3338 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011 Magnesium Diboride Wires With Nonmagnetic Matrices—AC Loss Measurements and Numerical Calculations Lauri Rostila, Eduard Demenˇ cík, Jan ˇ Souc, Silvia Brisigotti, Pavol Kovᡠc, Milan Polak, Giovanni Grasso, Mika Lyly, Antti Stenvall, Andrea Tumino, and L’ubomir Kopera Abstract—In the superconducting applications, the wires are exposed to time-varying magnetic field when the current changes. This generates losses which can be minimized by reducing filament size, twisting the wire, and increasing the transverse resistivity. However, the high losses of magnesium diboride wires often arise from magnetic sheath materials, and therefore, this work presents new type of wires with nonmagnetic matrix and multi-filamentary structure. The results of AC loss measurements, in external sinu- soidal magnetic field, are presented. Two samples were measured both in two temperature ranges, as two different set-ups were used, one with fixed LHe bath temperature 4.2 K. Second one enabled operation temperatures from 23 K up to the critical temperature of 39 K. Amplitude of magnetic field of the former set-up was up to 0.8 T and frequency range was from 0.1 to 1.4 Hz. In the latter one, the maximum amplitude was 28 mT, and the frequencies were 72 and 144 Hz. The results evidenced that the superconducting filaments were uncoupled and the measurements agreed with theoretical models based on this assumption. In practice, the uncoupling was modeled so that the net current in each filament was set to zero. Index Terms—Critical current density, electromagnetic mea- surements, finite-element methods, multifilamentary supercon- ductors, numerical simulation. I. INTRODUCTION T HE FAST development of wires has led to their commercialization. The wires are available in kilometer lengths and are relatively cheap both due to low material cost and straightforward manufacturing process, which have made them attractive for applications. However, especially AC loss properties need to be improved to make these wires more suitable for applications where electric current changes rapidly. Low loss properties are not only needed in power frequency Manuscript received August 02, 2010; accepted December 05, 2010. Date of publication January 20, 2011; date of current version May 27, 2011. This work was supported by NESPA, EU FP6 contract MRTN-CT-2006-035619. L. Rostila is with Columbus Superconductors S.p.A., 16133 Genova, Italy, and also with Electromagnetics, Tampere University of Technology, 33101 Tam- pere, Finland (e-mail: lauri.rostila@iki.fi). E. Demenˇ cík, J. ˇ Souc, and P. Kovaˇ c are with the Institute of Electrical Engi- neering, Centre of Excellence for New Technologies in Electrical Engineering, Slovak Academy of Sciences, 84104 Bratislava, Slovakia. S. Brisigotti, G. Grasso, and A. Tumino are with Columbus Superconductors S.p.A., 16133 Genova, Italy. M. Lyly and A. Stenvall are with Electromagnetics, Tampere University of Technology, 33101 Tampere, Finland. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2010.2099197 applications but also in magnets that are charged and discharged repeatedly. So, on one hand, low loss wires may not only be supposed for use in state-of-the-art applications, but on the other hand, they may enable new applications that have not been considered feasible before. Typically, high losses are due to ferromagnetic sheath mate- rials [1], [2] like Monel, Ni, or Fe. Therefore, the first step to reduce losses is to find a nonmagnetic replacement [3]–[5]. Re- cently, further steps have been taken with multi-filament con- ductors, that are twisted to limit the coupling losses [6], [7], which can be furthermore reduced with appropriate material choices and wire designs that aim at low transverse resistivity like in low loss LTS wire development [8]. Currently, measured AC loss data from wires is very limited. However, mag- netization losses of a single filament Ti wire were studied by Safran et al. [9] with the same setup that was used here, as well. For this work, we manufactured multi-filament wires with Ti matrix, which in principle, can be produced in kilometer lengths already. In addition to the low material cost, this type of wire is cheap, mechanically strong, light, and practically non-magnetic. The wire’s AC losses were measured with two techniques: cali- bration free electrical technique in which the sample is conduc- tion cooled and conventional electrical technique in which the cooling is based on LHe. For a comparison, the similar mea- surements were also made for loss optimized cable [6]. The measurements were compared to calculations obtained with modified Brandt’s method [10], in which -charac- teristics have been taken into account [11]–[14]. Filaments’ un- coupling was modeled so that the net current of each filament was set to zero, in the same way as presented in Ref. [15]. Some of the calculations were performed with finite element method (FEM) software using vector potential formulation [16]. II. WIRE DESCRIPTIONS Both samples were based on titanium which gives them suit- able magnetic properties for AC use, mechanical flexibility, and strength. Sample A, made by Columbus Superconductors, was composed of 19 Ti sheathed single-filament wires, which were bundled together into a Ti tube (see Fig. 1(a)). After several standings and heat treatments at 700 , the final form of the wire was obtained with superconducting cross-sectional area of 0.536 . Later, to stabilize the wire, some of the supercon- ducting strands can be replaced with copper ones. However, in this first trial, copper was not included to simplify the manu- facturing process, but in principle long length production is fea- sible. 1051-8223/$26.00 © 2011 IEEE