Norikiyo Koizumi

Test of the ITER central solenoid model coil and CS insert

The Central Solenoid Model Coil (CSMC) was designed and built from 1993 to 1999 by an ITER collaboration between the U.S. and Japan, with contributions from the European Union and the Russian Federation. The main goal of the project was to establish the superconducting magnet technology necessary for a large-scale fusion experimental reactor. Three heavily instrumented insert coils were built to cover a wide operational space for testing. The CS Insert, built by Japan, was tested in April-August of 2000. The TF Insert, built by Russian Federation, will be tested in the fall of 2001. The NbAl Insert, built by Japan, will be tested in 2002. The testing takes place in the CSMC Test Facility at the Japan Atomic Energy Research Institute, Naka, Japan. The CSMC was charged successfully without training to its design current of 46 kA to produce 13 T in the magnet bore. The stored energy at 46 kA was 640 MJ. This paper presents the main results of the CSMC and the CS Insert testing-magnet critical parameters, ac losses, joint performance, quench characteristics and some results of the post-test analysis.

Progress of the ITER central solenoid model coil programme

The worlds largest pulsed superconducting coil was successfully tested by charging up to 13 T and 46 kA with a stored energy of 640 MJ. The ITER central solenoid (CS) model coil and CS insert coil were developed and fabricated through an international collaboration, and their cooldown and charging tests were successfully carried out by international test and operation teams. In pulsed charging tests, where the original goal was 0.4 T/s up to 13 T, the CS model coil and the CS insert coil achieved ramp rates to 13 T of 0.6 T/s and 1.2 T/s, respectively. In addition, the CS insert coil was charged and discharged 10 003 times in the 13 T background field of the CS model coil and no degradation of the operational temperature margin directly coming from this cyclic operation was observed. These test results fulfilled all the goals of CS model coil development by confirming the validity of the engineering design and demonstrating that the ITER coils can now be constructed with confidence.

Test Results and Investigation of Tcs Degradation in Japanese ITER CS Conductor Samples

Japan Atomic Energy Agency (JAEA) has fabricated and tested the four conductor samples composed of high performance strands manufactured by the bronze-route process for the ITER Central Solenoid (CS) conductor. The current sharing temperature (Tcs) electrically assessed at 45.1 K and 10.85 T along the cycling loading at 48.8 kA and 10.85 T initially were 6.0 K and 6.1 K, and then 5.3 K and 5.5 K after 6000 cycles for the first SULTAN sample named JACS01, respectively. As results of second SULTAN sample named JACS02, the Tcs values initially were 7.2 K and 6.8 K, and then 6.6 K and 6.1 K after 10000 cycles for each conductor, respectively. The Tcs degradation was not saturated at the end of the test campaign. From the destructive observation, the large bending at the low transverse loading side in the high field zone was observed. The strand buckling and accumulating by slipping between the cable and the jacket are considered.

Ramp-Rate limitation due to current imbalance in a large cable-in-conduit conductor consisting of chrome-plated strands

The current distribution in the conductor, consisting of chrome-plated strands, was analysed assuming asymmetric strand transposition. The results show the circulation current is induced through the electrical joints at both ends of the conductor and electrical contact among the strands in the conductor. The current imbalance is produced as a result of the superimposition of the circulation and transport currents and becomes larger as the ramping rate increases. It was also found that the large current induced in the strands during a pulse charge cannot sufficiently be reduced at normal generation because of the induced voltage on these strands. The current flowing in the normal-state strands becomes larger for faster ramping. In addition, the effect of the non-uniform current distribution on the stability was experimentally investigated. The stability margin deteriorated when the current distribution in the conductor was not uniform. Moreover, the quench process in the ramp-rate limitation was considered. Since the coolant temperature is locally raised around the normal-state strands in the laminar-state coolant flow, the generation of the laminar flow region affects the ramp-rate limitation as a result of the current imbalance. From these results, it can be concluded that the current imbalance in the conductor has a very strong influence on the ramp-rate limitation.

Test of the ITER TF insert and Central Solenoid Model Coil

The Central Solenoid Model Coil (CSMC) was designed and built by ITER collaboration between the European Union, Japan, Russian Federation and the United States in 1993-2001. Three heavily instrumented insert coils have been also built for testing in the background field of the CSMC to cover a wide operational space. The TF Insert was designed and built by the Russian Federation to simulate the conductor performance under the ITER TF coil conditions. The TF Insert Coil was tested in the CSMC Test Facility at the Japan Atomic Energy Research Institute, Naka, Japan in September-October 2001. Some measurements were performed also on the CSMC to study effects of electromagnetic and cooldown cycles. The TF Insert coil was charged successfully, without training, in the background field of the CSMC to the design current of 46 kA at 13 T peak field. The TF Insert met or exceeded all design objectives, however some interesting results require thorough analyses. This paper presents the overview of main results of the testing - magnet critical parameters, joint performance, effect of cycles on performance, quench and some results of the post-test analysis.

First test results for the ITER central solenoid model coil

Abstract The largest pulsed superconducting coils ever built, the Central Solenoid (CS) Model Coil and Central Solenoid Insert Coil were successfully developed and tested by international collaboration under the R&D activity of the International Thermonuclear Experimental Reactor (ITER), demonstrating and validating the engineering design criteria of the ITER Central Solenoid coil. The typical achievement is to charge the coil up to the operation current of 46 kA, and the maximum magnetic field to 13 T with a swift rump rate of 0.6 T/s without quench. The typical stored energy of the coil reached during the tests was 640 MJ that is 21 times larger than any other superconducting pulsed coils ever built. The test have shown that the high current cable in conduit conductor technology is indeed applicable to the ITER coils and could accomplish all the requirements of current sharing temperature, AC losses, ramp rate limitation, quench behavior and 10 000-cycle operation.

Experimental results on instability caused by non-uniform current distribution in the 30 kA NbTi demo poloidal coil (DPC-U) conductor

Abstract Two 30 kA, NbTi Demo Poloidal Coils, DPC-U1 and DPC-U2, were fabricated and tested in the Demo Poloidal Coil project at the Japan Atomic Energy Research Institute. DPC-U1 and -U2 have a large current, forced flow cooling, cable-in-conduit conductor, which is composed of 486 strands. The strand surfaces are insulated by formvar to reduce coupling losses between the strands. DPC-U1 and -U2 reached their design current, but exhibited instability during charge, in many cases resulting in a coil quench. Such a quench occurred even at a current one-tenth of the conductor critical current. To clarify the cause of the instability, a detailed investigation on the quench current and normal voltage behaviour was carried out by charging the coil in several ways to the coil quench, and by measuring the stability of the coil at a current of 16–21.5 kA. These experimental results revealed the existence of non-uniformity of current distribution among the strands in the conductor, even under slow charging. This non-uniformity of current distribution caused the instability of the coil. The time constant of current redistribution is very large due to the insulation between the strands. However, if part of the conductor can be forced to go normal without coil quench occurring, a redistribution of current takes place and the current distribution becomes more uniform. It was then demonstrated that the current distribution could become uniform by applying heat to the conductor to generate intentional normalcy. Consequently, the possibility of stable operation of the DPC-U was suggested.

Current imbalance due to induced circulation currents in a large cable-inconduit superconductor

Abstract It has been previously reported by the authors that 30 kA NbTi pulsed coils (Demo Poloidal Coils; DPC-U1 and -U2) exhibit instability such as quenching at much lower currents than their critical level as a result of current imbalance in the conductor. In this paper, a theoretical study for such an imbalance in a large cable-in-conduit (CIC) conductor consisting of insulated strands is presented. This study indicates that significant circulation currents are induced in large CIC conductors, such as the conductor of DPC-U1 and U2, and remain for a long time because of the superconductivity of the strands. A large current imbalance is produced by superimposing the induced circulation current onto the transport current. It is also shown that the existence of an external field induces larger circulation currents, resulting in the larger current imbalance. For justification of these indications, characteristics of current imbalance are investigated from the experimental results. The magnitude of the current imbalance is evaluated as the ratio of the maximum strand current to the average strand current. This ratio was estimated to be 7.1 when DPC-U1 was charged singly, and reached about 15 when DPC-U1 was subjected to an external field from DPC-U2 and a test coil was installed between DPC-U1 and U2. Also, the time decay constant of the induced circulation currents was estimated to be around 2 h. These figures are interpreted by calculating the current distribution in the DPC-U1 conductor based on the assumption of asymmetric strand transposition of about 0.1% deviation in self inductances. It seems impossible to control such small asymmetry of the strand transposition in a commercial manufacturing procedure. Therefore, such instability as a result of current imbalance is inevitable in large CIC superconductors consisting of insulated stands. A similar instability may be caused in a large CIC superconductor when strands are coated with highly resistive material.

Numerical model using an implicit finite difference algorithm for stability simulation of a cable-in-conduit superconductor

Abstract A new numerical model and calculation method for stability simulation of a cable-in conduit conductor (CICC) has been developed. In this model, the governing equations are a one-dimensional fluid dynamics equation and heat conduction equations for strands and a conduit. Precise modelling of transient heat transfer from the strands to the coolant is the key to accurate simulation of stability. Modelling of the transient heat transfer coefficient is carried out more accurately than the conventional model by taking into account the variation in the heat flux and the temperature around the strands. The governing equations are made discrete by an implicit time-dependent finite different scheme to avoid any limitation from the CFL (Courant-Friedrichs-Lewy) condition. The finite different equation for the fluid is linearized according to a certain procedure used by Beam and Warming, resulting in a tridiagonal system. Therefore, no iteration is necessary to solve the fluid dynamics equation, in spite of the use of the implicit scheme. As a result, a large amount of CPU time could be saved. The simulated and experimental results of stability for a small CICC was compared for verification of the code. The results were in good agreement, resulting in a completion of the verification of the code.

Test Results of the Third Japanese SULTAN Sample

Many full size conductors for the ITER TF coils have been tested at the SULTAN test facility in Switzerland for conductor qualification. JAEA tested two samples and two kinds of Nb3Sn strands were evaluated through the tests. Now another sample named JATF3 has been tested, which uses two other kinds of Nb3Sn strands. The strands satisfy the critical current density Jc requirement, but results of the conductor test were lower than our expectation. After the test, JAEA has been investigating the reason by X-ray CT scan and destructive inspection, and has found a possible reason.