NbTi Cable-in-conduit Conductors Research Articles

Thermal-hydraulic analysis of the DTT CS and PF pulsed coil performance during AC operation

The EU DEMO fusion reactor, now entering in its conceptual design phase, will have to tackle several challenges, such as the long plasma pulse duration and the exhaust of very high heat fluxes from the plasma. The Divertor Tokamak Test (DTT) facility, a compact superconducting tokamak under construction at ENEA Frascati, will address the power exhaust problem in DEMO by testing several DEMO-relevant divertor solutions and operation scenarios.The DTT superconducting coils are the subject of the analyses presented here: as the tokamak must be very flexible to face different plasma scenarios, the design of the magnet system must be proven to be robust. In particular, the thermal-hydraulic performance of the coils operated in pulsed mode, namely the Central Solenoid (CS) and the Poloidal Field (PF) coils, will be analyzed. The CS is composed of 6 modules, layer-wound with Nb3Sn Cable-in-Conduit Conductors (CICCs), while the PF coils are pancake-wound. The two PF coils closest to the machine axis are wound using Nb3Sn CICCs, while the others adopt NbTi CICCs. All the coils are cooled by supercritical He (SHe) in forced flow.The pulsed operation of these coils induces AC losses, eroding their temperature margin: a detailed thermal-hydraulic model of the DTT pulsed coils is developed here using the 4C code. The model is then applied to the simulation of the two reference plasma scenarios, a single null and a double null, computing the minimum temperature margin. Different cooling options (single- or double-pancake) are investigated for the PF coils, while the sensitivity to the coupling time constant value is assessed for the CS.

Analysis of ITER NbTi and Nb3Sn CICCs experimental minimum quench energy with JackPot, MCM and THEA models

Cable-in-conduit conductors (CICCs) for ITER magnets are subjected to fast changing magnetic fields during the plasma-operating scenario. In order to anticipate the limitations of conductors under the foreseen operating conditions, it is essential to have a better understanding of the stability margin of magnets. In the last decade ITER has launched a campaign for characterization of several types of NbTi and Nb3Sn CICCs comprising quench tests with a singular sine wave fast magnetic field pulse and relatively small amplitude. The stability tests, performed in the SULTAN facility, were reproduced and analyzed using two codes: JackPot-AC/DC, an electromagnetic-thermal numerical model for CICCs, developed at the University of Twente (van Lanen and Nijhuis 2010 Cryogenics 50 139–148) and multi-constant-model (MCM) (Turck and Zani 2010 Cryogenics 50 443–9), an analytical model for CICCs coupling losses. The outputs of both codes were combined with thermal, hydraulic and electric analysis of superconducting cables to predict the minimum quench energy (MQE) (Bottura et al 2000 Cryogenics 40 617–26). The experimental AC loss results were used to calibrate the JackPot and MCM models and to reproduce the energy deposited in the cable during an MQE test. The agreement between experiments and models confirm a good comprehension of the various CICCs thermal and electromagnetic phenomena. The differences between the analytical MCM and numerical JackPot approaches are discussed. The results provide a good basis for further investigation of CICC stability under plasma scenario conditions using magnetic field pulses with lower ramp rate and higher amplitude.

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.

Results and analysis of the hot-spot temperature experiment for a cable-in-conduit conductor with thick conduit

In the design of future DEMO fusion reactor a long time constant (∼23s) is required for an emergency current dump in the toroidal field (TF) coils, e.g. in case of a quench detection. This requirement is driven mainly by imposing a limit on forces on mechanical structures, namely on the vacuum vessel. As a consequence, the superconducting cable-in-conduit conductors (CICC) of the TF coil have to withstand heat dissipation lasting tens of seconds at the section where the quench started. During that time, the heat will be partially absorbed by the (massive) steel conduit and electrical insulation, thus reducing the hot-spot temperature estimated strictly from the enthalpy of the strand bundle. A dedicated experiment has been set up at CRPP to investigate the radial heat propagation and the hot-spot temperature in a CICC with a 10mm thick steel conduit and a 2mm thick glass epoxy outer electrical insulation. The medium size, ∅=18mm, NbTi CICC was powered by the operating current of up to 10kA. The temperature profile was monitored by 10 temperature sensors. The current dump conditions, namely the decay time constant and the quench detection delay, were varied. The experimental results show that the thick conduit significantly contributes to the overall enthalpy balance, and consequently reduces the amount of copper required for the quench protection in superconducting cables for fusion reactors.

Test Results of a 100 kA NbTi CICC

For the secondary winding of the flux pump (super- conducting transformer) of the EDIPO test facility at CRPP Villigen, a NbTi cable-in-conduit conductor (CICC) was designed at CRPP and procured at the industry. From a left section of the CICC a SULTAN sample, nicknamed TRASEK, has been assembled and tested. In the conductor design, high Cu:non-Cu ratio in the strand (no copper segregation), low void fraction and SnAg5 coating of the strand surface are selected to allow smooth current redistribution and enhance the take-off voltage. The DC tests are carried out be- fore and after solder filling the sample termination and current sharing is observed up to 80 kA. Eventually, the DC conductor performance is limited by the self field of the sample in the U-bend region. In normal operation, the secondary coil of the transformer is not exposed to significant magnetic field sweeps. The AC loss is expected to be large according to the design choices (tin coating, low void fraction). The AC loss characterization in SULTAN includes over 400 load cycles. The pressure drop vs. mass flow rate is measured in SULTAN to complement the data base for friction factor of CICC's.

Optimization of ITER Nb3Sn CICCs for coupling loss, transverse electromagnetic load and axial thermal contraction

The ITER cable-in-conduit conductors (CICCs) are built up from sub-cable bundles, wound indifferent stages, which are twisted to counter coupling loss caused by time-changingexternal magnet fields. The selection of the twist pitch lengths has major implications forthe performance of the cable in the case of strain-sensitive superconductors, i.e. Nb3Sn, as the electromagnetic and thermal contraction loads are large but also for theheat load from the AC coupling loss. At present, this is a great challenge for theITER central solenoid (CS) CICCs and the solution presented here could be abreakthrough for not only the ITER CS but also for CICC applications in general.After proposing longer twist pitches in 2006 and successful confirmation by shortsample tests later on, the ITER toroidal field (TF) conductor cable pattern wasimproved accordingly. As the restrictions for coupling loss are more demandingfor the CS conductors than for the TF conductors, it was believed that longerpitches would not be applicable for the conductors in the CS coils. In this paper weexplain how, with the use of the TEMLOP model and the newly developed modelsJackPot-ACDC and CORD, the design of a CICC can be improved appreciably,particularly for the CS conductor layout. For the first time a large improvement ispredicted not only providing very low sensitivity to electromagnetic load and thermalaxial cable stress variations but at the same time much lower AC coupling loss.Reduction of the transverse load and warm-up–cool-down degradation can be reached byapplying longer twist pitches in a particular sequence for the sub-stages, offering a largecable transverse stiffness, adequate axial flexibility and maximum allowed lateral strandsupport. Analysis of short sample (TF conductor) data reveals that increasing the twistpitch can lead to a gain of the effective axial compressive strain of more than 0.3% withpractically no degradation from bending. This is probably explained by the distinctdifference in mechanical response of the cable during axial contraction for short and longpitches. For short pitches periodic bending in different directions with relatively shortwavelength is imposed because of a lack of sufficient lateral restraint of radial pressure.This can lead to high bending strain and eventually buckling. Whereas for cables withlong twist pitches, the strands are only able to react as coherent bundles, beingtightly supported by the surrounding strands, providing sufficient lateral restraintof radial pressure in combination with enough slippage to avoid single strandbending along detrimental short wavelengths. Experimental evidence of goodperformance was already provided with the test of the long pitch TFPRO2-OST2,which is still until today, the best ITER-type cable to strand performance everwithout any cyclic load (electromagnetic and thermal contraction) degradation.For reduction of the coupling loss, specific choices of the cabling twist sequence are neededto minimize the area of linked strands and bundles that are coupled and form loops withthe applied changing magnetic field, instead of simply avoiding longer pitches. In additionwe recommend increasing the wrap coverage of the CS conductor from 50% to at least 70%.A larger wrap coverage fraction enhances the overall strand bundle lateral restraint.The long pitch design seems the best solution to optimize the ITER CSconductor within the given restrictions of the present coil design envelope,only allowing marginal changes. The models predict significant improvementagainst strain sensitivity and substantial decrease of the AC coupling loss inNb3Sn CICCs, but also for NbTi CICCs minimization of the coupling loss can obviously beachieved. Although the success of long pitches to transverse load degradation was alreadydemonstrated, the prediction of the elegant innovative combination with low coupling lossneeds to be validated by a short sample test.

Stability Test of RHQT Nb3Al Cable-in-conduit Conductor

The Japan Atomic Energy Agency (JAEA) and National Institute for Material Science (NIMS) have been collaborating in the development of high-performance (i.e., more than 16 T and 80 kA) Nb3Al cable-in-conduit (CIC) conductor prepared using the rapid-heating, quenching and transformation (RHQT) process, and aiming for the application of this conductor to a demonstration plant. The technical issue relating to application of the RHQT Nb3Al strands to a fusion magnet is stabilization against perturbation. NIMS has developed a technique to attach a copper stabilizer using electroplating, and a sub-scale CIC conductor is developed using this conductor. JAEA performed a stability test of the developed CIC conductor to demonstrate the efficiency of this copper stabilization technique. The initial perturbation was applied via inductive heating, whose energy was calibrated using a calorimetric method. The measured stability margin is sufficiently high compared to that of a similar NbTi CIC conductor previously tested by the authors. In addition, a heat transfer coefficient to define the so-called limiting current is estimated to be about 1 kW/m2K, a sufficiently high value that is almost the same as that of a CIC conductor consisting of normal Cu stabilized strands. From these experimental results, it can be concluded that the copper stabilizer works efficiently from the viewpoint of stability, thus offering a solution to the remaining technical issues relating to the RHQT CIC conductor. On this basis, we can say that the RHQT Nb3Al CIC conductor is the most promising candidate for application to a magnet in the demonstration plant.

Study on the dynamic behavior of a current in cable-in-conduit conductors by using self magnetic field measurements

In order to understand the current behavior inside a cable-in-conduit conductor (CICC), self magnetic field measurements on the CICC were conducted. A prototype NbTi CICC, the configuration of which is a racket shape, for JT-60SA EF coil was used as a short conductor sample. By using Hall sensors arranged around the short conductor sample, the variation in the self magnetic field was measured. Taking into account the measurements, the current behavior inside the CICC was analyzed by using analytical models consisting of line currents. The analytical results indicate that the current distribution in the cross-section of the CICC would be non-uniform before a normal propagation, and the current distribution would be improved after the normal propagation.

Stability and Quench Test for NbTi CIC Conductor of JT-60SA Equilibrium Field Coil

The EF coil conductors of JT-60SA are designed with the NbTi cable in conduit conductor because the maximum magnetic field goes up to 6.2 T. The prototype NbTi conductor was developed and tested to confirm the capability of the real conductor. The prototype conductor was proven to have enough Tcs margin under the operating conditions investigated in previous test. In this study, the quench test was operated to measure the stability margin and the normal state propagation. Firstly, the MQE under the Tcs margin of 0.2 K was found to be 80 mJ/cc-strand which is almost the same as that of the previous CIC conductor to result in enough stability margins on EF coil operation. Secondly, the propagation velocity to upstream direction and downstream direction were found to be about 0.47 m/s and 0.54 m/s, respectively. Finally, the quench analysis was conducted to calculate the maximum temperature during quench. The analysis results showed that the maximum temperature reached about 70 K, which is within the permissible value of 150 K for the EF coils.

Application of the ENEA Joint Concept to NbTi CIC Conductors

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In superconducting coils for fusion applications, the cable-in-conduit (CIC) conductor is the most adopted solution. ENEA reported last year about a newly developed compact joint, successfully tested for <formula formulatype="inline"> <tex Notation="TeX">${\rm Nb}_{3}{\rm Sn}$</tex></formula> conductors in a configuration suitable for the EFDA Dipole which is currently in construction phase. The main advantages of this new concept are: low room occupancy, low price and easiness of manufacturing process. In the present paper, new results coming from the characterization of NbTi joint samples are presented, suggesting new applications for this innovative junction. A joint layout optimization study by means of an electrical simulation code is included as well. </para>

New Considerations About Stability Margins of NbTi Cable in Conduit Conductors

The design of NbTi cable in conduit conductors (CICCs) according to the criterion of the limiting current often leads to high copper content, with a Cu/nonCu ratio inside the multifilamentary composites in the range of 6 to 8. This high copper content makes NbTi multifilamentary composites difficult and expensive to manufacture, especially if the required filament diameter is in the range of 5-10 mum. Moreover, a detailed analysis of the cable in conduit stability for expected disturbances or heat deposition on a long length of conductor shows that copper plays, in practice, no role in the case where current is uniformly distributed. An analytical approach is given to demonstrate this statement and propose a cheaper and reliable method to identify the drivers of the stability margins. Certainly copper is needed in large amounts for protection, but this copper can be segregated in dedicated strands. Typical copper content of 60% to 70% in the strands is sufficient for intrinsic dynamic stability and to stabilize the CICC against short mechanical disturbances in the range of 1 ms for the time constant affecting short lengths of CICC (10 mm). Stability issues in NbTi CICCs might also be linked to current distribution effects where contact resistance between strands plays a leading role.

Simulation of the ITER Poloidal Field Coil Insert DC performance with a new model

The Poloidal Field (PF) Coil Insert is made from a NbTi cable in conduit conductor and has been subjected to tests in the Central Solenoid Model Coil facility at JAEA in Japan. For the interpretation of the voltage tap signals from these tests, we adapted the JackPot model – which was used previously to analyse short sample tests – to simulate also the model coil experiments. A key ingredient of JackPot is that the local magnetic field on the superconducting strands and the inter-strand contact resistances all depend on the “trajectories” of the strands within the cable. These trajectories are precisely calculated, ensuring a realistic distribution of magnetic field- and contact resistance values. The results of the model calculations show that the applied joints are most likely responsible for the poor performance of short samples of similar PF conductors in earlier experimental tests. The model predicts that the influence of the joints is significantly less pronounced for the Poloidal Field Coil Insert.

Quench Characteristics of an NbTi CICC With Non-Uniform Current Distribution

The effect of non-uniform current distribution (NUCD) on stability and quench characteristics of an NbTi/Cu cable-in-conduit conductor (CICC) with non-insulated or bare strands has been investigated by artificially introducing NUCD among the strands using a sophisticated current feeder system with thin-film resistive heaters. The stability margin or minimum quench energy (MQE) has been found to decrease significantly, especially in the transition region between so-called well-cooled and ill-cooled regions, due to the introduction of NUCD. The experimental results of stability margin with uniform current distributions have been compared with numerical calculations using the 1-D finite element code GANDALF, whereas the non-uniform current distribution cases have been treated by another code developed by the authors, which models the last two twisting stage of the conductor with uniform contact resistances between sub-cables. The numerical calculation results also simulate the similar behaviors as found in experiments. The quench propagation velocities deduced from voltage tap signals are also discussed.

Design, Production and QA Test Results of the NbTi CIC Conductors for the W7-X Magnet System

The magnet system of the W7-X Stellarator consists of 50 non-planar and 20 planar coils. The superconductors for both types have the same design, which is a Cable-in-Conduit (CIC) conductor of 243 NbTi strands. Namely the non-planar coils asked for a conductor, which can be easily wound to a complex shape, but has high mechanical strength to resist the large magnetic forces acting during magnet operation. This has been achieved by jacketing the cable into an Aluminium conduit by a co-extrusion process. The used 6063 Aluminium alloy is very soft in the extruded state, but gets high strength properties after winding during a precipitation hardening treatment at about 160°C. The production of the needed 390 conductor lengths (including spares) and the related QA tests are nearly completed and a large number of coils are fabricated. Some of them were already subjected to a cold test at CEA Saclay, where the conductor behaved as expected from short sample measurements.

Status of the KSTAR PF6 and PF7 Coil Development

The KSTAR superconducting magnet system consists of 16 TF (Toroidal Field) and 14 PF (Poloidal Field) coils. Both of the TF and PF coil systems use internally-cooled Cable-In-Conduit Conductors (CICC). The PF coil system, which consists of 8 coils in the CS (Central Solenoid) coil system and 6 outer PF coils, provides 17 V-s and sustains the plasma current of 2 MA for 20 seconds inductively. PF 1-5 coils use Nb/sub 3/Sn CICC in an Incoloy 908 conduit and PF 6-7 coils use NbTi CICC in a modified stainless steel 316LN(316LN+). The fabrication of PF7L/U & PF6L/U coils is completed at present. The engineering issues and fabrication process, which are related with the KSTAR PF 6-7 coil development, are presented and discussed in this paper. TF and PF coils are in the fabrication stage for KSTAR completion in the year 2006.

Reconstruction of Current Unbalance in Full-Size ITER NbTi CICC by Self-Field Measurements

Methods have been developed to study the distribution of the transport current among the strand bundles of cable-in-conduit conductors (CICC) by using self-field measurements with Hall probe arrays. The unbalance in the transport current is mainly caused by the unavoidable nonuniformity of the joints and can be a reason for a change in the voltage-temperature characteristic and consequently of the temperature margin. In the present study we focus on the reconstruction of the current unbalance in a full size NbTi CICC tested in the SULTAN test facility. The crucial point in the reconstruction procedure is the proper choice of the reference self-field profile corresponding to a uniform current distribution. To achieve this uniform distribution, the conductors were driven far into the current sharing regime. The self-field profile corresponding to the highest achieved voltage level was taken as a reference. This assumption is supported by the modeling of current-sharing runs in the CUDI-CICC network model. Furthermore, local redistribution effects were observed in the conductors at high currents. The current transfer associated with this redistribution is analyzed as well.

Effect of Cyclic Loading and Conductor Layout on Contact Resistance of Full-Size ITER PFCI Conductors

An evolution of the contact resistances (R/sub c/) of two full-size ITER (International Thermonuclear Experimental Reactor) cable-in-conduit conductors (CICC) aimed for the Poloidal Field Conductor Insert (PFCI) was studied in the Twente Cryogenic Press. The conductors are distinguished by the presence of stainless steel wraps around the last stage sub-cables in one of the conductors. The R/sub c/ and AC loss were measured on the conductors being in the "virgin" state and after they have been loaded 40,000 times with a peak force of 315 kN/m. The test results are compared with those, obtained on sections from the same conductor lengths, in the SULTAN test facility. The consequences of the cyclic loading and the sub-cable wraps on the R/sub c/ and the coupling AC loss are discussed in view of the previously published results on full- and sub-size ITER NbTi CICCs. Special emphasis was paid to the measurements of the contact resistances between individual sub-cables and the conduit, since the knowledge of them is important for proper interpretation of the DC test results. As a final goal, the test outcome will be used as a reference for calibration of the PFCI AC loss performance.

Electromagnetic Performance of Sub-Size NbTi CICC's Subjected to Transverse Cyclic Loading

It was demonstrated previously that transverse cable loading due to electromagnetic forces in coils has a strong impact on the inter-strand contact resistance R/sub c/ of poloidal field (PF) cable-in-conduit conductors (CICC) for the International Thermonuclear Experimental Reactor (ITER). Continuing the study, two NbTi medium-size CICC's were subjected to cyclic loading up to 40,000 cycles in the Twente Cryogenic Press in order to simulate transversal forces on the strands and to verify their influence on the conductors' contact resistances R/sub c/ and AC loss behavior. The results are presented and compared with the data obtained on the other section of the same conductor lengths in the SULTAN test facility and on full size ITER cables tested in the cryogenic press.

Change of interstrand contact resistance and coupling loss in various prototype ITER NbTi conductors with transverse loading in the Twente Cryogenic Cable Press up to 40,000 cycles

The multistrand NbTi conductors for the Poloidal Field (PF) Coils of the International Thermonuclear Experimental Reactor (ITER) are subjected to heavy transverse loading due to the Lorentz forces in the coils. The current in the multistage Cable-In-Conduit Conductors (CICC) exceeds 50 kA and the magnetic field reaches up to more than 6 T for a few tens of thousands of pulses. The large transverse forces, accumulating from strand to strand over the cable cross-section, cause a severe deformation of the cable bundle inside the conduit and this goes along with electromagnetic, mechanical, and thermohydraulic effects. In order to study the electromagnetic and mechanical behaviour in more detail, a Cryogenic Cable Press is build to simulate the effect of the Lorentz forces on a conductor comparable to the present design for ITER in magnet operating conditions. The magnetisation of the conductor (and from this the coupling loss expressed in nτ) and the interstrand resistance ( R c) between various strands and strand bundles inside the cable can be measured along the loading history, starting at virgin condition and accordingly subjected to various loads. The results, all obtained on eight full-size ITER type NbTi conductor samples with a variety of cable strand layout and coatings, are reported here. A consistent correlation is found between the experimental AC loss and interstrand contact resistance ( R c) results. It is also observed that there is a strong impact of cyclic loading on the AC loss and R c which may change up to orders of magnitude. The variation of the AC loss due to transverse cyclic loading of CICC conductors in ITER coils can be accomplished by reducing the void fraction. The results point out that cyclic loading with a significant number of cycles, sufficient to reach a saturation after having passed the peak transverse resistance, should be included in next tests on large NbTi CICC's and PF Model Coils as the AC loss and ability of current sharing among strands will vary along the loading history.

Electrical characteristics for full size NbTi cable in conduit conductor

Abstract In the ITER research program framework a full size cable in conduit NbTi short length conductor has been manufactured. The conductor is composed of two straight parallel legs with a copper joint between them at one extremity. The legs are based on different strands, Ni plated the first, CuNi internal barrier the second. The system is fully instrumented; in particular several voltage taps and thermometers measure the conductor behaviour and allow following the superconducting/normal transition. Full sets of experimental runs have been performed at the SULTAN facility obtaining a complete conductor characterisation. In this paper experimental data about the V – I characteristic as a function of the more relevant physical parameters have been investigated.