Keeman Kim

Results of a New Generation of ITER TF Conductor Samples in SULTAN

A new generation of ITER TF conductor samples has been assembled and tested in SULTAN in 2007 following a common procedure agreed among the ITER parties. The test results of six SULTAN samples, made of twelve conductor sections manufactured in Europe, Japan, Korea and Russia, are reported here. The conductor layout reflects the ITER TF conductor design, with minor differences for the Nb3Sn strand characteristics, void fraction and twist pitch. The object of the test is a straight comparison with the ITER requirement of 5.7 K current sharing temperature at 68 kA current and 11.3 T field. A broad range of behavior is observed.

Results of the TF conductor performance qualification samples for the ITER project

The performance of the toroidal field (TF) magnet conductors for the ITER machine are qualified by a short full-size sample (4 m) current sharing temperature (T-cs) test in the SULTAN facility at CRPP in Villigen, Switzerland, using the operating current of 68 kA and the design peak field of 11.8 T. Several samples, including at least one from each of the six ITER Domestic Agencies participating in TF conductor fabrication (China, European Union, Japan, Russia, South Korea and the United States), have been qualified by the ITER Organization after achieving T-cs values of 6.0-6.9 K, after 700-1000 electromagnetic cycles. These T-cs values exceed the ITER specification and enabled the industrial production of these long-lead items for the ITER tokamak to begin in each Domestic Agency. Some of these samples did not pass the qualification test. In this paper, we summarize the performance of the qualified samples, analyze the effect of strand performance on conductor performance, and discuss the details of the test results.

Strain dependence of critical current in internal tin process Nb/sub 3/Sn strands

The development of a high performance superconducting magnet requires a thorough understanding of the strain effect in the critical current density (J/sub c/) of Nb/sub 3/Sn strands. A modified version of the WASP device is used for the investigation of the strain effect. The spring is made of BeCu alloy and is capable of applying strain up to 0.7% reversibly at 4.2 K. Several types of high performance Nb/sub 3/Sn strands (J/sub c/>1000 A/mm/sup 2/ at 12 T and 4.2 K) have been developed using internal tin process. A comparative study on the axial strain dependence of the critical current is performed.

A Variable Temperature Walters Spiral Probe for the Critical Current Measurement of Superconducting Strands

We have developed a probe for the critical current measurements of low temperature superconducting strands at various field, temperature and strain. A 30 cm-long strand sample is soldered on a Walters spiral made of beryllium copper alloy and compressive or tensile axial strain can be applied up to 0.7%. Temperature control ability was tested using a MgB2 wire up to 30 K. Three cernox sensors are attached on a spiral adjacent to the strand and the temperature of the spiral is controlled within plusmn50 mK of target temperature during the critical current measurement up to 80 A. Extensive critical current measurements have been carried out for an internal-tin processed Nb3Sn strand. It was found that the measured field, temperature and strain dependence of the critical current for the internal-tin Nb3Sn strand is in agreement with the recent scaling law based on strong coupling theory of superconductivity.

Development and Sultan Test Result of ITER Conductor Samples of Korea

As a way to improve performance of ITER TF conductors, two types of cable-in-conduit conductors were developed in Korea with variations of conduit thickness resulting in the different void fraction of the conductors. The estimated void fractions of the conductors are 31% and 33%. Here we report the details of the TF conductor development and the performance test result of them carried out in SULTAN. Regarding the conductor development, the internal-Sn-processed strand characteristics, strand cabling, twist pitch and characteristics of the conduits for the conductors are presented. For an extended understanding of the conductor design and performance, the SULTAN test results are presented and the effect of the void fraction variations is discussed based on the results.

Effects of Cr Diffusion on

Nb3Sn strands which will be used for the ITER TF coils should be chrome plated thickness of 1.8 mum to reduce AC loss and improve the thermal stability of cable. During heat treatment, the RRR value can be affected by Cr diffusion. In this work, the effect of Cr diffusion was systematically studied, using the KSTAR and ITER candidate strands. For the KSTAR strand, the Cu/non-Cu ratio was systematically varied from 1.53 to 0.69 by chemical etching whereas the samples were heat treated by the same schedule. For the ITER candidate strand, on the other hand, the Cu/non-Cu ratio was fixed to the specification value of 1.0, and the variation of the RRR value was studied with respect to the heat treatment schedule, especially the retention time of the 650degC plateau was varied from 100 to 200 hours. It was also compared with the Cr diffusion distance obtained from EPMA (electron probe microanalysis). We found that the RRR value of the ITER candidate strand becomes lower than the ITER requirement of 100, if the heat treatment at 650degC is longer than 200 hours. The RRR can be reached up to 161 when the duration of 650degC plateau is reduced to 100 hours, while satisfying all other ITER requirements including the critical current density.

RRR

We find a simple expression for the field dependences of the n-value for Nb3Sn strands which gives reasonable fits to empirical data. It is also found that a relationship between the n-value and the critical current, which is obtained from an assumed reduction of the flux line lattice shearing pinning force due to thermal activation, can describe the field dependence of the critical current. Using the relation between the n-value and the critical current, we propose a scaling law which we have called the Kramer model including thermal activation. It is shown that the proposed scaling law can explain the critical current data at 4.2 K, even at high field near the upper critical field. It is also compared with Ekin’s strain scaling law.

Values of Cr-Plated Nb

According to the pre-qualification program for the production of Korean conductor for ITER TF magnet, a CICC(Cable In Conduit Conductor) sample for SULTAN test was developed based on the ¿Option 2¿ specification which is the improved one among the two established baseline designs. For the assessment of performance, SULTAN test of the conductor sample was carried out in CRPP employing recently updated instrumentations on the sample and the joint of two conductor pieces, which has been discussed and modified for the proper estimation of . As a course of the analysis, the assessment of values was presented using the basic protocol of data reduction, and the discussion was made on the standard process of analysis. In addition, we investigated some exposed problems in the data manipulation such as non-linear voltage slopes being possible to be related to conductor characteristic itself and the initial offset problem in the calorimetric method. On the presentation of the test results, we also report the manufacture of the conductor with the qualification of the components including carefully designed Nb3Sn internal-tin superconducting strands for an improvement of conductor performance.

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The Republic of Korea is participating in the ITER project to construct and operate the ITER tokamak for the purpose of demonstrating the feasibility of fusion power. ITER Korea, the implementing agency for the Republic of Korea that is procuring items for the ITER project on behalf of the Korean government, has established domestic and international contracts for the procurement of cable-in-conduit conductor (CICC) that will be used in the ITER toroidal field (TF) magnets. The CICC for the ITER TF magnets is made of superconducting and copper strand cable inside a cylindrical stainless steel jacket, and is designed to operate at a nominal peak field of 11.8 T at 4.5 K with 68 kA of nominal operating current. Recently, the first CICC from Korea, which will be installed in an ITER TF magnet, has been manufactured and tested including testing performed on a 4 m sample near ITER operating conditions at the CRPP-EPFL SULTAN facility in Villigen, Switzerland under the coordination of the ITER International Organization. This paper provides a brief description of the CICC along with preliminary results of the tests and the conductor performance characteristics derived from the results.

Sn Strands Fabricated by Internal-Tin Process

Two identical conductor samples were fabricated from 110 m length cable prepared for qualification of the manufacturing process for Phase II procurement of ITER TF conductor. Superconducting strand characteristics, cabling specifications, and the stainless steel jacket sections are described. Sample assembly and instrumentation of various voltage taps and temperature sensors have been prepared according to specified procedures for conductor performance qualification. The performance test program which was agreed to by the SULTAN working group was applied to the conductor samples. To assess the current sharing temperature , standard analysis procedures were adopted. The of both samples at 68 kA with a background field of 10.78 T after a 1000 cyclic load are well above the acceptance criteria. Behavior of individual “star” voltage taps which are located at different positions and which are possible origins of rather large discrepancies in the of identical samples is discussed. The effective strain and the transition index of the samples are obtained from the experimental data.