Toroidal Field Model Coil Research Articles

The SPARC Toroidal Field Model Coil Program

The SPARC Toroidal Field Model Coil Program

Design, Fabrication, and Assembly of the SPARC Toroidal Field Model Coil

Design, Fabrication, and Assembly of the SPARC Toroidal Field Model Coil

Experimental Assessment and Model Validation of the SPARC Toroidal Field Model Coil

Experimental Assessment and Model Validation of the SPARC Toroidal Field Model Coil

50-kA Capacity, Nitrogen-Cooled, Demountable Current Leads for the SPARC Toroidal Field Model Coil

50-kA Capacity, Nitrogen-Cooled, Demountable Current Leads for the SPARC Toroidal Field Model Coil

A 20-K, 600-W, Cryocooler-Based, Supercritical Helium Circulation System for the SPARC Toroidal Field Model Coil Program

A 20-K, 600-W, Cryocooler-Based, Supercritical Helium Circulation System for the SPARC Toroidal Field Model Coil Program

Building the Runway: A New Superconducting Magnet Test Facility Made for the SPARC Toroidal Field Model Coil

Building the Runway: A New Superconducting Magnet Test Facility Made for the SPARC Toroidal Field Model Coil

Radiation Resistant Insulation Systems for the ITER Toroidal Field Coils

Insulation systems for fusion magnets have to operate under intense radiation, mainly fast neutrons and γ-radiation. At the ITER design fluence of 1x10 22 m -2 (E>0.1 MeV), conventional epoxy resins, which were used, e.g., for the ITER TF model coil, start to disintegrate. As a consequence, research programs were launched under EFDA, F4E and ITER contracts in order to find radiation harder resins as well as to investigate the properties of cyanate esters (CE) and CE/epoxy blends. The CE content was varied in the range between 100 % and 20 %. Test materials, consisting of wrapped R-glass fiber / polyimide tapes and VPI impregnated with various resins, were produced by European industry and tested at 77 K. Tensile and short beam shear tests were carried out to characterize the mechanical properties prior to and after irradiation to fast neutron fluences of up to 5x10 22 m -2 (E>0.1 MeV) in the TRIGA reactor (Vienna). In addition, tension-tension fatigue measurements were performed in the load-controlled mode to simulate the pulsed operation conditions of ITER. It turned out that these CE based insulation systems offer excellent properties at the ITER design fluence including an adequate safety margin, especially when a mixing ratio of 40% CE and 60 % epoxy resin is employed. Slight modifications of the resin, mainly regarding an extension of the pot-life to more than 100 hours, were made in view of the dimensions and the complexity of the ITER TF coils. The corresponding results confirm their excellent performance and will be reported. Radiation resistant materials have also been developed by US and Japanese Industry. The qualification of these insulation systems is currently under way and comprises 40:60 CE / epoxy blends from different suppliers (Huntsman, CTD). Test materials were fabricated under the same conditions in order to provide a comparable data base. The results will be presented and discussed.

Internal Transient Over-Voltages in Large Fusion Coils

Magnets show a linear voltage distribution along the windings for slow changing currents. This simple behavior may be lost when fast transients occur, e.g. for fast discharge or earth fault events. Such transients may become critical with respect to the magnet lifetime if they are not considered in design and acceptance tests. The problem of internal transient over-voltages can become more essential for large magnets with high number of turns because the critical frequencies for transients are lowered due to the large number of capacitances and inductances of windings and winding packs. At KIT large fusion magnets such as POLO and the ITER Toroidal Field Model coil (TFMC) have been investigated in the large coil testing facility TOSKA. For these coils direct measurements of internal over-voltages were possible and could be compared to calculations of transient voltages. As a consequence a set of transient voltage calculations was executed at KIT for ITER coils. An overview of calculations and results for ITER TF and PF coils is given and consequences for high voltage testing are discussed.

Transient Electrical Voltages Within ITER Poloidal Field Coils

High-voltage insulation co-ordination of the coil system is essential for reliable operation of ITER. Transient electrical excitations occur on the terminals of superconducting coils, for example in case of a fast discharge of the coils and if a failure occurs within the components of the electrical circuit. Transient voltage on the coil terminals may lead to non-linear voltage distribution and oscillations within large superconducting coils because of their large dimensions and high number of turns. Thus, the internal voltages may even be higher than the voltages on the coil terminals. This effect depends on the rise time of the excitations and was measured within the ITER Toroidal Field Model Coil which is about three times smaller than the ITER toroidal field (TF) coil. The calculations of the transient electrical behavior of the coils provide a basis for high-voltage insulation co-ordination and definition of test voltages and waveforms. The high voltage tests during the manufacturing process will control the quality of the electrical insulation and ensure reliable operation of the coils. The transient electrical behavior of ITER poloidal field (PF) coils was calculated using the PF 3 and PF 6 coils as examples. Maximum voltages for ground, layer and turn insulation were calculated for four scenarios. Results of these calculations are presented in detail in this paper.

Characterization of Bonded Glass/Polyimide Tapes for the ITER TF Coil Insulation

The insulation of the ITER TF coils consists of multiple wrapped layers of glass fiber polyimide sandwich tapes. In order to simplify the fabrication process, the polyimide tape can be bonded to the glass fiber tape, mainly to avoid misalignments of the polyimide tape. However, due to the intense neutron and γ-radiation exposure, bonding agents have to be radiation resistant, otherwise the composite will be severely damaged, as was observed for the ITER TF model coil insulation. Two bonded glass fiber polyimide tapes (40 mm wide, ~0.175 mm thick) were supplied by Arisawa, Japan, and Advanced Composites Group, United Kingdom. Test materials were fabricated based on the build-up of the ITER TF turn insulation and vacuum pressure impregnated with a 40:60 cyanate ester/epoxy blend supplied by Huntsman, Switzerland. The mechanical properties of these materials were characterized prior to and after reactor irradiation to a fast neutron fluence of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> (E >; 0.1 MeV), i.e. twice the ITER design fluence. Static and dynamic tensile tests were carried out at RT and 77 K. The Arisawa tape showed excellent mechanical properties.

Thermal-Hydraulic Simulation of 80 kA Safety Discharge in the ITER Toroidal Field Model Coil (TFMC) Using the 4C Code

The toroidal field model coil (TFMC) was extensively tested in 2001-2002, in the TOSKA facility at the Karlsruhe Institute of Technology, Germany, up to a maximum transport current of 80 kA and a peak magnetic field of ~ 10 T. Here, we apply the 4C code, which was developed for the analysis of thermal-hydraulic transients in superconducting magnets, including winding, structures, and cryogenic circuit, to the analysis of the TFMC safety discharge from 80 kA. The code is able to reproduce the main experimental features, despite the limitations due to no more fully available information about the details of the cryogenic circuit, which play here a much more important role than at lower currents.

Validation of the 4C Thermal-Hydraulic Code Against 25 kA Safety Discharge in the ITER Toroidal Field Model Coil (TFMC)

The first step of the validation of the recently developed thermal-hydraulic code 4C is presented. We study the transient evolution during a so-called standard 25 kA safety discharge of the International Thermonuclear Experimental Reactor (ITER) Toroidal Field Model Coil (TFMC), tested in 2001–2002 in the TOSKA facility at the Karlsruhe Institute of Technology, Germany. The computed outlet He temperatures from both winding and case cooling channels, as well as the local maximum temperature increase of the structures, are shown to be in good agreement with the measured data.

Axial Compression Tests on ITER-TF Conductor Samples

As demonstrated in the -superconductor cable of the ITER TF Model Coil (TFMC) has an average longitudinal stiffness inside the stainless steel jacket of about 1/10 of that calculated supposing all components are fully bonded. This conductor was wound towards the inner circumference of the grooves of a stainless steel mould and heat treated in it showed an elongation of 0.048% after cooling to ambient temperature . This can only partially be due to the lower expansion coefficient of than the other constituents. The stiffness of three conductors (copper dummy, non-reacted and reacted TFMC) was nearly the same and independent of the constituents. It is expected that by reducing the void fraction the stiffness would increase, having the limit at zero void fraction equal to the average of the fully bonded constituents involved. We investigated pieces from the TFMC conductors (34% void) and 2 different ITER TF Coil conductors with 29.7% and 28.3% void fraction and different twist pitches. Heat treatments on short samples were performed and the change of length of the cable and the jacket were investigated. Compression tests just on the cables inside the jacket were performed. From these tests the stiffness of the ITER TF Coil conductor samples were found to remain in the same level. Nevertheless, the samples with lower twist pitches show higher yield strength in axial compression. Acoustic emission records on reacted samples containing showed that cracks occur already at small compression levels which the cable sees inside the jacket during cool down from reaction to room and helium temperature.

Axial compression test on ITER-TFMC conductors at room temperature

The superconductor used for the ITER TF Model Coil (TFMC) consists of 720 twisted Nb 3Sn-strands and 360 copper strands, which are cabled around a central spiral and are surrounded by a stainless steel jacket. The conductor is heat-treated at 650 °C in a stainless steel mould. After cool down to room temperature the conductor is found to be elongated by 0.45 mm/m, which can be attributed to the lower shrinkage of Nb 3Sn. This means that the jacket is under tensile stress and the cable under longitudinal compression after cool down. This effect aggravates upon cooling further to the operating temperature. The cable and the jacket can be interpreted as a double spring system, the jacket being the expansion spring and the cable being the compression spring. In order to evaluate and possibly predict such elongations, it is necessary to know the Young's modulus of the cable. Therefore, we investigated samples cut from a dummy conductor, a not heat-treated and a heat-treated TFMC conductor by mechanically compressing them. From the stress–strain diagram the elastic modulus of the cables was determined and found to be about one-tenth of that calculated assuming all components of the cable being fully bonded. The stiffness of the cables turned out to be fairly independent of the state of the cable as all three cables show almost the same modulus. The mechanical compression of the cable is about 8.5 times larger than the measured elongation of the jacket.

Investigation on Thermal Strain in Full Size ITER Conductors

The TF magnetic system of the international thermonuclear experimental reactor (ITER) will be composed of 18 superconducting coils assembled in a torus. The design of these large coils relies on the use of a circular cable-in-conduit conductor (CICC) composed of a cable of about one thousand twisted Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn and copper strands embedded in a thin steel jacket. The different thermal contractions of the materials composing this conductor during the cool down from the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn heat treatment reaction temperature (about 920 K) to the operating temperature (about 5 K) result in a compressive strain on the superconducting Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn filaments which leads to a degradation of the conductor critical current. The qualification of the ITER conductor prototypes will be performed by testing short straight full size samples in the unique European test facility SULTAN at PSI (Switzerland). A comparison of the performance of the toroidal field model coil (TFMC) conductor in a short straight sample and in the coil has shown indications that the sample performance is better, apparently because the thermal strain in Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn is lower. It looks possible that this could have been caused by a slippage between the cable and the jacket in the joints at the sample ends during the heat treatment. In this case, these kind of short straight samples could not be relevant for conductor qualification. An investigation on the thermal strain by destructive examination of different TFMC conductor samples, including one leg of the TFMC short sample tested in 1999 in the SULTAN facility, was performed at CEA Cadarache. The paper reports on the different strain measurements, proposes an analytical approach of the results in order to conclude on the relevance of straight samples.

Results of R&amp;D on ITER-TF Winding Critical Issues

New technical issues are initiated in the ITER toroidal field (TF) coil manufacturing because of the scale-up by about 3 times from the TF model coil. The major issues are feasibilities of high accuracy, automatic winding, precise prediction of conductor elongation after heat treatment to insert the conductor in narrow groove of a radial plate, and development of insulation system, which can sustain irradiation of a neutron fluence of 1022 n/m and, in addition, be impregnated in a suitable duration. One of major parts of automatic winding machine, a bending roller head, was developed and trial winding using short sample conductor was successfully carried out. The elongations of the TF conductor after the heat treatment is evaluated to be in plusmn0.03% In addition, results of impregnation test using short samples indicate that it will take about 60 h to impregnate TF coil double-pancake by the method developed in the model coil project. New impregnation technique is under development to shorten the impregnation period.

Stability of Superconducting Magnet for KSTAR

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The magnet system comprises sixteen toroidal field (TF) coils and fourteen poloidal field (PF) coils for KSTAR (Korea Superconducting Tokamak Advanced Research) Tokamak. All the TF coils and 10 PF coils <formula formulatype="inline"> <tex>$({\hbox{PF}}1\sim 5\ {\hbox{UL}})$</tex></formula> are made of <formula formulatype="inline"><tex>${\hbox{Nb}}_{3}{\hbox{Sn}}$</tex></formula> strands and 4 PF coils <formula formulatype="inline"><tex>$({\hbox{PF}}6\sim 7\ {\hbox{UL}})$</tex> </formula> are made of NbTi strands. </para>

Manufacturing of the ITER TF Full Size Prototype Conductor

The experience gained in the past for the ITER toroidal field model coil conductor and the results obtained so far have led to the definition of an upgraded full size prototype conductor, based on advanced Nb3Sn strand, and entirely manufactured in the European Union (EU). Samples for the characterization in the Sultan facility have been prepared by Luvata (Italy) following the conductor layout defined by ITER. ENEA was responsible for conductor fabrication. Since the conductor layout was new, a full size copper dummy conductor has been preventively produced for the setting of the cabling and jacketing tools. Then, a total of four full size superconducting cables have been prepared by using Nb3Sn advanced strands produced by Oxford Instruments (OST) and European advanced superconductors (EAS), by internal tin and bronze technology, respectively. The details of manufacturing procedures will be described in this paper.

Influence of cable layout on the performance of ITER-type Nb3Sn conductors

In 2005-2006, the European Union tested in the SULTAN facility (CRPP Villigen, Switzerland) four ITER-type conductors assembled in two samples. Each conductor made use of an advanced Nb3Sn strand specially developed by European companies for the ITER Toroidal Field (TF) coil conductor. The cable layout of these conductors was the one of the ITER TF Model Coil conductor. In 2007, the European Union tested in SULTAN four other conductors within two other samples as prototypes of the updated ITER TF conductor. These conductors differed mainly from the former by their cable layout whereas identical or similar Nb3Sn strands were used. The paper reports on the current sharing temperature measurements of the second samples under ITER relevant operating conditions. The effect of cycling (1000 cycles) in current under full magnetic field was also investigated. The analysis of the test results is performed with respect to the strand properties using the usual smeared models of the cable. These test results were found to be significantly better than those obtained on the first conductors. The paper discusses the possible causes as well as the consequences of such a performance scattering.

Development of large current superconductors using high performance Nb3Sn strand for ITER

In the ITER model coil tests, degradations of the critical current were observed in Nb3Sn cable-in-conduit conductors when a large transverse electromagnetic force was applied to them. Both mass production technique of strand having high critical current enough to recover the degradation and also trial fabrication of the ITER-TF cable and conduit section were successfully performed. These results demonstrate feasibility of fabrication of the TF conductor using the high performance Nb3Sn strands. In addition, critical current degradation by spatially periodic transverse load was both experimentally and numerically studied. The simulation results show that the transverse resistance among the filaments is significantly small compared to that calculated from bronze resistivity. It is explained by the effect of superconducting current across the filament cross-section. This current however disappears when a large bending strain is applied to the strand, resulting in deterioration of the transverse resistance. It suggests that small degradation of the TF model coil (TFMC) conductor is not attributed to sufficiently low resistance among filaments as is expected in the optimized design of the ITER conductor but high resistance of the TFMC strand critical current against the transverse load. On the other hand, the developed strand has almost the same resistance against the transverse load as the TFMC strand. Therefore, the small degradation of the TF conductor, i.e., stable operation of the TF conductor is preliminarily anticipated.