bending process at room temperature for three types of Nb3Sn composite wires. To evaluate effects of change in three directional strain, the von Mises strain was introduced. As a result, the minimum von Mises strain against the applied tensile strain corresponded to Jcm; low strain shows Jcm enhancement by the pre-bending for Cu-Nb reinforced Nb3Sn wire. It was found that there is a direct correlation between the minimum von Mises strain and Jcm related to the pre-bending.

This paper presents the electric insulated type detection method of the temperature rise in the Bi2223/Ag tape using the time–frequency visualization of AE signals based on the wavelet transform. At the high operation temperature, the specific heat and the normal zone propagation velocity of the Bi2223/Ag tape are high and low, respectively, and therefore the local and excessive temperature rise is easy to generate in the superconducting-to-normal transition. As a result, the Bi2223/Ag tape is degraded or burned. In the proposed method, AE signals accompanying with the temperature rise are measured by an AE sensor, and furthermore AE signals are precisely and visually measured and estimated by the time–frequency visualization using the wavelet transform. Experimental results show the feasibility of the proposed method as the detection method of the temperature rise in the Bi2223/Ag tape.

We have been developing a magnetic field measurement system with two-axis hall-probes to study the critical current density distribution or magnetic properties of high- T c superconductors. In this study, the current distributions at corners of the meander type resistive fault current limiters were measured by hall probe with high spatial resolution. The both of axial and radial magnetic field components ( B x and B z ) according to the direct (DC) and alternating (AC) transport currents were measured by hall probe to estimate the current density distribution with high accuracy. And, it is confirmed that the measured radial components are very useful to estimate the current distribution.

Reducing AC loss in a superconducting apparatus is one of the most important issues, and the precise measurement and estimation of AC losses are essential to reduce them. The four-terminal method is universally used as an electric measurement method of AC losses for superconducting tapes and wires. In this method, noise and inductive voltage superposed on the terminal voltage of the superconductor are eliminated by a lock-in amplifier and cancel coil, respectively, and then measurement of very small resistive voltage is achieved. However, using this conventional method, a plurality of measuring instruments and apparatuses are needed, and therefore the measuring system becomes complicated and much time is consumed in the calibration process. In this paper, we present a simple and precise measurement system based on an active power detection method, which is proposed as a quench detection method. The proposed system consists of a small number of instruments and apparatus and is less susceptible to noise. Its usefulness is verified by comparing the proposed method and the conventional four-terminal method in measuring the AC transport current loss of a Bi2223/Ag tape.

AC transport current loss (self field loss) of ex-situ-processed MgB2 superconducting tapes with Ni sheath was measured under different conditions of transport current value, transport current frequency, and temperature. Experimental results were compared with Norris' theoretical curves and previous measured losses of MgB2 tapes with CuNi and stainless-steel sheaths, which were larger than the theoretical ones and previous data. The loss vs. normalized transport current (transport current normalized by critical current) curves varied with temperature and the loss at 9 K was larger than that at 14 K. The measured loss approached Norris' curves in the high normalized transport current region. In order to gain insight into these origins, FEM analysis of magnetic flux concentration by Ni and estimations of hysteresis loss of ferromagnetic nickel-sheath and of homogeneity of the core were carried out. As a result, the hysteresis loss of Ni was very small based on the estimation and the loss measurement of Ni. Main contributions to large loss are considered to be self-field enhancement due to magnetic flux concentration by the Ni-sheath and critical current density distribution of the MgB2 core caused by the inhomogeneity of critical current density and critical temperature

It is known that current sharing effects of the stabilizer are limited in cryocooling conditions being different from poolboiling. Therefore we have studied stability under it, focusing on minimum quench energy (MQE) of Nb/sub 3/Sn superconductors under various conditions of the applied magnetic field, temperature, and the transport current, for various volume fractions and configurations of Cu stabilizer. From the experimental results, the larger the external Cu volume fraction, the higher the MQE was obtained. All MQE data were summarized by only one line in characteristics of normalized MQE and normalized B(B/B/sub c2/). The obtained MQE behavior was to be characterized by only one empirical equation. It is thought that the obtained formula is useful for a standard of designing the Nb/sub 3/Sn superconductor and its coil under cryocooled condition.

The measurement and analysis of AC transport current loss (self-field loss) in MgB2 superconducting composites were studied. In experimental results, the normalized Ic dependency of AC loss in the MgB2 deviated from Norris' theoretical curve. Then a numerical analysis was performed by taking into consideration the model of a round superconducting wire that consisted of three concentric layers. The analysis clearly showed the experimentally obtained results, such as that the phenomenon originated in the inhomogeneity of the critical current density in the wire rod section.

In this paper, we present an electric quench detection method without a central voltage tap which may cause the short-circuit of the lead-wires from the voltage taps in the quench detection of a large AC superconducting coil. In this method, an inductive voltage detection coil is used instead of the central voltage tap. The inductive voltage detection coil is electrically insulated from a superconducting coil and therefore the lead-wires do not break down. Through the quench detection tests for a Bi-2223/Ag HTS coil, we show the feasibility of the proposed method for detecting the quench of the large AC superconducting coil.

The authors have been developing a compact power supply with a single-phase Bi2223 high temperature superconducting (HTS) transformer. The conventional transformer has an iron-core for enhancing magnetic coupling between its primary coil and secondary one. However, the iron-core has great majority of size and weight of the transformer and therefore it is desirable to be removed for a smaller and lighter transformer. In this paper, the authors propose an air-core HTS transformer for a more compact power supply than the conventional one. As experimental results, it is shown that appropriate design of the air-core transformer has a possibility to decrease the weight and volume of the large AC current supply.

The authors have been developing a hybrid single-phase Bi2223 high temperature superconducting (HTS) transformer used in the AC current source with rated current of over 500A. Its primary coil is a copper coil and secondary coil is a Bi2223 HTS coil. In this paper, the authors propose a new detection method of normal transitions in the secondary coil by using the active power method and a magnetic flux detection coil attached on the inside of the secondary coil. In the proposed method, the normal transitions are detected by measuring active power dissipated in the secondary coil, and induced voltage of the magnetic flux detection coil by a primary and leakage flux of the secondary coil enables to calculate the active power dissipated in only the secondary coil. As experimental results for a hybrid single-phase Bi2223 HTS transformer, it was found that the proposed method enabled to detect the normal transitions in its secondary superconducting coil.

The normal zone propagation velocities in the high TC superconductors are slow at the high operation temperature and therefore the local and excessive temperature rise generates when the superconducting-to-normal transition occurs, and then the superconductors are degraded or burned. Therefore it is essential to detect the temperature rise in the high TC superconducting coils as soon as possible to protect the coils. In general, the temperature rise can be detected by many voltage taps and thermocouples, however, the taps and thermocouples make the electric insulation of the windings degraded. The author has presented the time–frequency visualization of AE signals as a method to detect the temperature rise with non electric contact and the usefulness for Bi2Sr2Ca2Cu3Ox (Bi2223) superconductors has been presented. In this paper, the usefulness of the proposed method for YBa2Cu3Ox (YBCO) superconductors is verified and the experimental results show that the proposed method can detect the temperature rise in a YBCO coil.

This paper presents the results of the quench detection tests of a high Tc superconducting coil (HTS coil) wound with Bi-2223 tape, impregnated by epoxy resin and cooled by LN/sub 2/ using a partial active power detecting method we have proposed as a new type quench detecting method. The tests were carried out for DC and AC current. The results showed that the proposed method was useful for the quench detection of the HTS coil.

AE (Acoustic Emission) techniques are universally used for investigating and watching the deformation and crack of various materials because they enable the non-destructive and continuous measurement. However, it is not always easy to clarify the main cause or mechanism of AE signal generation by its analysis and an AE sensor is generally vulnerable to the noise, and therefore advanced measurement and estimation techniques of AE signal are needed. In this paper, the authors present time-frequency visualization of AE signal using the wavelet transform. The proposed method enables especially precise measurement of the frequency band and generation time of AE signal and gives a clue in advanced AE techniques.

When a quench occurs in a superconducting magnet, excessive joule heating in normal region may damage the magnet. It is important to detect the quench in the magnet as soon as possible and discharge magnetic energy stored in the magnet. The authors have proposed a quench detection/protection system based on active power method which detects an active power dissipated in the magnet after the quench. In this paper, the authors propose a method to decide an appropriate criterion to protect the magnet from the excessive joule heating by using 1D heat equation. The method can determine a safe criterion for the quench detection/protection within a short time. The authors show the usefulness of the method through experimental results for high temperature superconducting coils.