Test results on the first 50 kA NbTi full size sample for the International Thermonuclear Experimental Reactor

Abstract

Within the framework of the R&D studies for the International Thermonuclear Experimental Reactor (ITER) project, the first full size NbTi conductor sample was fabricated industrially and tested in the SULTAN facility (Villigen, Switzerland). This sample (PF-FSJS), which is relevant to the poloidal field coils of ITER, is composed of two parallel straight bars of conductor, connected at the bottom through a joint designed according to the CEA twin-box concept. The two conductor legs are identical except for the use of different strands: a nickel plated NbTi strand with a pure copper matrix in one leg, and a bare NbTi strand with a copper matrix and internal CuNi barrier in the other leg. The two conductors and the joint were extensively tested as regards DC and AC properties. This paper reports on the test results and analysis, stressing the differences between the two conductor legs and discussing the impact of the test results on the ITER design criteria for the conductor and joint. While joint DC resistance, conductors and joint AC losses fulfilled the ITER requirements, neither conductor could reach its current sharing temperature at relevant ITER currents, due to instabilities. Although the drop in temperature is slight for the CuNi strand cable, it is more significant for the Ni plated strand cable.

Implications of NbTi Short-Sample Test Results and Analysis for the ITER Poloidal Field Conductor Insert (PFCI)

“…For a sketch of the PFCI as well as other information, which will not be repeated here, we refer to [4]. The main objectives of the PFCI test can be summarized as: (1) establish the DC current sharing limits of the conductor, especially where they are driven below the usual 10 level and compare to predictions based on short (and sub-size) sample tests; (2) measure the overall AC (coupling) losses at various operating conditions (and again compare to short sample measurements); (3) check the effect of cycling on (1) and (2), especially noting the very long term cycling effects predicted by mechanical simulations; (4) investigate stability to fast ramping of the current; (5) investigate the quench behavior.…”

“…The problem of a sudden quench of the conductor during tests is somehow ubiquitous in the case of NbTi CICC and related to a high magnetic field gradient on the conductor cross section, due to the self field [1], [2], [7]. When this phenomenon occurs at temperature/current values lower than expected from single strand measurements, as in the PFCI-FSJS, it may be qualitatively explained as a local quench induced by a highly uneven current distribution, with a strand overload (at locally high electric field) that can hardly be re-distributed because of a low inter-strand voltage.…”

<tex>$rm J_rm C(rm B,rm T)$</tex>Characterization of NbTi Strands Used in ITER PF-Relevant Insert and Full-Scale Sample

“…Two dedicated PF conductor samples (PF-FSJS and PFCI) were tested [9], [10] in SULTAN facility (Villigen, Switzerland).…”

Preparation of the ITER Poloidal Field Conductor Insert (PFCI) Test

“…At present, the knowledge of the behavior of ITER-type NbTi CICC is mainly based on the test of sub-size samples, and of short full-size samples and joints [6]- [8]. Among the interesting results of these tests (including some still open problems), we may quote the so-called sudden quench of the conductor at currents (and/or temperatures) lower than expected from the single strand performance [9].…”

“…1. Also for the first time, an ITER-PF-type joint [10] will be tested in the PFCI in time-varying magnetic field acting both in axial direction, i.e., in the "plane" of the joint (as was already the case in [6]) and in radial direction, i.e., orthogonal to that "plane"-a completely new entry for this type of joint (although ITER-CS-type joints, US design, were tested in the past in both parallel and transverse pulsed fields, at the MIT Pulse Test Facility [11]). …”