Alain Lacaze

Reduction of AC losses in ultra-fine multifilamentary NbTi wires

The authors present AC loss measurements for a series of wires in which the proximity effect has been greatly reduced by increasing the distance between the filaments while retaining high values of the overall critical current density, typically 3000 A/mm/sup 2/ at 0.5 T. For the best performing wires, 50-Hz AC losses in W/(Am) (=(losses in W/m/sup 3/)/(overall J/sub c/ in A/m/sup 2/)) for small test coils are a factor of ten lower for 0.5-T peak field and a factor of three lower at 1-T peak field. For wires with negligible proximity coupling, losses at low fields are reduced compared to that calculated by the Bean model, due to the small number of flux lines that thread across the width of each filament. >

Protection of superconducting AC windings

Protection schemes for superconducting AC windings are proposed which are applicable when the conductor is made up of several wires. Attention is first given to active protection, which involves ultrarapid quench detection. It is based on the measurement of the current passing through the central resistive wire and/or of unbalanced currents in the different superconducting wires. About 20 ms after detection, a fast circuit breaker switches off the current. Also considered is a complementary passive protection, which is provided by the resistance developing during normal phase propagation. This behavior naturally induces a certain amount of current reduction. Experimental results show how this effect can be magnified through an adequate definition of the superconducting and resistive wires assembled in the conductor. >

Thermo-electromagnetic stability of ultrafine multifilamentary superconducting wires for 50-60 Hertz use

The authors present calculations of the theoretical thermoelectromagnetic stability of ultrafine multifilamentary superconducting wires. These calculations take into account the self-field effect of the transport current. The authors then compare theoretical and experimental results for conductors comprising typically several hundred thousand 0.17- mu m diameter filaments. These first measurements of the electromagnetic stability of multifilamentary wires under AC conditions have given encouraging results and have shown good agreement with theory. The results are pertinent to the development of Nb-Ti ultrafine filamentary wires for 50-60-Hz applications. >

A new step in AC superconducting conductors

High current AC superconducting cables are made of numerous co-assembled wires. The assembly mode may be a source of performance degradation, due to different causes, such as wire deformation, mechanical instabilities, heating, anomalous current distribution among the different wires, longitudinal self magnetic field, inadequate junctions. These possible causes have been systematically investigated. Significant results are presented for possible applications of high-performance multi-kA conductors. >

Thermo-Electromagnetic Stability of Ultrafine Multifilamentary Superconducting Cables for Industrial Frequency Use

The development from 1983 onwards of Nb-Ti ultrafine multifilamentary wires for 50—60 Hz applications has opened up many interesting perspectives in electrotechnology. The AC losses have been greatly reduced due to decreasing the filament diameter to values of 0.1 to 0.2 µm and by using a highly resistive CuNi matrix with a 30% nickel content. The low thermal conductivity and specific heat of this CuNi matrix and the very high critical current densities induce a very acute problem of thermoelectromagnetic stability. As the electromagnetic diffusivity is much higher than the thermal diffusivity, the stability is governed by an adiabatic criterion. The last experiments about various coils and different assembly configurations of industrial cables give us a large panel of results which permit us to check the validity of the theoretical calculations.

Magnetization and critical current density of ultra-fine multifilamentary superconducting wires

Multifilamentary superconducting wires with a greatly reduced level of losses have been produced with lengths of several tens of kilometers. In spite of the reduction of the filament diameter, proximity effects are avoided, and the best possible use of the reversible motion of the flux lines is made, so that the hysteretic losses are reduced. The concepts lead to a realization of conductors comprising filaments of Nb-Ti (0.1-0.2- mu m diameter) embedded in a highly resistive CuNi matrix. In order to characterize the possible application to industrial power systems, it is necessary to investigate the losses in submicrometric filaments. It is possible to determine the correct value of the critical current density with critical current measurements and magnetization curves on such wires.

Performances of Industrial Superconducting Wires for 50–60 Hz Applications

Multifilamentary superconducting wires with a greatly reduced level of losses have been produced in lengdis of several tens of kilometers. In spite of the reduction of the filament diameter, proximity effects are avoided and we make die best possible use of die reversible motion of the flux lines, so that die hysteretic losses are lower. The reduction of losses due to induced currents can be obtained by choosing a twist pitch as short as possible. These concepts lead us to realize, first at a small scale, then at an industrial scale, conductors comprising filaments of Nb-Ti with a diameter of 0.1 to 0.2 μm and a highly resistive CuNi matrix. The twist pitch of these filaments is four times the diameter of die conductor. Some coils have been tested at the frequency of 50 Hz and show very reduced losses.

Effects of Heat-Treatments and of Processing Parameters on the Microstructure of Multifilamentary Nb-46.5 wt% Ti Superconductors

The superconducting behaviour of the conventional Nb-Ti wires is governed by their internal microstructure 1,2,3,4,5,6 whereas that of the ultra-fine filaments seems to arise from surface pinning at the boundaries between the filaments and the matrix 7. Therefore, it is particularly pertinent to examine the effects of the manufacturing process both on the internal and on the interfacial structure of the filaments. The authors have recently demonstrated that the Alsthom manufactured wires are highly polyphased 6,8 which can contain among other phases, significant amounts of interfacial intermetallic compounds and of internal α″ martensite.