Yasunori Mawatari

Critical current density and third-harmonic voltage in superconducting films

When a sinusoidal drive current I0 cos ωt flows in a small coil close to the surface of a superconducting film, third-harmonic voltage V3 cos(3ωt+θ3) is induced in the coil if the film causes a nonlinear response. We have developed an approximate theoretical method yielding the relationships among I0, V3, and θ3, thus providing the scientific basis for a widely used inductive method for measuring the critical current density Jc in large-area superconducting films. Our results show that V3 is near zero when I0 is smaller than a threshold value Ic0∝Jcd, where d is the film thickness. When I0>Ic0, on the other hand, the third-harmonic voltage is expressed as V3 exp(−iθ3)=ωIc0G(I0/Ic0), where G(x) is a scaling function determined by the configuration of the coil. We demonstrate the scaling law of V3/Ic0 vs I0/Ic0 in a YBa2Cu3O7−δ film.

AC losses in a finite Z stack using an anisotropic homogeneous-medium approximation

A finite stack of thin superconducting tapes, all carrying a fixed current I, can be approximated by an anisotropic superconducting bar with critical current density Jc = Ic/2aD, where Ic is the critical current of each tape, 2a is the tape width, and D is the tape-to-tape periodicity. The current density J must obey the constraint , where the tapes lie parallel to the x axis and are stacked along the z axis. We suppose that Jc is independent of field (Bean approximation) and look for a solution to the critical state for arbitrary height 2b of the stack. For c<|x|<a we have J = Jc, and for |x|<c the critical state requires that Bz = 0. We show that this implies in the central region. Setting c as a constant (independent of z) results in field profiles remarkably close to the desired one (Bz = 0 for |x|<c) as long as the aspect ratio b/a is not too small. We evaluate various criteria for choosing c, and we show that the calculated hysteretic losses depend only weakly on how c is chosen. We argue that for small D/a the anisotropic homogeneous-medium approximation gives a reasonably accurate estimate of the ac losses in a finite Z stack. The results for a Z stack can be used to calculate the transport losses in a pancake coil wound with superconducting tape.

Nondestructive determination of current–voltage characteristics of superconducting films by inductive critical current density measurements as a function of frequency

A nondestructive inductive method was developed to determine the current–voltage characteristics of superconducting films by measuring the critical current density Jc as a function of the frequency of ac magnetic fields. In well-known inductive methods that measure the Jc distribution of large-area films, Jc is measured at a frequency and the electric field E induced in the superconductor is proportional to the frequency; therefore, the E versus current density relation can be determined by measuring Jc at various frequencies. Current–voltage characteristics of YBa2Cu3O7 films measured by this method agree well with those determined by measuring magnetic-field-sweep rate dependence of magnetization.

Inductive measurements of third-harmonic voltage and critical current density in bulk superconductors

We propose an inductive method to measure critical current density Jc in bulk superconductors. In this method, an ac magnetic field is generated by a drive current I0 flowing in a small coil mounted just above the flat surface of superconductors, and the third-harmonic voltage V3 induced in the coil is detected. We present theoretical calculation based on the critical state model for the ac response of bulk superconductors, and we show that the third-harmonic voltage detected in the inductive measurements is expressed as V3=G3ωI02/Jc, where ω/2π is the frequency of the drive current, and G3 is a factor determined by the configuration of the coil. We measured the I0–V3 curves of a melt-textured YBa2Cu3O7−δ bulk sample, and evaluated the Jc by using the theoretical results.

AC loss evaluation of thin superconducting wires with critical current distribution along width

AC losses are analytically and numerically evaluated in thin superconducting wires with cross sections of various shapes for the application of either transport currents or transverse magnetic fields perpendicular to the broadest face. It is assumed that their electromagnetic response is determined by Beans critical state model, in which the critical current density is independent of the magnitude of local magnetic field. New theoretical expressions of the AC losses are derived for a rhomboid strip, whose sheet critical current varies from centre to edges linearly, and they are compared with conventional results for rectangular and elliptical strips. The transport current losses of wires with finite thickness are also calculated numerically by evaluating the penetration of magnetic flux into them on the basis of the minimization of magnetic energy in addition to the perpendicular field losses. These obtained results are compared with each other for different shapes of cross sections, and the influence of sheet critical current distribution along the wire width on the AC losses is discussed.

Hysteretic ac loss of polygonally arranged superconducting strips carrying ac transport current

The hysteretic ac loss of a current-carrying conductor in which multiple superconducting strips are polygonally arranged around a cylindrical former is theoretically investigated as a model of superconducting cables. Using the critical state model, we analytically derive the ac loss Qn of a total of n strips. The normalized loss Qn∕Q1 is determined by the number of strips n and the ratio of the strip width 2w to the diameter 2R of the cylindrical former. When n⪢1 and w∕R⪡1, the behavior of Qn is similar to that of an infinite array of coplanar strips.

Critical current properties and vortex-glass-liquid transition in Ag-sheathed Bi-2223 tapes

Abstract The magnetic field and temperature dependence of the critical current density Jc and the voltage-current characteristics of a Bi-2223 tape in a magnetic field parallel to the c-axis are studied. The critical current density decreases with increasing field in a power-law manner at low magnetic fields, and decreases exponentially at high fields. These field dependences suggest the validity of the brick-wall model proposed by Malozemoff, in which the critical current density is limited by the grain boundaries perpendicular to the c-axis. The magnetic field dependence of the macroscopic pinning force density, Fp = JcB, shows clear scaling behaviour for different temperatures. Furthermore, we investigated the temperature dependence of the voltage-current characteristics precisely, and a critical scaling analysis demonstrates the existence of a quasi-two-dimensional vortex-glass-liquid transition, which has been observed recently in Bi-2223 epitaxial films. The observed glass-liquid transition temperatures Tg were found to be much higher than those in the epitaxial films.

Flux pinning centres correlated along the c-axis in PLD-YBCO films

Flux pinning centres having correlation along the c-axis in epitaxial YBCO films were investigated by measuring the magnetic-field angle ? dependence of the critical current density JC and by observing the microstructure. First, c-axis oriented YBCO films were prepared on four different substrates at three different target-to-substrate distances D by using a pulsed-laser-deposition method. Films deposited on LaAlO3 (100) and CeO2/Al2O3 with large D (112 or 142 mm) showed narrow peaks in the JC versus ? curves when the magnetic field was applied parallel to the c-axis (B//c). In these films, a high density (up to 5.5 ?m?1) of planar defects, most probably stacking faults parallel to the c-axis, were evident. The ratio of JC (B//c) to JC (B//a?b) significantly increased as the density of the planar defects increased. In contrast, films deposited on SrTiO3 (100), LaAlO3 (100) and NdGaO3 (110) with small D (50?60 mm) showed high, broad JC peaks around B//c, and had a high density (up to 430 ?m?2) of nanometre-sized precipitates that were elongated along the c-axis. The ratio of JC (B//c) to JC (B//a?b) increased as the density of these small elongated precipitates increased. In conclusion, planar defects such as stacking faults parallel to the c-axis, and the small elongated precipitates are effective as c-axis correlated flux pinning centres.

Predicted field-dependent increase of critical currents in asymmetric superconducting nanocircuits

The critical current of a thin superconducting strip of width

Hysteretic ac losses in power transmission cables with superconducting tapes: effect of tape shape

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