Riccardo Tebano

Current profiles and ac losses of a superconducting strip with an elliptic cross-section in a perpendicular magnetic field

We study the case of a hard type II superconductor in the form of a strip with an elliptic cross-section when it is placed in a transverse magnetic field. We approach the problem in two steps, both based on the critical-state model. First, we calculate numerically the penetrated current profiles that ensure complete shielding in the interior, without assuming an a priori form for the profiles. In the second step we introduce an analytical approximation that asumes that the current profiles are ellipses. Expressions linking the sample magnetization to the applied field are derived covering the whole range of applied fields. The theoretical predictions are tested by comparison with the experimental data for the imaginary part of ac susceptibility.

Current density distribution in BSCCO-2223 tapes in overload conditions

The current density distribution in HTS tapes for power transmission applications plays a key role for the determination of the conductor dissipation both in normal and in overload conditions, as occurring for example during transmission lines faults. We have tested the response of Ag/Bi2223 tapes in dc and ac self field transport current in order to determine the repartition of current between the superconducting material and the silver sheath, in particular in the high current range. The experimental results are compared with the results of a numerical calculation model able to determine the tapes self-field current density distribution both above and below Ic. The same model is also used to calculate the tapes AC losses. The results are compared with the measured values.

Design and development of a prototype of hybrid superconducting receiver front-end for UMTS wireless network: first results and application perspectives

In this paper we will describe the main results of the activity aimed to develop a prototype of superconducting receiver front-end (HTS RFE) for specific application in UMTS network. The system has been specifically designed to reach very low noise figure on the receiving channel to exploit the best performance in terms of sensitivity of the up-link channel.

Higher harmonics in the voltage on a superconducting wire carrying AC electrical current

The problem of determining the harmonic content in the voltage that appears on a superconducting wire carrying cosine-like AC current was resolved theoretically, using two approaches. First, the Fourier components of the voltage spectrum were found by numerical integration. Importance of individual terms was established, leading to two conclusions: a) it is the cosine component of the 3rd harmonic that represents the bulk of harmonic distortion, b) for the practical purposes it is sufficient to consider higher harmonics with n ≤ 7. Then, the analytical formulas were derived. While for the sine components a general expression containing an infinite series was found, closed-form formulas were derived for the cosine components of the harmonics 1, 3, 5, 7. Consequences of the results to the experimental technique used to study the AC transport properties of superconductors are discussed.

Generation of higher harmonics in voltage on superconducting wire carrying cosine-like AC current

Hard superconductors are known to exhibit nonlinear and hysteretic magnetic properties. Then, the electrical field on the surface of a wire made from such material carrying AC current, will be nonharmonic also in the case when the excitation current is purely cosine-like. We show the harmonic spectra of this signal computed in both numerical and analytical way, following the critical state model with constant j/sub c/. Odd harmonics only are present in the signal. Assuming the cosine-like transport current, the sine components are negligible in wide range of current amplitudes, except the fundamental sine component. The ratio between harmonic components remains constant up to the current amplitudes close to the critical current. Theoretical results are compared with experiments carried out on two samples: multifilamentary tape from HTS Bi-2223 in metallic matrix, and sintered tubes of Bi-2223. Measured content of higher harmonics laid below theoretical results for both the samples, indicating that the simple model with constant j/sub c/ does not sufficiently describe the flux pinning in these materials.

Critical current and ac susceptibility in superconducting tapes with elliptical cross-section

The current penetration in an infinite superconducting tape with elliptical cross-section in the critical state as a response to a perpendicular applied field is described by two models. One is a numerical model based on energy minimization after each field change. The second model is a simple analytic approximation for the flux fronts obtained by the numerical model. The magnetization, the ac susceptibility and the associated ac losses can be calculated. We provide an empirical expression for the position of maxima in the imaginary part of ac susceptibility. Then, the value of the critical-current density in a superconducting tape with elliptical shape of any dimensions can be extracted from ac susceptibility data.

Eddy current loss in Ag-sheathed BSCCO tapes in the AC transport regime

Experimental investigations on Ag-sheathed BSCCO tapes show that losses in the ac-current transport regime are not of purely hysteretic nature. A contribution due to the eddy currents in the silver matrix is present and detectable in the low current limit. In the present paper we extend an existing model in order to account for this contribution. The tape-superconducting core is regarded as a strip or as an ellipse. The expressions we obtain are analyzed and compared to experimental data.

Low frequency impedance of a round superconducting wire

Abstract We calculated the electric field E on the surface of a straight superconducting wire with circular cross-section carrying AC transport current I=Iacosωt. Performing the Fourier analysis of E, we found that both components of the first harmonic have the same form: the critical current Ic in prefactor and the rest depending on the ratio F=Ia/Ic. The in-phase component leads to the classical result of loss calculation, while the out-of-phase component was derived for the first time. Thus the wire can be symbolized by a complex self-inductance L1(I)=L1′(I)−jL1″(I) where L1′ represents the reactive power while L1″ the losses. When the lock-in amplifier, used to sort out the components of the first harmonic, is utilized in the wide-band mode, it allows one to determine the magnetic flux penetrated in the wire volume at two significant moments of the AC cycle: at zero current (remanent flux) and at the amplitude value of current.

NUMERICAL INVESTIGATION ON AC PROPERTIES IN HIGH TC SUPERCONDUCTING TAPES

Reduction of AC losses for large-scale applications of superconductors is a critical issue. Therefore, the quantitative evaluation of AC losses is important for the development of superconductors and their applications to electrical power systems. The development of numerical models that simulate the electromagnetic phenomena inside superconductors allows to understand the electromagnetic behavior of superconductors and to evaluate the AC loss properties. Following an approach proposed by Brandt in several papers, a numerical model was developed in order to study the AC properties of superconducting tapes in different geometrical arrangements and with time dependent current and magnetic field. Here we show an example for simple single-layer model cables to show how this rather simple and versatile numerical approach allows optimizing configurations for actual operational conditions.

Analysis of the DC and AC behaviour of Bi2Sr2Ca2Cu3Ox tapes arranged in z-stack configuration

Abstract The DC and AC behaviour of Ag-sheathed Bi 2 Sr 2 Ca 2 Cu 3 O x (BSCCO-2223) tapes, arranged in a z-stack configuration, are investigated. The number of tapes in the stack can be varied, from one (single tape) to four, as well as the inter-tape separation. Tapes are isolated and connected in series to ensure a uniform current distribution among the tapes. The critical currents and the average AC losses per tape as a function of the transport current are measured. By increasing the number of tapes or/and reducing the separation between them, the critical current of each tape is decreased while the average losses are increased. Tapes in a stack are found to be electromagnetically coupled, the degree of this coupling being dependent on the separation among them. Theoretical models are provided to explain the experimental results.