AC loss characteristics of YBCO conductors carrying transport currents in external AC magnetic fields
Introduction
AC superconducting power apparatuses such as power cables and transformers are most promising applications of high temperature superconductor (HTS) because costs to cool the AC losses in the HTS conductors in those apparatuses are much low compared with low temperature superconductor AC power apparatuses. However, even in HTS, AC losses in superconductors are the major losses dissipated in the apparatuses and dominate the efficiency and the economic feasibility of the apparatuses. Therefore, it is important to have correct and accurate data of the AC losses.
YBCO coated tape conductors potentially have high critical current density (more than 106 A/cm2 at 77 K) and are valuable HTS conductors for power applications. A YBCO tape conductor usually has very high aspect ratio and its AC losses strongly depend on the angle between the face of the tape conductor and an external magnetic field.
Superconductors used in the apparatuses are exposed to external AC magnetic field and, therefore, it is necessary to have data of the AC losses in the superconductors carrying AC transport current in AC magnetic fields with various directions.
AC losses in superconductors are composed of two kinds of AC losses, magnetization losses due to the external magnetic field and the transport current losses due to the transport current. Methods to measure the total AC losses that are the addition of the magnetization and transport current losses are categorized in two methods, electric [1], [2], [3] and calorimetric ones [4]. Generally, electric methods have high sensitivity and resolution and can measure the components of magnetization losses and transport current losses separately. However, data obtained by the electric method can be wrong if electro-magnetic environment around the sample conductors is not well defined. Therefore, in the case of using an electrical method it is necessary to prove its validity. On the other hand, calorimetric methods are less sensitive and have lower resolution but give actual losses regardless the electro-magnetic environment around the sample, although a proper method for calibration is necessary.
We have developed an electric method to measure the AC magnetization and transport current losses in a short sample of HTS tape subject to the external AC magnetic field perpendicular to the tape face [1]. Applying this electric method to the calibration, we measured the dependence of the total AC losses in a YBCO tape conductor on the angle between the direction of the external magnetic field and the tape face by a calorimetric method. The angular dependence of the AC losses was studied already by electric measurement [3]. We conducted the calorimetric measurement to make sure the validity of the results obtained by the electric measurement. We also investigated the loss characteristics in the case that the external magnetic field is almost parallel to the tape face, a difficult case for the electric measurement. The calorimetric method used in this work was similar to the method developed by Ashworth [4] except the calibration method.
Section snippets
Sample arrangement and experimental set-up
Fig. 1 illustrates an arrangement of a measurement sample to measure the AC losses by the calorimetric method together by the electric method. A rectangular pick-up coil is for the magnetization loss measurement and the calibration and spiral voltage leads is for transport current loss measurement [5]. A thermo-couple is placed on the face of the sample HTS tape to measure the temperature rise and the part of the tape where the thermo-couple is placed is covered by polystyrene thermal
Critical current measurement
Dependence of the critical current Ic on the angle of external DC magnetic field BmDC were measured and is shown in Fig. 3. Ic is dependent on the angle α and BmDC.
AC loss measurement
Changing the angle α, Qm and Qtotal were measured by calorimetric method using the same calibration curves used to obtain the data shown in Fig. 2. Qt was also measured by the electric method (Qm can not be measured electrically except α = 90°). Fig. 4 shows α dependence of Qm plotted against Bm Fig. 5 shows Qm plotted against Bmn the
Concluding remarks
There were works on electric measurements of the dependence of total AC losses in YBCO conductors on the angle of external AC magnetic field [3]. However, there might be some uncertainty in determination of the total losses by adding the transport current and magnetization losses which were measured separately. Very accurate agreement of the phases of the transport current and external AC magnetic field is required and it is rather difficult to guarantee the phase agreement. Therefore, we
Acknowledgment
This work is supported in part by Grant-in-Aid for Scientific Research (S) the Ministry of Education, Culture, Sports, Science and in part by the Ministry of Economy, Trade and Industry through ISTEC as the US–Japan Joint Study of AC Losses in High Temperature Superconductivity.
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