Demo Poloidal Coil Research Articles

Ｎｂ３Ｓｎ線材の現状と将来展望―発見から５０年を記念して―核融合用Ｎｂ３Ｓｎ導体の開発

The progress of Nb3Sn conductor development for fusion power applications at JAERI is described. The superconductor development was carried out in connection with the research and development for four fusion coil development projects: Phoenix, Cluster test, Demo Poloidal Coil and ITER-EDA. In the Phoenix project, the reliable production of multi-filamentary Nb3Sn strands was established during the development work towards realizing a 18 T coil, and an 18 T generation coil (BC-18) with multi-filamentary Nb3Sn strands was successfully operated at 4.2 K for the first time in the world. In the Cluster Test project, a pool-cooled Nb3Sn conductor with a high heat-flux surface, and the first 1-m class bore Nb3Sn coil (TMC) in the world was constructed and operated up to 12 T. In the Demo Poloidal Coil project, which was constructed to demonstrate the possibility of a poloidal coil, a forced-cooled Nb3Sn conductor was developed. The Nb3Sn coil (DPC-EX) constructed with it was pulse-operated up to 7 T in 0.5 s. Finally, in the ITER-EDA project a CICC-type 13 T-46 kA Nb3Sn conductor was developed. A Central Solenoid Model Coil (CSMC) was fabricated using it and successfully operated up to 13 T with 0.4 T/s.

ＩＴＥＲ‐ＣＳモデル・コイルとＣＳインサート・コイルの実験結果 ＣＳモデル・コイルの圧力損失及び流量配分

International Thermonuclear Experimental Reactor (ITER) Engineering Design Activities (EDA) have been performed in collaboration with Japan, the European Union, the Russian Federation, and the United States of America. In ITER EDA, the Central Solenoid Model Coil (CSMC) and CS insert coil have been fabricated to demonstrate the realization of ITER superconducting magnet. The conductor used in the CSMC and CS insert coil is a forced flow cable-in-conduit conductor (CICC) cooled by supercritical helium. For CICC, a pressure drop characteristic is one item of important information from the viewpoint of understanding of heat transfer in the conductor and thermodynamics design of a cryogenic system with a helium circulation pump for coil cooling. The ITER conductor has a central channel made of a spiral tube of thin stainless steel tape and a spring tube made of INCONEL in the bundle having about 1, 100 superconducting strands. The helium gas flow into the bundle and central channel in parallel and the pressure drop characteristic are determined from flow balance between the bundle region and the central channel because of different flow friction factor characteristics of both. In the past, pressure drop measurement for various bundle types of CICC composed of a few hundreds strands, such as Demo Poloidal Coil (DPC), has been well performed at 4-K supercritical helium. Recently, the friction factors of the bundle region and the central channel of the ITER conductor have been measured at room temperature. The pressure drop characteristics of an ITER relevant sub size conductor, such as QUELL have been measured at 4K. However, there are no measurement results in the practical coil operation conditions (4-K supercritical helium condition) for an ITER full-size conductor. In a CSMC experiment, the pressure drop characteristic of ITER full size conductor has been measured in a practical operation condition for the first time at an ITER CS model coil test facility in the Naka Fusion Research Establishment, Japan Atomic Energy Research Institute (JAERI).

Effect of non-uniform current distribution on the stability in Nb-Ti cable-in-conduit superconducting conductor

30kA-NbTi Demo Poloidal Coil (DPC-U) exhibited instability such as the conductor quenches at 40% of the rated current which is still expected conductor critical current, instability was caused by the non-uniform current distribution in the DPC-U conductor whose strands were insulated from each other. To investigate the instability of the DPC-U conductor, a stability experiment of a subsize conductor consisting of 27 strands was performed and the effect of the current imbalance on the stability was investigated. The current imbalance was forcibly established in the conductor using two current sources in this experiment. The experimental results indicate that the stability margin decreases as the current imbalance becomes larger and that the stability margin of the conductor is governed by the stability of the strand with the highest current in the conductor. Also, it is confirmed from the experimental results that the instability of DPC-U has to be attributed to the current imbalance in the conductor.

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

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.

Stabilized operation of 30 kA NbTi Demo Poloidal Coil (DPC-U) with uniform current distribution in conductors

Two 30 kA NbTi Demo Poloidal Coils (DPC-U1, -U2) were fabricated and tested in the Demo Poloidal Coil Project. DPC-U1 and -U2, referred to collectively as DPC-U, exhibited instability in pulsed and even in d.c. charge, such as coils quenching at much lower current than the design current which itself is still below the conductor critical current. It was found from the previous paper published by the authors that non-uniform current distribution is established in a large cable-in-conduit conductor, such as the DPC-U conductor, whose strands are electrically insulated from one another, due to an imbalance in the inductances of the strands. Also, it was clarified that the instability of DPC-U is caused by the current imbalance. However, it was also shown that the imbalanced distribution could be made uniform by creating a small resistive zone in the conductor by applying inductive heating pulses. In this paper, it is shown that the coil exhibiting instability due to imbalanced current distribution could be operated stably using this effect. In addition, it is shown that the DPC-U strands exhibited no deterioration in their designed critical current. This result indicates that the instability of DPC-U is entirely attributable to the current imbalance.

Effects of cyclic pulsed operation on the coil performance in the Nb/sub 3/Sn Demo poloidal coil (DPC-EX)

The effect of cyclic pulsed operation on the coil performance was studied from the electrical and mechanical points of view in the second experiment of the Nb/sub 3/Sn Demo Poloidal Coil (DPC-EX). Current sharing temperatures, ac losses, displacements, and strains were measured before and after cyclic pulsed operation, which consisted of about 350 cycles. No degradation of the electrical and mechanical performance of the DPC-EX was observed in the range of these cycles.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Experimental results on instability caused by non-uniform current distribution in the 30 kA NbTi Demo Poloidal Coil (DPC-U) conductor

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.

AC loss performance of 1-m-bore, large-current Nb/sub 3/Sn superconducting coils in JAERI Demo Poloidal Coil project

The Demo Poloidal Coil (DPC) project at JAERI started in 1985, and four forced-flow, large-current-capacity, 1-m-bore Nb/sub 3/Sn coils have been fabricated and tested. Two of them, DPC-EX and US-DPC, have been tested in pulsed charging at several teslas per second. Their coupling loss time constants were measured to be 2 ms, and quite good AC loss characteristics were obtained owing to chromium plating on the surface of their strands. The other two, DPC-TJ and the Toroidal Model Pancake (TMP), were designed to be DC coils, and revealed a much larger coupling time constant (a few seconds). An increase in the coupling time constant of the bundle-type conductor with regard to a coil current was observed. >

Test results of the toroidal model pancake for ITER/FER toroidal field coils

Development of a hollow cooling monolithic (TMC-FF) conductor has been carried out by Japan Atomic Energy Research Institute (JAERI) since 1988. Manufacture of the toroidal model pancake (TMP) made of the TMC-FF conductor was completed in August, 1991, after trial manufacturing and verification tests. Experiments on the TMP started in January, 1992, in the Demo Poloidal Coil (DPC) test facility. Results of cooldown, pressure drop, stability, and mechanical performance of the TMP are presented. >

Stability performance of the DPC-TJ Nb/sub 3/Sn cable-in-conduit large superconducting coil

The DPC-TJ test coil was developed in the collaboration between the Japan Atomic Energy Research Institute (JAERI) and Toshiba for the Demo Poloidal Coil (DPC) project at JAERI to obtain key technologies required for the construction of superconducting coils of a fusion experimental reactor. The DPC-TJ is a 1-m-bore, 40-A/mm/sup 2/ forced-flow cooled superconducting coil wound with a double-walled 24-kA, 12-T (NbTi)/sub 3/Sn cable-in-conduit conductor. The DPC-TJ was tested to investigate stability performance at operating conditions between 15 kA, 2.3 T and 30 kA, 4.5 T. Another test was carried out with a 1/18 reduced-scale model of the DPC-TJ conductor to investigate stability margins at the designed field of 12 T. Results indicated the limiting current of the DPC-TJ was around 25 kA. The DPC-TJ coil was stably charged up to 30 kA without any normal transitions because only a small mechanical disturbance could occur in the newly developed type of conductor.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Experimental results of stability and current sharing of NbTi cable-in-conduit conductors for the poloidal field coils

Japan Atomic Energy Research Institute has developed and tested the 30-kA NbTi Demo Poloidal Coils whose stored energy and inner diameter were 30 MJ and 1 m, respectively. The conductor of these coils consists of Formvar-coated NbTi strands inside the stainless steel conduit. During the charging test, an instability was observed. Stability and current sharing of the cable-in-conduit conductor were studied to solve the problem. These experimental results clarified that the cause of the instability was the low stability margin of the conductor due to the nonuniformity of current distribution between the strands. Therefore, the surface of the strand of the new conductor was designed to be chromium instead of the Formvar. Two types of the new 30-kA conductors were fabricated. One of these conductors was tested, and good results were obtained. These new conductors can be used for the conductor of the outer ring coils of the International Thermonuclear Experimental Reactor (ITER) with minor modifications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

３０ｋＡ‐ＮｂＴｉ実証ポロイダル・コイル（ＤＰＣ‐Ｕ）の不安定現象と安定性実験結果

30kA, NbTi Demo Poloidal Coils (DPC-U1, U2) were fabricated and tested in the Demo Poloidal Coil (DPC) project at Japan Atomic Energy Research Institute (JAERI). The DPC-U has large forced-flow cable-in-conduit conductor and its strand surface is insulated by the formvar. The DPC-U exhibited an instability in pulse and even in DC charging, such as the DPC-U had quenches at the 1/10th current of the conductor critical current. Detailed investigations on the quench currents and stability experiments were carried out in order to clarify the cause of the instability. These experimental results showed existence of the non-uniformity of current distribution in the conductor even in DC charging. And it could be concluded the instability was caused by the non-uniform current distribution and long time constant of current redistribution because of insulation between the strands, However, it was also shown the current distribution could be uniform, depositing energy on the conductor by the inductive heater without quench. Therefore it was suggested stable operation of the DPC-U was possible using inductive heater.

AC loss results of the Nb/sub 3/Sn Demo Poloidal Coil (DPC-EX)

AC losses in the DPC-EX, whose purpose is to demonstrate the applicability of Nb/sub 3/Sn conductor to poloidal coils for Tokamak fusion machines, were measured, and the results were analyzed. The coil was fabricated with a Nb/sub 3/Sn cable-in-conduit conductor and has a 1 m inner diameter. The AC loss was 9.4 kW for the pulse operation of a 0-17 kA-0 cycle with a ramp time of 1 s, corresponding to a 6.7-T maximum field and a 6.7-T/s pulse rate. It was 0.14% of the magnetic stored energy. The effective Nb/sub 3/Sn filament diameter was estimated as 32 mu m and the effective time constant of the coupling current within the strand was calculated as 2 ms on the assumption that coupling current between strands is negligible. The limitation of coil current due to AC losses is discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Experimental results of the Nb/sub 3/Sn demo poloidal coil (DPC-EX)

In order to demonstrate the applicability of a Nb/sub 3/Sn conductor to pulsed poloidal coils for Tokamak fusion machines, the Nb/sub 3/Sn Demo Poloidal Coil (DPC-EX) has been fabricated and tested. DPC-EX, whose inner diameter is 1 m, consists of two double pancakes made by a react-and-wind technique. The coil has a flat cable-in-conduit conductor cooled by forced-flow helium. DPC-EX has been set up in the Demo Poloidal Coil Test Facility (DPCF). In the operation of the coils in series, DPC-EX has been ramped up to 17 kA in 1 s. The magnetic field at this point was 6.7 T, and pulsed operation at 6.7 T/s was thus demonstrated. An average current density of 37.2 A/mm/sup 2/ in the winding was achieved in this operation, and the AC loss of the coil was confirmed to be quite small.

Development of new cryogenic steels for the superconducting magnets of the fusion experimental reactor.

The Japan Atomic Energy Research Institute (JAERI) has successfully developed new cryogenic steels for the superconducting magnets of the Fusion Experimental Reactor (FER) in collaboration with 4 steel companies. JAERI required a strength-toughness combination (σy>1 200 MPa, Klc>200 MPa√m, called JAERI box) that was beyond the capabilities of the existing austenitic stainless steels for cryogenic use. These requirements were determined from stress analyses, properties of the superconducting materials, crack propagation analyses, coil operation, etc. The new cryogenic steels were developed using tension, Charpy impact, and fracture toughness tests, step by step. The largest 4-K testing machine in the world, of maximum load 100 t, was installed at JAERI in order to evaluate the mechanical properties of large specimens and to investigate specimen size effects. The newly developed steels supplied from industrial heats (5-50 t) were named "Japanese Cryogenic Steels" (JCS), and their mechanical properties satisfied requirements mentioned the above. These steels have been already used in superconducting coils, such as JAERI's Demo Poloidal Coils (DPC).This paper reviews the development of the JCS for the superconducting magnets of the FER.

Recent progress in the demo poloidal coil program

The fabrication of two 30-kA NbTi pulsed coils and one 10-kA Nb/sub 3/Sn coil with a total stored energy of 40 MJ is in progress as a part of the DPC (Demonstration Poloidal Coil) Program. All the Nb-Ti superconducting strands have been fabricated, and their loss time-constant has been measured at 0.32 ms at 7 T, which is well below the initial target of less than 1 ms. A novel winding technique is described for the large current conductor which provides good mechanical contact. >

Research and development of superconducting magnet for fusion power.

This paper describes mainly technological achievment of superconducting magnet for fusion power for the latest 10 years in Japan. The magnet development had been devoted to tokamak Fusion Experimental Reactor (FER). The major results obtained up to now are as follows. In toroidal coil program, 12 T field generation, which is requested in a reactor toroidal coil, was realized with 6 kA multifilamentary Nb3Sn conductor in 1 m bore. For scaling-up of toroidal coil, half size coils of FER, LCT coils were tested up to 9 T. In poloidal coil program, Demo Poloidal Coil project is now under way and coil testing will be started in spring of 1989. The stored energy of this coil is around 40 MJ. In cryogenic technology program, fabrication and operation of large helium refrigerator technology were well established. A supercritical helium pump of 500 g/s was tested for forced flow coil.

Research and Development of Superconducting Magnet for Fusion Power

This paper describes mainly technological achievment of superconducting magnet for fusion power for the latest 10 years in Japan. The magnet development had been devoted to tokamak Fusion Experimental Reactor (FER). The major results obtained up to now are as follows. In toroidal coil program, 12 T field generation, which is requested in a reactor toroidal coil, was realized with 6 kA multifilamentary Nb3Sn conductor in 1 m bore. For scaling-up of toroidal coil, half size coils of FER, LCT coils were tested up to 9 T. In poloidal coil program, Demo Poloidal Coil project is now under way and coil testing will be started in spring of 1989. The stored energy of this coil is around 40 MJ. In cryogenic technology program, fabrication and operation of large helium refrigerator technology were well established. A supercritical helium pump of 500 g/s was tested for forced flow coil.

Thermal design and verification tests of the Nb-Ti demo poloidal coils (DPC-U1, U2)

For the development of a 30-MJ, 7-T/s pulsed coil, a 30-kA cable-in-conduit conductor with low pulsed loss was designed and fabricated. The Japan Atomic Energy Research Institute performed verification tests to measure the temperature increase when a high-energy disturbance of 300 mJ/cc-strand is put into the superconducting strands and also to measure the stability margin of the conductor, which is increased by a factor of 1.65 when the He flow rate is increased from 0 to 1.0 g/s at the nominal current under 7 T. A possible mechanism to explain this improvement is discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Stress analysis and mechanical tests for Demo Poloidal Coil

Japan Atomic Energy Research Institute is developing the Demonstration Poloidal Coils which consist of two Nb-Ti pulse coils and a Nb/sub 3/Sn pulse coil using a forced-flow conductor. Three-dimensional and two-dimensional finite-element models were used for the stress analysis of the coil system and each pancake, respectively. The coil system, consisting of three coils, was analyzed to estimate the deformation and the stress on coils and support structures for the mirror and cusp operation modes. The maximum stress appeared at the innermost turn. This was found to be due to the inner pressure increase generated by the coil quench. The calculated stresses were within the allowable stress limits for the Japanese Cryogenic Steel used. >