S. Shimamoto

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.

Construction of ITER common test facility for CS model coil

Japan Atomic Energy Research Institute is constructing the International Thermonuclear Experimental Reactor common test facility for the Central Solenoid Model Coil which is around 180 tons, a forced-flow cooled magnet with the maximum pulsed operation of 2 T/s and generates the rated magnetic field of 13 T at 48 kA with stored energy of 668 MJ. The test facility consists of a coil vacuum chamber, a cryogenic system with the 5-kW refrigerator and 500-g/s cryogenic pump, two pairs of 50-kA current leads, two DC power supplies (50 kA and 60 kA) and two JT-60 pulsed power supplies (50 kA, /spl plusmn/4.5 kV and /spl plusmn/40 kA, /spl plusmn/1.5 kV). The facility will be demonstrating the refrigeration and operation of a fusion pulsed magnet and the design and construction will accumulate experience towards the construction of ITER.

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

Abstract 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.

Propagation Velocity of the Normal Zone in a Cable-in-Conduit Conductor

The normal zone propagation velocity in a cable-in-conduit conductor has been experimentally investigated so as to obtain a data base for the design of fusion superconducting magnets. The cable-in-conduit conductor used for this experiment consisted of eighteen NbTi/Cu composite strands inserted in a stainless steel conduit. An inductive heater of over 40 mm length was positioned in the center of the conductor of 26 m length to create an initial normal zone. The measurement of the propagation velocity of the normal front was carried out by observing the appearance of the resistive voltage on each of taps attached to the conductor. The results show that the propagation velocity of the normal zone in the cable-in-conduit conductor increases with the elapsed time. This is in contrast to what happens in more conventional conductors, potted or pool- cooled conductors for example. Hence, the propagation velocity in the cable-in-conduit conductor is proportional to the 0.6th power of the elapsed time and to the 2.8th power of the transport current, at an initial pressure of 1 MPa, a magnetic field of 7 T, and a mass flow rate of zero. The transport current was varied from 1.5 – 2.0 kA, which corresponded to a current density of 54 – 71 A/mm2 within the cable space.

Experimental results of the Nb3Sn demo poloidal coil (DPC-EX)☆

Abstract The aim of the development of DPC-EX is to demonstrate the applicability of an Nb 3 Sn conductor to pulsed coils for tokamak fusion machines. The DPC-EX, whose inner diameter is 1 m, consists of two double pancakes fabricated by a react-and-wind technique. The conductor is a forced cooled, flat, cable-in-conduit conductor. The DPC-EX has been installed between two Nb-Ti demo poloidal coils (DPC-U1 and U2). In the series operating mode (DPC-EX, DPC-U1 and U2), DPC-EX was ramped up to 17 kA in 1 s and ramped down to zero in 1 s after a flat top time for 1 s, without normal transition. The maximum magnetic field and the maximum pulse field were 6.7 T and 6.7 T s −1 , respectively. The current density in the winding was 37.2 A mm −2 at an operating current of 17 kA. The ratio of a.c. losses to the stored energy was ≈ 0.14% during pulsed operation. After more than 50 cycles of pulsed operation, no damage could be found in the DPC-EX. The stability test indicates that DPC-EX has a high stability margin. These results demonstrate the possibility of high field (12–14 T) poloidal coils, as required in the FER and ITER.

Mechanical Evaluation of Nitrogen-Strengthened Stainless Steels at 4 K

Because the structural materials of large superconducting magnets for fusion machines suffer high magnetic stress at 4 K, the knowledge of their mechanical properties at cryogenic temperature is highly desirable. Austenitic stainless steels have good properties for use in superconducting fusion magnets, but their relatively low yield stress at 4 K is serious drawback. Though some studies1,2 show this shortcoming can be overcome by adding nitrogen and carbon, there is as yet no reliable data base regarding the structural properties of the improved steels. During the design of the Japanese LCT coil, it became clear that the yield stress of the structural steel must be greater than 700 MPa at 4 K. For this reason, JAERI began to evaluate the mechanical properties of nitrogen-strengthened stainless steels at 4 K.

The transverse stress effect on the critical current of jelly-roll multifilamentary Nb/sub 3/Al wires

Experiments were conducted to determine the effect of transverse compressive stress (TCS) on the critical current of jelly-roll multifilamentary Nb/sub 3/Al wire (0.8-mm dia.) for magnetic flux densities up to 12 T. For comparison, identical experiments were performed for bronze-process Ti-alloyed multifilamentary Nb/sub 3/Sn wire (1.0-mm dia.). Although the unstressed critical current density of Nb/sub 3/Al was inferior to that of (NbTi)/sub 3/Sn at high fields, under applied TCS Nb/sub 3/Al exhibited less critical current degradation than (NbTi)/sub 3/Sn. For example, at 12 T and 150 MPa. TCS-induced critical current degradation was approximately 20% for Nb/sub 3/Al, whereas it was approximately 65% for (NbTi)/sub 3/Sn.

Design and fabrication of superconducting cables for ITER central solenoid model coil

The Nb/sub 3/Sn cable is being fabricated for the central solenoid (CS) model coil under the ITER Engineering-Design Activity. The cable consists of about 1000 strands whose diameter is 0.81 mm. The design current is 48 kA at a magnetic field of 13 T. The 0.6-GJ CS model coil is operated in a pulse mode (0.5 T/s). The first trial fabrication of a 100-m dummy cable and a 20-m superconducting cable was completed successfully. The second trial fabrication of a 1000-m dummy cable was performed to establish the stable manufacturing procedure in January, 1995. The authors measured the AC losses of the full-sized conductor and could determine the cable coupling time constant. They analyzed the heat generation of the CS model coil and calculated the temperature rise of the cable for the model coil.

Design consideration of the ITER-TF coil with a react-and-wind technique using Nb/sub 3/Al conductor

The ITER-TF coil requires a generation of 12.5 T in a size of 12 m/spl times/18 m. For such a high field-large coil, the applicability of Nb/sub 3/Al conductors was considered with a react-and-wind technique which realizes high reliability and low cost for the coil fabrication. The maximum bending strain on the Nb/sub 3/Al conductor in use of this technique is 0.39%. I/sub c/ degradation due to the strain is expected to be below 5%. The limiting current is estimated as 65 kA for 60 kA operation current. AC loss in the TF coil with Nb/sub 3/Al conductor is almost the same as with Nb/sub 3/Sn conductor.