L. Zani

J/sub C/(B,T) characterization of NbTi strands used in ITER PF-relevant insert and full-scale sample

In the framework of the R&D for the ITER Poloidal Field Coils (PFC), a testing campaign was achieved on NbTi strands used in PFC-relevant modules. Three candidate NbTi strands have been used for manufacture of two Full-Size Joint Samples (PFIS and PF-FSJS) and the PF Conductor Insert (PFCI): - one strand manufactured by Europa Metalli (Italy), Ni plated and 0.81 mm diameter; - one strand manufactured by Alstom (France), with a CuNi barrier and 0.81 mm diameter; - one strand manufactured by Bochvar VNIINM (Russia), Ni plated and 0.73 mm diameter. An extensive test campaign has been performed at GHMFL (CNRS, Grenoble) in 2003 using the CEA VTC facility at temperatures ranging from 4.2 K to 7.5 K and at magnetic fields ranging from 2.5 to 10.5 T. The results showed that all three strands have quite close performances in the ITER PFC operating range despite their structural differences and that due to the critical current density curvature with temperature, they fail to meet the 1.5 K PFC temperature margin criterion by 0.2 to 0.3 K. Additionally, applications of corresponding fits to PF-FSJS tests showed consistent results and finally a brief discussion on fitting formula is presented.

Completion of TF Strand Production and Progress of TF Conductor Manufacture for JT-60SA Project

In the framework of the JT-60SA project, aiming at upgrading the present JT-60U tokamak, Europe, as part of its in-kind contribution within the Broader Approach agreement, will provide the toroidal field (TF) magnet system. For this purpose, Fusion for Energy is committed to procure about 29 km of TF conductor. The TF conductor is cable-in-conduit type and includes 486 strands (2/3 NbTi-1/3 copper) wrapped with a thin stainless steel foil and compacted into a rectangular stainless steel jacket. The procurement is split into two main contracts: one for TF strand manufacturing and the other for TF conductor cabling and jacketing. TF strand manufacture was completed while the TF conductor one is being finished. In the present paper, we draw an overview of both productions emphasizing on the quality control (QC) approaches and on aspects relevant to risk management of the JT-60SA tokamak operation. For the NbTi strand, the complete production overview is provided including extensive statistical considerations on NbTi strand critical performances ( IC, TCS). For the TF conductor, the overview also deals with collected results from acceptance tests and peripheral tests led for consolidating the QC (hydraulic tests, nondestructive examination, full-size sample cold tests in SULTAN facility).

Characterization of transport properties variations with magnetic field and temperature of ITER-candidate NbTi strands

Abstract While the International Thermonuclear Experimental Reactor (ITER) conceptual design retained the Nb 3 Sn for toroidal field (TF) and central solenoid (CS) coils, the low working field (around 6 T) promoted the choice of NbTi for the poloidal field (PF) coils. EU has carried out the experimental study of industrial NbTi strands and cables with different internal structures in order to choose the one which generate the lowest losses when used in the PF operating conditions (i.e. pulsed field). CEA has contributed to this project through the experimental study of the transport properties variations with respect to magnetic field and temperature of two candidate strands. One of them contains an internal CuNi barrier and the other is Ni-plated. A homemade cryostat is used to control the temperature of the sample which is wound on a VAMAS-like mandrel. J c measurements are presented here and subsequently the parameters deduced from scaling laws and their variation with temperature between 4.2 and 7 K and with field up to 11 T. A comparison between the two strands characteristics and ITER PF coils criteria is also discussed. The results are in good agreement with literature and lie inside an acceptable range in spite of some discrepancy with the ITER PF criteria: a recent thermo-hydraulic simulation confirmed it. In the future, this study, completed by AC losses measurements on cabled strands, should help to optimise the strands performances below the ITER PF security margins.

A New Design for JT-60SA Toroidal Field Coils Conductor and Joints

The upgrade of JT-60U to JT-60 Super Advanced (JT-60SA), a fully superconducting tokamak, will be performed in the framework of the Broader Approach (BA) agreement between Europe (EU) and Japan. In particular, the Toroidal Field (TF) system, which includes 18 coils, is foreseen to be procured by France, Italy and Germany. This work covers activities from design and manufacturing to shipping to Japan. The present paper is mainly devoted to the analyses that lead to the conductor design and to the technical specifications of the joints for the JT-60SA TF coils. The conductor geometry is described, which is derived from Cable-In-Conduit concept and adapted to the actual JT-60SA tokamak operating conditions, principally the ITER-like scenario. The reported simulations and calculations are particularly dealing with the stability analysis and the power deposition during normal and off-normal conditions (AC losses, nuclear heating). The final conductor solution was selected through a trade-off between scientific approach and industrial technical orientation. Besides, the TF system connections layout is shown, derived from the industrially assessed twin-box concept, together with the associated thermo-hydraulic calculations ensuring a proper temperature margin.

Manufacture and test of NbTi subsize joint samples for the ITER poloidal field coils

In the framework of the design and the development of the ITER PF coils, two subsize joint samples (scale /spl sim/1/6) made of two different NbTi strands were manufactured at CEA/Cadarache (France) and were tested in the JOSEFA test facility at CEA. The characterization of joints was achieved in the areas of resistance T/sub CS/, pulsed field losses, and thermodynamic stability. The experimental results were interpreted with regard to the ITER specifications.

Influence of bending strain on the critical properties of jacketed Nb3Sn strands for ITER

The tests analysis of the International Thermonuclear Experimental Reactor (ITER) model coils showed lower performances than expected. For that reason, a specific program has been launched in Europe for investigating the influence of bending strain on the transport properties of high-performances Nb3Sn strands inside cable-in-conduit conductors when experiencing significant Lorentz forces. To this purpose, an experimental campaign has been initiated on single Nb3Sn strands jacketed inside a 0.2 mm thick stainless-steel (SS) tube in order to be tested in mechanical ITER Toroidal Field (TF) Coils relevant conditions. In this article we describe in a first part the procedure investigated at CEA to impose a controlled pure bending strain upon jacketed stands and we present the results of the qualification tests carried out in the ENEA facility to assess the reliability of the chosen method. In a second part we propose a new model to simulate the strands behaviour experiencing bending strain together with an initial compressive strain. To understand our results, a key parameter considered is a specific weighting function relying on strand interfilament current redistribution capacity. At the end, this model enables us to explain more reliably the experimental results but also shows discrepancies which requires further improvements and extra refinements

Study of Current Distribution in ITER TFMC NbTi Busbar III

In the framework of the development of High Temperature Superconducting current leads (HTSCL), one demonstrator was tested at Forschungszentrum Karlsruhe (FZK) in 2004 together with a conventional current lead. Both were connected by a superconducting short circuit NbTi conductor, referred as busbar III (BBIII). Here the BBIII was used for current distribution measurements. In addition to a 64 Hall Probe (HP) system, two additional 4-HP heads from CEA were assembled at both ends of the BBIII. This system, already successfully used during the TFMC test phase II, was now adapted by FZK to the present BBIII set-up. Electrical tests were performed for conductor currents up to 80 kA without background magnetic field. In this paper, the analysis of the current distribution is presented using two methods: (1) The current barycenter excursion was investigated in various runs. A comparative study is presented between resistive and inductive hypotheses where the current is supposed to be uniformly distributed. In particular, we show that the two hypotheses lead to different results. (2) A global model of the current bundles distribution between four artificial subcables is investigated. More quantitative results show current imbalances between subcables to be either significant or weak, depending on whether the resistive or the inductive hypothesis is considered. A discussion on why the second solution is thought to be more likely is presented. In addition, a global comparison with results previously obtained during the TFMC tests will be shown

Effect of joint quality on conductor short sample performance

Full size conductor short samples for the International Thermonuclear Experimental Reactor (ITER) are composed of two straight conductors connected together at one end, and to the facility current leads at the other ends. The quality of these electrical connections was early suspected to play a role in the measurement of the conductor performance, which has been confirmed by recent test results. In order to investigate this phenomenon, CEA developed an experimental program, within EFDA task ELRES, to study the effect of well calibrated joint defects on subsize NbTi conductor performances. Two types of NbTi strands associated with two different cable void fractions were used in order to vary the interstrand resistances. Experimental results clearly show that joint defects degrade measured conductor critical currents, except when sufficiently low interstrand resistances allow a possible current redistribution among strands. Analysis of Hall probe signals show evidence of current redistribution among subcables during the tests.