Test Results of a NbTi Wire for the ITER Poloidal Field Magnets: A Validation of the 2-Pinning Components Model

Abstract

A two-components model has been recently developed, for describing the normalized bulk pinning force curves and the critical current density of NbTi strands over a wider B-T range with respect to conventional single-component models. The model was previously successfully applied to data collected on several NbTi commercial strands, with different size, Cu:nonCu ratio, filament diameter and layout, thus confirming the presence of two different pinning mechanisms in conventionally processed NbTi wires. For a further validation, we have extensively tested a strand recently produced by the Chinese Company Western Superconducting Technologies for the ITER Poloidal Field (PF) magnets PF2 to PF5, and applied the model to these data. In order to take into account the observed non-scaling with temperature of the reduced pinning force curves, the model has been updated, including the observed difference in the temperature dependences of the two components contributing to the overall bulk pinning force. The importance of testing wires over very wide temperature ranges is evidenced, and the good agreement between experimental and fit results validate the proposed formulation, which can be regarded as a reliable tool for the description of NbTi performances, to be used in the design of superconducting magnets. From the phenomenological point of view, it is shown that at low temperatures, the two pinning mechanisms contribute to the bulk pinning force, resulting in a pinning force peaking at a reduced field . As the temperature increases, the pinning force peak moves to lower fields, indicating that the low field component pinning mechanism becomes dominant.

Cable-in-conduit conductors: lessons from the recent past for future developments with low and high temperature superconductors

“…Regarding superconducting properties of NbTi strands, we have in the past highlighted [150] that the properties at 4.2 K do not necessarily guarantee performances at higher temperatures and fields, for example at 6 T-6.5 K, which is instead the typical range of interest for fusion magnets, and where NbTi is characterized by low critical current densities. In these applications, we thus pointed out [151] that appropriate extrapolation formulas and qualification tests should be considered in the design phase, in order to avoid over-estimation of performances [152].…”

Minimum quench power dissipation and current non-uniformity in international thermonuclear experimental reactor type NbTi cable-in-conduit conductor samples under direct current conditions

“…Since they are wounded out of the same cable, the dissipation spot lengths of PFIS and PFCI are found, not surprisingly, to coincide (~ 8.4 mm). The dissipation spot length of the PFCN1 is instead lower (~ 4.0 mm), which is likely to be connected to the sharper V-I transition of its strands [27] and the different conductor geometry. The dissipation spot lengths have been used in the following analysis to normalize the strand power dissipation at quench of the different samples.…”

Starting EU Production of Strand and Conductor for JT-60SA Toroidal Field Coils

“…A-the experimental qualification of candidate optimized NbTi strand designs using pre-production strands B-the definition of strand final design C-the validation of strand design and production process on a first billet At stage A, the performances of nine strands manufactured from a short-size billet with various designs and fabrication paths were ranked by mean of electro-magnetic measurements performed at CEA and ENEA cryogenic installations ( [4], [5]), some being also eliminated.…”