SULTAN Test Facility Research Articles

Modelling and analysis of quench in the 15-kA HTS conductor

High Temperature Superconductors (HTS) are very promising materials for possible application in future fusion magnets, with significant R&D progress on HTS conductors in recent years. However, since geometric and thermo-physical characteristics of HTS and LTS conductors differ significantly, some doubts have arisen if the approaches successfully used in numerical simulations of the thermal–hydraulic behavior of LTS conductors would be sufficient also for HTS, particularly in cases when fast transient processes (such as e.g. quench) are considered. In order to provide data for better understanding of the quench phenomenon in HTS conductors as well as for testing different numerical approaches and proper tuning of the numerical codes, a dedicated experimental campaign (Quench Experiment) was carried out at the SULTAN test facility within the international collaboration between the EUROfusion consortium and China. Our present study is a part of the work on analysis and interpretation of the data collected during this experiment. Simulations of the chosen experimental run were performed using two THEA models with different levels of complexity. Uncertain model parameters (thermal resistances and copper RRR) were explored across a wide range. Our goal was to identify the possibly simple model that accurately reproduces the experimental results.

Measurements of AC Loss Evolution in ITER TF Conductors

AC loss in the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cable-in-conduit conductors (CICC) usually decreases after the conductor undergoes electromagnetic cycling. This can be attributed to the increase of inter-strand resistance due to the detachment of the strand-bonding during cyclic loading. In the years 2018 to 2020, ITER launched a series of test campaigns in SULTAN test facility, whose aim was to study the T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> degradation of TF conductors of all manufactures. Along the main scope of the project, an accompanying study of the AC loss evolution has been measured on two samples. The AC loss was measured prior to any electromagnetic loading, after 1, 5, 50 and 1000 cycles, and at the very end also after a thermal cycle to room temperature. Sinusoidal AC loss was measured in the frequency range of 0.1 to 1.0 Hz. The measured AC evolution will help to predict heat load generated in the TF coils during the initial phase of ITER operation. It may also serve as an input to analysts for deducing the evolution of inter-strand resistance during electromagnetic cycling. The main observation is that already after the first electromagnetic cycle the AC loss reduction is significant, namely 80-90% lower compared to the initial conductor state.

Co-Wound Superconducting Wire for Quench Detection in Fusion Magnets

High temperature superconducting (HTS) materials are widely utilized in various design proposals for fusion magnets, resulting in enhanced performance of the machines compared to the past. However, a reliable quench detection in HTS conductors remains an open issue. Using a co-wound superconducting wire of high normal state resistance as an electrically insulated and thermally coupled sensor provides strongly increased sensitivity for the voltage-based quench detection methods. Furthermore, resistance of the wire can be practically proportional to the size of the normal zone, even though the location of the hot-spot cannot be identified. We present adaptation of this method for fusion conductors by considering various wire options, such as MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wires in a highly resistive matrix, non-stabilized Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires and (K,Na)-Ba122 wires. The insulated wire of a small diameter (<1 mm) can be embedded in the steel jacket, thus barely affecting the conductor design and manufacturing aspects. Alternatively, if installed within the cable space, the wires might even allow monitoring of quench dynamics among the strands. Our first experimental demonstration is planned in a sub-scale ReBCO cable-in-conduit sample, which will be tested in the SULTAN test facility recently upgraded for DC operation in resistive samples with the transport currents up to 15 kA and maximum voltage of 10 V.

An analytical model for coupling losses in large conductors for magnetic fusion

During the operation scenarios of large superconducting tokamaks such as JT-60SA or ITER, disruptions or VDE (Vertical Displacement Events), AC losses are deposited in the superconducting magnets. Due to these AC losses, the temperatures of the Cable in Conduit Conductors (CICC) increase, reducing the temperature margins. It is of highest importance for the operation to correctly estimate AC losses and conductor temperatures. Due to the fast transient nature of those events, an important component of the losses is the coupling losses.The approach for coupling losses estimation, using a unique effective time constant “nτ” is not completely adequate to calculate these losses.For this purpose, since 2010, a heuristic analytical model called MPAS (Multizone PArtial Shielding), has been proposed. It is now currently used by several teams.For each conductor of the coils of a tokamak, the parameters of the MPAS model and nτ are estimated based on the characterization of the conductor AC losses under a sinusoidal excitation at different frequencies. This process is extensively conducted in existing facilities such as the SULTAN test facility at SPC (Swiss Plasma Center), on the dipole and press of Twente University or on JOSEFA dipole test facility at IRFM (Research Institute for Magnetic Confinement) at Cadarache.This characterisation leads to the constitution of a database for the different conductors used in the tokamak. This has been pushed to the uttermost within the ITER qualification and QA/QC (Quality Assurance, Quality Control).The methodology for estimating the MPAS parameters in relation with the measured AC losses is described in details and illustrated for two conductors. Using this database, it is then possible to calculate the coupling losses during ITER module discharges and scenarios. Estimation of coupling losses are compared to experimental results in recent experiments on ITER CS (Central Solenoid) modules.

T cs degradation of ITER TF samples due to fast current discharges

Direct current tests performed in the past on the conductor samples of the toroidal field (TF) ITER coils revealed degradation of current sharing temperature, T cs. The degradation progresses with repetitive electromagnetic loading, and also with thermal cycles between 4.5 K and room temperature. This feature was observed on short samples in SULTAN test facility (EPFL-SPC, Switzerland) as well as in TF Insert Coil tests in CSMC test facility (Naka, Japan). We present three independent observations suggesting that initiation of sample quench followed by a fast current discharge, which normally complements every I c and T cs test in both SULTAN and CSMC, enhances the T cs degradation rate. The exact mechanism of this contribution to the degradation remains unidentified.

Inter-Layer Joint of Nb3Sn React&amp;Wind Cables for Fusion Magnets

The Swiss Plasma Center (SPC) has developed a layout of Toroidal Field (TF) coil for EUROfusion DEMO tokamak, based on the reference tokamak baseline of 2015. Each TF coil winding pack is wound with graded Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors and consists of 12 single layers, connected in series by means of inter-layer graded joints. The design of inter-layer joints takes into account the react-and-wind (R&W) manufacturing technique for fabrication of TF coil winding pack, i.e., the inter-layer joint is prepared with use of two already heat treated Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors. The development, preparation and test of inter-layer joint at SPC is performed in frame of R&D program for TF coil of EUROfusion DEMO tokamak. The high-grade Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn TF conductor, operating at 63.3 kA and 12.2 T (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> >6.5 K) was tested at SPC, and afterwards was used for fabrication of inter-layer joint. The developed TF inter-layer joint is an “overlap-type” joint, which can be fit within the dimensions of TF winding pack. Each end of two conductors is copper cladded by a thermal-spraying technique and then bonded together along the surfaces of cladded copper with use of a high-frequency inductor. This paper describes the design of inter-layer joint, process of joint fabrication and test results obtained at SPC at the SULTAN test facility.

AC Loss Measurement of the DEMO TF React&amp;Wind Conductor Prototype No. 2

The EUROfusion DEMO is being designed as the fusion machine to be built after ITER. During the preconceptual phase, several design options are investigated by theoretical analyses as well as tests on newly developed conductor prototypes. One design option for the toroidal field magnet (TF) and central solenoid (CS) is based on flat Nb 3 Sn forced-flow conductors made with react&wind technique. The usage of these conductors simplifies some steps in the conductor and coil manufacturing, and together with graded layer-winding allows approx. 50% reduction of the required amount of Nb 3 Sn compared to ITER-like design based on wind-react-insulate pancake-winding. Two full-size prototype cables for DEMO TF coil were manufactured, jacketed and tested in several test campaigns in SULTAN test facility. The DC results for the second prototype, RW2, rated for 63 kA at 12.3 T, were presented and published in 2018. The AC loss data were collected over several test campaigns performed on various assemblies of RW2 at AC field parallel and perpendicular to the broad cable side. The measurements done at 4.5 K and 20 K allow us to decompose the AC loss contributions originating from the bundle of superconducting strands (hysteresis and coupling loss) and copper-matrix stabilizer located around the cable. The AC loss for sinusoidal and trapezoidal field variations are presented and discussed. The low AC loss of the flat cable makes the cable an attractive choice for the central solenoid operating in a pulse mode.

Preliminary Tests of Soldered and Diffusion-Bonded Splices Between Nb3Sn Rutherford Cables for Graded High-Field Accelerator Magnets

High-field accelerator magnets of the next generation will require coil grading in order to decrease the amount of superconductor and hence the magnet cost. The joints between Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor grades must have low electrical resistance (<;1 nΩ) and reasonably good mechanical strength. The joints are classified as “Wind & React” because they will be assembled during the coil winding, prior to the heat treatment. Various techniques have been initially considered for the preparation of joints between Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Rutherford cables, including soldering, diffusion bonding with and without interleaved copper foil, electroplated cables (to “smooth” the surface of the cable overlap), and crimped cables. Initial investigations included examinations of cross-sections (cut by electro-erosion). In a second stage, joint samples are prepared and assembled for test in the SULTAN test facility, with background field up to 11 T and operating current up to 14 kA. Besides the resistance test results, the selection of the most promising joint layout will also consider the restriction imposed by the coil manufacturing, e.g., available space for assembly, access after heat treatment, applicable quality control, etc.

Development of cable-in-conduit conductor for ITER CS in Japan

The National Institutes for Quantum and Radiological Science and Technology has the responsibility to develop a cable-in-conduit conductor (CICC) for the ITER central solenoid (CS). Qualification tests of CICCs fabricated in the initial development stage were carried out at the SULTAN test facility; the superconducting performance (Tcs = current-sharing temperature) was found to be degraded by the repeated cyclic loading that simulates realistic ITER operating conditions. From destructive examination and neutron diffraction tests, this degradation appears to result from bending strain on the strands generated by electromagnetic forces. In response, the cabling of the CICC was optimized by shortening the twist pitch to make it stiffer against electromagnetic forces. No Tcs degradation of the optimized CICC was seen in the subsequent SULTAN test; further, a CS insert (CSI) test was performed at the CS model coil test facility, which included hoop strain for a more realistic simulation of ITER conditions. Good performance was also achieved in the CSI test.

Inter-Layer Joint for the TF Coils of DEMO—Design and Test Results

In summer 2015, a new reference baseline is issued for the DEMO EUROfusion tokamak. Thereafter, the toroidal field (TF) coils have been updated with the new layout of the react-and-wind conductor proposed by the Swiss Plasma Center (SPC). Each TF coil consists of 12 single layers of graded Nb3Sn conductors connected in series by “invisible” inter-layer joints fully embedded in the winding pack. The high-grade Nb3Sn conductor operates at 63 kA, 12.2 T with Tcs > 6.5 K. The new prototype of the high-grade cable had been manufactured by ENEA at TRATOS, Italy, and delivered to SPC: 20 m of Nb3Sn and about 10 m of copper dummy conductor. The SULTAN conductor sample consisting of two identical, beforehand heat-treated conductors was prepared and tested at SPC in May–June 2017. Upon the completion of comprehensive conductor tests, the one of those conductors was used for manufacturing of the designed at SPC potentially applicable inter-layer TF joint. The conductor was cut, and then, joined again in accordance with the developed at SPC design of inter-layer joint. The SULTAN sample comprising the earlier tested non-destroyed conductor and the earlier tested, but then cut and joined conductor was re-assembled and tested in the SULTAN test facility at SPC in August 2017.

Theoretical analysis for the mechanical behavior caused by an electromagnetic cycle in ITER $$\hbox {Nb}_{3}\hbox {Sn}$$ Nb 3 Sn cable-in-conduit conductors

The central solenoid (CS) is one of the key components of the International Thermonuclear Experimental Reactor (ITER) tokamak and which is often considered as the heart of this fusion reactor. This solenoid will be built by using \(\hbox {Nb}_{3}\hbox {Sn}\) cable-in-conduit conductors (CICC), capable of generating a 13 T magnetic field. In order to assess the performance of the \(\hbox {Nb}_{3}\hbox {Sn}\) CICC in nearly the ITER condition, many short samples have been evaluated at the SULTAN test facility (the background magnetic field is of 10.85 T with the uniform length of 400 mm at 1% homogeneity) in Centre de Recherches en Physique des Plasma (CRPP). It is found that the samples with pseudo-long twist pitch (including baseline specimens) show a significant degradation in the current-sharing temperature (Tcs), while the qualification tests of all short twist pitch (STP) samples, which show no degradation versus electromagnetic cycling, even exhibits an increase of Tcs. This behavior was perfectly reproduced in the coil experiments at the central solenoid model coil (CSMC) facility last year. In this paper, the complex structure of the \(\hbox {Nb}_{3}\hbox {Sn}\) CICC would be simplified into a wire rope consisting of six petals and a cooling spiral. An analytical formula for the Tcs behavior as a function of the axial strain of the cable is presented. Based on this, the effects of twist pitch, axial and transverse stiffness, thermal mismatch, cycling number, magnetic distribution, etc., on the axial strain are discussed systematically. The calculated Tcs behavior with cycle number show consistency with the previous experimental results qualitatively and quantitatively. Lastly, we focus on the relationship between Tcs and axial strain of the cable, and we conclude that the Tcs behavior caused by electromagnetic cycles is determined by the cable axial strain. Once the cable is in a compression situation, this compression strain and its accumulation would lead to the Tcs degradation. The experimental observation of the Tcs enhancement in the CS STP samples should be considered as a contribution of the shorter length of the high field zone in SULTAN and CSMC devices, as well as the tight cable structure.

Investigation of RF Nb-Ti PF 1&amp;6 Coils Conductor Performances in the SULTAN Test Facility

This paper summarizes the work on the manufacturing and testing of conductor samples for ITER poloidal field 1&6 coils. The work was carried out by the IO and multiple institutions in the EU, Switzerland and Russia. Five samples were manufactured and tested in the SULTAN test facility. Analyses of dc performances, ac, and mechanical losses test results have been performed. All measurements were carried out before and after cyclic mechanical load created by the electromagnetic forces. DC test results of all samples tested demonstrated that the conductor performances are not affected by cycling mechanical load and meet the required value of current-sharing temperature T cs ≥ 5.8 K at magnetic field B max = 6.4 T and operating current I op = 48 kA. Measurements of the ac and mechanical losses were performed by gas flow calorimetric method. AC loss measurements were carried out at different operating currents and background magnetic fields. AC loss of tested samples showed no change under cycling mechanical load, but showed a correlation of ac loss versus background magnetic field. Mechanical loss, as had been expected, decreased significantly after first ten cycles then reaches a stabile value. The value of mechanical loss even in virgin condition does not affect workability of a magnet.

Completion of the Commissioning of the EDIPO Test Facility

The main coils of the new European DIPOle (EDIPO) test facility, which is able to provide a background magnetic field up to 12.35 T, were commissioned at the Center for Research in Plasma Physics in July 2013. A field homogeneity of +/-1% was achieved over a 900-mm length, as compared to SULTAN's 10.9 T over 425 mm. To complete the commissioning of the test facility, the new superconducting transformer has been operated in 2015, and a high-current NbTi sample, which was earlier tested in the SULTAN test facility, has been tested in EDIPO under the same conditions as in SULTAN to benchmark the results. During the initial commissioning of the 100-kA transformer in 2013, the primary coil was found to be insufficiently cooled due to the debonding of the primary winding from its former that also served as the liquid helium cryostat. The NbTi primary coil was therefore modified to be in direct contact with liquid helium. At the next attempt of commissioning, in March 2014, a mechanical failure due to the imperfect alignment of the primary and secondary windings caused major damage. A replacement primary coil, which was wound in Summer 2014, was used in the third commissioning campaign in November 2014, but it failed due to the poor impregnation of the NbTi winding. Eventually, the original primary winding was repaired and commissioned in April 2015. The commissioning included a calibration of the current meter for the secondary winding and the high-current test, i.e., up to 95 kA at various background fields, of the “Trasek” conductor sample, which is a rectangular cable-in-conduit conductor made of 324 SnAg-coated NbTi strands. An excellent agreement was found between the results obtained from the two facilities. The sample holder unit is also equipped with a high-temperature superconducting (HTS) adapter and a counterflow heat exchanger to allow testing in EDIPO of HTS high-current samples over a broad range of operating temperatures, from 4.5 up to 50 K.

Statistics of Test Results for the ITER Nb3Sn and NbTi Conductors in the SULTAN Facility

The series production of Nb3Sn and NbTi cable-inconduit conductors (CICC) for the winding of various coils in the International Thermonuclear Experimental Reactor (ITER) magnet system is being completed in 2015. The tests of the ITER conductor samples are running in the SULTAN test facility in Villigen, Switzerland, in the scope of a framework contract, flanked by six bilateral agreements between IO and the Domestic Agencies (DAs) of China, EU, Korea, Japan, Russian Federation, and USA. The key acceptance parameter of the tests for the conductor samples made from the series production ITER CICCs is the current sharing temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> at the nominal operating field and current after applied electromagnetic cyclic loadings at the nominal field and sinusoidal swing of conductor current from zero to nominal one. The T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> is defined as the maximum temperature at which the conductors operate (nominal current and field) before developing an electric field of 10 μV/m. This paper gathers the latest test results of the ITER series production CICC. The qualified conductor units will be used in the winding of the toroidal field and poloidal field coils, as well as the central solenoid. The test results for the series production conductors are compared with the results of the conductors tested at the early stages of their development. The performance variation among the suppliers and the performance evolution since the start of the conductor production are discussed as well.

Twin-box ITER joints under electromagnetic transient loads

The ITER Toroidal Field (TF) coil winding packs are designed to be wound in double-pancakes. The twin-box joint provides the electrical pancake-to-pancake connection between the two Nb3Sn 68kA conductors and electrical coil-to-bus bar connection between the TF coil terminations and NbTi 68kA bus bars conductors.The twin-box full size joint sample connecting two Nb3Sn conductors (pancake-to-pancake joint) was prepared in order to qualify the TF joint assembly in SULTAN Test Facility. The original goal of the test program was the measurement of joint resistance at different operating conditions. The accidental dump of the SULTAN background field caused a noticeable increase of resistance due to the induced electromagnetic transient load. The TF joint test was continued in order to investigate a change of joint resistance following electromagnetic transient loading which was triggered by intentional dump of the background field. Also, a dependence of the joint resistance on current was observed; in order to explore the origin of resistance change, additional experiments were performed with a modified (artificially degraded) TF twin-box joint.The test results of a TF twin-box joint after electromagnetic transient loading and performance of the joint after a modification are presented in this paper. The performance of Nb3Sn 68kA conductor observed during those tests is highlighted as well.

Residual Resistances Ratio in NbTi Strands Extracted From the ITER PF1/6 Conductor Sample After SULTAN Tests

The residual resistance ratio (RRR) is an important parameter affecting the stability of superconductors and the quench protection properties of magnets. During the manufacture and testing of cable-in-conduit-conductors, RRR may change noticeably. Recently the RRR changes of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands for conductors of the ITER toroidal coils have been studied during the manufacturing process, heat treatment and SULTAN testing. Here we present the similar study of RRR of Nb-Ti strands used for conductors of the ITER poloidal coils. RRRs of the strands were measured in as-received condition and at different stages of the PF conductor manufacturing. RRR of strands extracted from ITER PF conductor sample after electromagnetic and thermal cycling in the SULTAN test facility was also measured. For this purpose the strand samples were extracted from different locations in the cross-section of the PF conductor and from different positions along the axis after testing in the SULTAN. The results about the RRR change during the manufacturing process and after the SULTAN tests and also the details of experiments are presented and discussed.

DC Performance Results Versus Assessment of ITER Main Busbar NbTi Conductors

Four ITER main busbar (MB) conductor samples were tested in the SULTAN test facility (Centre de Recherches en Physique des Plasma, Ecole Polytechnique Federale de Lausanne, Switzerland) between 2011 and 2013. The MB conductors are NbTi-based cable-in-conduit conductors (CICCs), and they will become part of the feeder system of the ITER magnets. The measured dc performance of the four MB samples varied significantly, supposedly depending on the design of the bottom terminations. Two out of three samples with a U-bend box, made of a continuous conductor section, exhibit approximately 0.5 K lower current-sharing temperature Tcs than the sample with a solder-filled bottom joint, consisting of two straight conductor sections. We assess the theoretically expected Tcs performance of the MB conductor based on the characterization of individual NbTi strands, on the ITER NbTi scaling law, and on the magnetic field distribution across the cable cross section. The magnetic field in a SULTAN sample consists of three components, namely, the background SULTAN field, the self-field generated by the current in the conductor under test, and the magnetic field generated by the current in the return conductor. Taking into account all the three components, we calculate the average electric field in the cable and determine Tcs as in the experiment, namely, as the temperature at which the electric field reaches the critical value of Ec=0.1 μV/cm. The theoretically assessed Tcs confirms that the MB sample with solder-filled joint behaves as expected.

Analysis of Internal-Tin &lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;$\hbox{Nb}_{3}\hbox{Sn}$&lt;/tex-math&gt;&lt;/inline-formula&gt; Conductors for ITER Central Solenoid

Japan Atomic Energy Agency (JAEA) is procuring 100% of the ITER Central Solenoid (CS) conductors. The CS conductor is required to maintain the performance under 60000 pulsed electromagnetic cycles. JAEA tested two internal-tin Nb3Sn conductors for the CS at the SULTAN test facility. As a result of destructive examination, the twist pitches of both of the cables satisfied requirements of the ITER Organization (IO). The current sharing temperatures Tcs of each sample were 6.6 and 6.8 K before cyclic operation, and the Tcs values were 6.8 and 6.9 K after 9700 electromagnetic cycles, including three warm-up/cooldowns, respectively. The Tcs performance of both samples satisfied the IO requirement. The ac losses of CSKO1-C and CSKO1-D were approximately half of typical bronze-route CS conductors at 2 and 9 T. The ac loss at 45.1 kA after the cycling was 1.5 times higher than that without the transport current. An almost constant strain of the jacket was observed after the test as a result of the residual strain measurement. Therefore, the deformation of the cable might have been homogeneous along the conductor axis. Because of the higher Tcs of CSKO1-D than CSKO1-C, JAEA started the manufacturing of the CS conductor with the same specification as CSKO1-D.

Analysis of Nb3Sn Strand Microstructure After Full-size SULTAN Test of ITER TF Conductor Sample

The study of defects generated in superconducting filaments of Nb3Sn strands under electromagnetic and thermal cycling was carried out for the TFRF3 cable-in-conduit-conductor (CICC) sample that passed final testing inthe SULTAN test facility. The TFRF3 sample was manufactured forthe qualification of the RF Toroidal Field (TF) CICC. The strand samples were taken from different locations in the cross–section of TFRF3 and different positions along its axis in relation to background magnetic field. Qualitative and quantitative analysis of defects were carried out using metallographic analysis of images obtained by Laser Scanning Microscope. We analyzed number, type, and distribution of defects in filaments of the Nb3Sn strand samples extracted from different petals of TFRF3 in dependence on thestrand location in the cross–section (the center of petal, nearby the spiral, nearby the outer jacket) in the high field zone (HFZ). The results about the defects amount and their distribution are presented and discussed.

Transverse heat transfer coefficient in the dual channel ITER TF CICCs Part II. Analysis of transient temperature responses observed during a heat slug propagation experiment

A heat slug propagation experiment in the final design dual channel ITER TF CICC was performed in the SULTAN test facility at EPFL-CRPP in Villigen PSI. We analyzed the data resulting from this experiment to determine the equivalent transverse heat transfer coefficient hBC between the bundle and the central channel of this cable. In the data analysis we used methods based on the analytical solutions of a problem of transient heat transfer in a dual-channel cable, similar to Renard et al. (2006) and Bottura et al. (2006). The observed experimental and other limits related to these methods are identified and possible modifications proposed. One result from our analysis is that the hBC values obtained with different methods differ by up to a factor of 2. We have also observed that the uncertainties of hBC in both methods considered are much larger than those reported earlier.