Heekyung Choi

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.

Relation between the critical current and the n value of ReBCO thin films: A scaling law for flux pinning of ReBCO thin films

Detailed field and angle dependencies of the critical current and the n value for a SmBCO coated conductor have been measured. It was found that the field dependence of the n value can be fitted by an empirical power law with three parameters including the irreversibility field. We also found that there is a correlation between the critical current and the n value which can be described by the Kramer model including thermal activation. The model fits the field dependence of the empirical critical current data at various angles and temperatures with three fitting parameters, the pinning force maximum, the g factor, and the upper critical field. The upper critical field found from fitting was higher than the irreversibility field, and the angular dependence of the upper critical field is in agreement with the Tinkham model. The pinning force maxima do not show a correlation with the upper critical or the irreversibility fields, which is attributed to the difference in the pinning mechanism with a variation ...

The Effect of Plastic Deformation on Low Temperature Mechanical and Magnetic Properties of Austenite 316LN Tube for ITER TF Conductor

While the manufacturing of superconducting CICC (cable in conduit conductor) and winding conductors into winding packs for ITER TF magnets, cold work is unavoidably applied on the jacketing tube. This article investigates the effect of plastic deformation on low temperature mechanical and magnetic properties of austenite 316LN stainless steel tube for ITER TF conductor. To simulate the manufacturing process of TF magnet, tubes were compacted, extended to 2.5% in longitudinal direction and heat treated. After each step, specimens were sectioned using wire cutting EDM, and tensile tests at 4 K, magnetic susceptibility measurements, and hardness tests have been carried out on those specimens. Remarkable reductions of elongation at failure have been observed as the amount of cold work is increased through compaction and extension. Further, correlations among results of tensile tests, magnetic susceptibility measurements and hardness tests are presented.

Overview of Conductor Production for ITER Toroidal Field Magnet in Korea

The ITER toroidal field (TF) conductor is made up of superconducting Nb3Sn and copper strands assembled into a multistage, rope-type cable inserted into a conduit of butt-welded stainless steel jacket sections. For the ITER Project, the Korean Domestic Agency (KODA) took the responsibility of the procurement of 27 superconducting conductors for the ITER TF magnets. After concluding the Procurement Arrangement (PA) with the ITER International Organization in May 2008, KODA has been implementing the PA through four major industrial contracts: (1) Nb3Sn strand, (2) cable, (3) stainless steel jacket sections, and (4) jacketing. Prior to the production of conductors required for the TF coils, one 760-m-long copper dummy conductor and one 100-m-long superconducting conductor were fabricated for manufacturing process qualification. As of June 2013, 16 TF conductors were successfully manufactured. The full-size conductor performance tests in the SULTAN facility yielded very high performance. This article describes the technical requirements of the TF conductor and how KODA has been manufacturing the conductors with a high-level quality assurance/quality control system. It also presents the results of acceptance tests, including those of the SULTAN test.

Prototype

The ITER tokamak being constructed in southern France will incorporate a central solenoid (CS) coil consisting of 6 modules stacked one on top of the other. Each module will be constructed from round-in-square Nb3Sn based superconducting cable-in-conduit conductor (CICC). Production of CS conductor for the ITER tokamak had been delayed due to discoveries of faster than expected performance degradation. Investigations into mitigating this shortfall have included using internal-tin (IT) route Nb3Sn strand in place of the bronze (BR) route strand that was originally proposed. Prototype ITER CS conductor samples have been produced using IT route Nb3Sn strand in conjunction with cabling optimization, and conductor performance tests show that they satisfy ITER performance requirements. One of the suppliers of IT route ITER Nb3Sn strand, which will be supplying superconducting strand for ITER CS conductor, is Kiswire Advanced Technology Ltd. (KAT) of Daejeon, Korea. The characteristics of their prototype Nb3Sn strand for ITER CS conductor are presented. Furthermore, performance degradation of the CS strand due to possible strand deformations during the cabling process was also investigated. The results show similarities as well as differences with recent results obtained with BR route Nb3Sn strand. However, direct interpretation of the strand deformation versus performance results may not lead to correct predictions of cable performance because of the strand contact interaction mechanism investigated.

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

Manufacturing of superconducting Nb3Sn based Cable-in-Conduit Conductor (CICC) for Toroidal Field (TF) coils for the ITER tokamak under the responsibility of the ITER Domestic Agency for the Republic of Korea, ITER Korea, has been in progress since early 2012. Production of sufficient CICC needed to fabricate the first TF coil solely using Korean (KO) conductor has been achieved. Furthermore, low temperature performance verification tests near ITER operating conditions on a second CICC sample, following tests on a sample from the first KO CICC destined for an ITER TF coil, have recently been performed at the EPFL-CRPP SULTAN facility in Villigen, Switzerland. This paper provides an overview on all CICC that is destined for the first ITER TF coil to be fabricated with KO CICC, along with a brief summary of the results of the low temperature performance verification tests on KO CICC samples.

Superconducting Strand for ITER CS Coil Conductor Produced in Korea Using the Internal-Tin Route

The Korean Domestic Agency (KODA) for the International Thermonuclear Experimental Reactor (ITER) project has taken the responsibility of the procurement of cable-in-conduit conductors for the ITER toroidal field magnet. While procuring 27 conductor unit lengths, which are the Korean sharing, 10 conductors have gone through the superconducting performance test, under low temperature and high magnetic field, as one of the final acceptance tests. The sample assembly and instrumentation of various voltage taps and temperature sensors have been done according to the specified procedure, which had been agreed between domestic agencies and the ITER International Organization. The performance test program, which has also been agreed, was carried out. For the assessment of the current sharing temperature (Tcs), the standard analysis procedure has been adopted. Tcs of all samples, at 68 kA and a background field of 10.78 T, after 1000 cyclic current loads are well above the acceptance criterion. By analyzing the electric field as a function of temperature, the effective strain (εeff) and transition index (n-value) were reduced, and the behavior of εeff will be discussed in terms of the electromagnetic load, i.e., I × B. The effective strain was compared with the physical strain and results with a numerical simulation.

Manufacturing and Preliminary Performance Expectations of

We have developed an apparatus to investigate the effect of transverse stress under axial strain on the critical current of superconducting strands. An internal tin Nb3Sn wire is soldered on a C-shape ring (called Pacman) made of 2% beryllium doped copper alloy and axial strain from -0.7 to 0.7% can be applied. The actual strain on a sample was estimated from a comparison with the critical current measurement results using a Walter spiral probe. Reversible transverse stress effects on the critical current were studied in this work and the transverse pressure was applied up to 40 MPa under ±0.5% axial strain. The critical current was initially increased about 3% as transverse load was applied and then decreased almost linearly under compressive axial strain. Similar behavior is observed under tensile axial strain but the critical current decreases rather sharply at higher load. A unified description based on 3 dimensional deviatoric strain was difficult for the sample studied in this work.