F. Bellina

Electromagnetic analysis of superconducting cables and joints in transient regime

The paper deals with the electromagnetic analysis of the superconducting multistrand cables used to manufacture magnets for fusion research. An analysis method is described, in which the cable is represented as an equivalent lumped network, whose parameters are automatically computed starting from the cable geometrical data. The network is then solved in transient regime.

Superconductive cables current distribution analysis

A computational tool is being developed for the analysis of superconductive magnets, combining detailed descriptions of termination joints and cables to thermo-hydraulic (TH) models. In parallel, an experiment (Stability Experiment Upgrade—SexUp) has been designed with the target to study the current distribution in cable in conduit conductors (CICC). Finally, the establishment of a reliable method for the measurement of the current distribution profile on the cable cross section is being implemented on the ITER toroidal field model coil (TFMC).

Electromagnetic Analysis of the Voltage-Temperature Characteristics of the ITER TF Conductor Samples

The measurement of the current sharing temperature of a Cable in Conduit Conductor is a complex task. The voltage traces measured on the conduit (jacket) of these conductors during tests, give V-I characteristics that significantly differ from the typical transition from the superconducting to the resistive state measured on single strands. In these measurements, after an initial ramping of the transport current up to the test value, the temperature is increased by steps until the resistive transition occurs. However, even in the first phase of the current ramping, when the cable is still fully or almost superconductive, early voltages are measured along the jacket at different angular positions around the cable. In particular, this was observed in the ITER TF conductors recently tested in the SULTAN facility at Villigen PSI, Switzerland. These samples showed significant ramps of voltages taken in the proximity of the joints and terminations. The present paper gives a possible qualitative and quantitative explanation for this mechanism. Numerical simulations are also shown which reproduce the scattering of the voltages measured around the jacket at a given location, showing the effect of the voltage taps position along the jacket in a six sub-cable model.

Current distribution measurement on the ITER-type NbTi bus bar III

The Bus Bar III (BBIII), fabricated within the Toroidal Field Model Coil Task of the International Thermonuclear Experimental Reactor (ITER), was tested at the Forschungszentrum Karlsruhe, Germany, in the spring of 2004. The BBIII consists of an approximately 7 m long NbTi dual-channel conductor with a thick square stainless steel jacket, cooled by forced flow supercritical He. It was energized with currents up to 80 kA and operates in its self magnetic field (up to /spl sim/0.8 T). The BBIII was instrumented with Hall-probe heads and arrays, voltage rings and longitudinal voltage taps for electro-magnetic measurements, in order to get experimental data to be used for the validation of a recently developed hybrid thermal-hydraulic electro-magnetic code (THELMA), as well as for the assessment of the possibility of performing a reliable reconstruction of the current distribution in the conductor cross section under controlled conditions. In the tests, current ramps at different rates were applied to characterize the conductor time constants, while two different resistive heaters (one upstream of the BBIII inlet, another one directly on the BBIII jacket) were separately operated in order to approach current sharing in the conductor and to observe the related current re-distribution. In this paper, a summary of the collected experimental results is presented, with particular emphasis on those aspects more relevant for the forthcoming THELMA analysis.

Analysis of the optimal location of magnetic field probes for the determination of the current distribution inside S/C cables

During the tests on superconducting cables, the magnetic field is often measured by means of pick-up or Hall probes and these data are used to reconstruct the current distribution inside the cable. The efficiency and the accuracy of the reconstruction procedure depend on the algorithm adopted and on the location of the field probes. For a given geometry of the lines of current, the determination of the current distribution can be mathematically formulated as a linear inverse problem, in which the residual between the measured and the computed field values at the probe locations is minimised. A linear least-squares problem results, which is solved by means of the truncated singular value decomposition technique, applied to the matrix G which relates the unknown currents to the computed magnetic field values. The condition number of G is proposed here as an index of the quality of the location of the probes: a good location corresponds to a well conditioned G, while a bad location gives a worse conditioned matrix. In this paper a reconstruction procedure is presented and a criterion for the optimal probe positioning is shown.

The THELMA Model of the Joint Resistance Distribution Tests with JORDI

In a cable-in-conduit conductor (CICC) operating in DC mode, an uneven current distribution can be present, due to the non-uniform contact resistance between the individual strands and the resistive part of the termination. In the JORDI (JOint Resistance Distribution) facility at CRPP, the contact resistance distribution among the sub-cables (channels) of a large CICC termination was measured in 2006 under conditions of balanced, impressed currents. The voltage was measured between each channel and the mid-plane of the connection with the facility current lead, so a set of channel equivalent resistances were obtained, which depend on the contact non uniformity. This experiment has been simulated with the THELMA code, to investigate its capability to reproduce the equivalent resistance distribution. To this purpose, the termination has been modeled as an equivalent lumped network, in which all the channels are individually represented. The paper describes the model, compares the results of the simulations with the measured values and discusses the influence that the model geometrical and electrical parameters have on the resistance distribution.

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

As the test of the PFCI is foreseen in 2006 at JAERI Naka, Japan, it is essential to consider in detail the lessons learned from the short NbTi sample tests, as well as the issues left open after them, in order to develop a suitable test program of the PFCI aimed at bridging the extrapolation gap between measured strand and future PF coil performance. Here we consider in particular the following issues: 1) the actual possibility to quench the PFCI conductor in the TCS tests before quenching the intermediate joint, 2) the question of the so-called sudden or premature quench, based on SULTAN sample results, applying a recently developed multi-solid and multi-channel extension of the Mithrandir code to a short sample analysis; 3) the feasibility of the AC losses calorimetry in the PFCI

Analysis of the ITER

This paper presents the models of three new-generation high-performance samples of ITER full-size Nb3Sn Cable-in-Conduit conductors (CICCs). These samples were recently tested in SULTAN, at CRPP (Switzerland), and demonstrated the technical feasibility of CICCs that satisfy the ITER specifications. The improvements of these samples regard not only the cable strand and the cabling parameters, but also the manufacturing details of their SULTAN samples. A set of changes have been adopted on the SULTAN sample termination and joint, to try to achieve a more uniform current distribution among the strands and a more accurate measurement of the strand longitudinal voltages. This paper presents a comparative numerical analysis of the effects that these different joint technologies have on the DC current distribution among the strands of the SULTAN samples cable. A correlation between the current distribution in the sample and the resistance distribution at the joint is also shown and discussed.

{\hbox{Nb}}_{3}{\hbox{Sn}}

In this paper, we discuss the predictive analysis performed in support of the test program of the International Thermonuclear Experimental Reactor (ITER) poloidal field conductor insert (PFCI). A subset of the test program items was considered, with particular emphasis on DC performance and AC losses. The results and implications of the comparison of selected predictions from different laboratories will be presented.

SULTAN Sample Test Conditions With Different Joint Technologies

Bus Bar III (BBIII) is a curvilinear NbTi full-size ITER-type multistrand superconducting cable 7 m long, tested in 2004 at Forschungszentrum Karlsruhe, Germany. The sample was cooled with a forced helium flow and was equipped with many electromagnetic and thermal-hydraulic sensors, to acquire as wide as possible an experimental database, aimed at getting a benchmark for the validation of the THELMA code, a computer tool for the analysis of CICC prototypes and magnets, recently developed and presently under validation. This code can compute the current distribution among the macrostrands used to represent the cable, taking into account their superconducting behavior, as well as the temperature and the magnetic field distribution. The paper deals with the application of THELMA to some tests on BBIII useful for the electromagnetic model validation and presents in detail the analysis setting-up, the results, its critical aspects and limits. A comparison is shown between the measured and the computed waveforms of the electromagnetic data