S.S. Shen

Measurements of stability of cabled superconductors cooled by flowing supercritical helium

The concept of stability in superconductors cooled by forced flow of supercritical helium is somewhat different from conductors cooled by pool-boiling helium. The crucial point is whether such a conductor can recover from a large deposition of energy before the cryogen is heated to a level that prohibits recovery. In an investigation of stability of forced flow conductors it is usually necessary to use indirect methods of heating the conductor initially, such as by an external pulse coil. However, in using such indirect methods it is essential to determine accurately the time development and magnitude of energy deposition. We use ac loss techniques to examine pulse coil heating and compare those results with extensive measurements on a specially constructed sample containing an embedded heater.

Stability tests of the Westinghouse coil in the International Fusion Superconducting Magnet Test Facility

The Westinghouse coil is one of three forced-flow coils in the six-coil toroidal array of the International Fusion Superconducting Magnet Test Facility. A discussion is presented of results taken when the coil was tested both individually and in the six-coil array. The tests covered charging to full design current and field, when measuring the current-sharing threshold temperature using resistive heaters (installed to simulate nuclear heating), and when measuring the stability margin using pulsed inductive heaters (used to facilitate stability testing). It was found that at least one section of the conductor exhibits a very broad resistive transition (resistive transition index=4). The broad transition, though causing the appearance of voltage at relatively low temperatures, does not compromise the stability margin of the coil, which was greater than 1.1 J/cm/sup 3/ of strands. In another, nonresistive location, the stability margin was between 1.7 and 1.9 J/cm/sup 3/ of strands. This is from six to ten times larger than the value obtained by analyzing the current-sharing threshold. The cause of this discrepancy has been traced to failure of the conductor to obey the ideal critical-state theory. >

Magnetization of in situ multifilamentary superconducting Nb 3 Sn-Cu composites

Magnetic properties are reported for in situ superconducting Nb 3 Sn composites that have exhibited attractive electrical properties and superior mechanical characteristics. Magnetization measurements were conducted up to 4 T at 4.2 K on a variety of samples of different sizes and twist pitches, and the results are presented in absolute M-H curves and losses per cycle. It is observed that the magnetization of such composites is generally proportional to the size of the wire (∼0.25 to 0.51 mm) rather than the fiber size ( \sim10^{-7} m), which indicates a strong coupling effect among Nb 3 Sn fibers.

Preliminary results of the partial-array LCT coil tests

The Large Coil Task (LCT) is a collaboration between the US, Euratom, Japan, and Switzerland for the production and testing of 2.5 × 3.5 m bore, superconducting 8-T magnets. The definitive tests in the design configuration, the six coils arrayed in a compact torus, will begin in 1985. Partial-array tests are being done in 1984. In January the initial cooldown of two coils was aborted because of helium-to-vacuum leaks that developed in certain seal welds when the coil temperatures were 170 to 180 K. In July three adjacent coils (designated JA, GD, CH) were cooled, and in August two were energized to the limits of the test facility. An overview of the results is presented, including facility, cooldown, energization, dump, recovery from intentional normal zones, strain, and displacement, for operation up to 100% of design current but below full field and stress. These initial results are highly encouraging.

SIMULATION OF THE QUENCHING OF AN INTERNALLY COOLED SUPERCONDUCTING MAGNET

Stability measurements on cable-in-conduit internally cooled superconductors have shown that the heat transferred to helium earlier will cause a pressure rise and induce a transient flow inside the conduit. In turn, this will enhance the heat transfer and help the superconductor to recover to its superconducting state. However, if the heat input is very high or if the heated length is very long, the pressure rise can be tremendous and the helium expulsion can be excessive. This paper describes the experimental results of simulating the quench of an entire hydraulic path. The measured thermal expulsion of helium and peak pressure during the quench are compared favorably with a similarity theory and a simple scaling relation.

Interaction of transport current and transient external field in composite conductors

Investigations of transient field losses in superconducting composites carrying transport current are presented. The magnetization and terminal voltage of a variety of composites have been measured as a function of transport current and external field. The losses are analyzed as a sum of magnetization losses and those due to dynamic resistivity. Results are presented for slowly changing external fields where the magnetization losses are purely hysteretic and for higher \dot{B} where coupling losses are important.

The IEA Large Coil Task test results in IFSMTF

The Large Coil Task (LCT) is an international collaboration under the auspices of the International Energy Agency (IEA) among the United States, EURATOM, Japan, and Switzerland to develop large superconducting toroidal field magnets for tokamak fusion reactors. Six 2.5-m*3.5-m bore coils capable of producing 8 T were fabricated, three by the US and one each by the other participants, and assembled in a toroidal array in the International Fusion Superconducting Magnet Test Facility (IFSMTF) at the Oak Ridge National Laboratory (ORNL). The coils were widely different in design with three cooled by pool-boiling helium at atmospheric pressure and three cooled for forced-flow helium at supercritical pressure (1.5 MPa). An overview is given of the various single-coil and six-coil array tests, to design point and beyond, and also the symmetric torus tests that were performed. All six coils exceeded the design goals, both as single coils and in six-coil toroidal tests. In the symmetric torus set, a maximum field of 9 T was reached in all coils simultaneously. Only brief summary is given of the specific thermal and mechanical experiments that were also conducted. >

First results of the full-array LCT coil tests

The international Large Coil Task (LCT) has designed, built, and is testing six different toroidal field coils. Each has a 2.5- × 3.5-m D-shaped bore, a current between 10 and 18 kA, and is designed for stable operation at 8 T. Three coils are bath-cooled; three are cooled by forced flow of helium at supercritical pressure. One uses Nb 3 Sn; the others NbTi. The test coils are equipped with voltage, temperature, magnetic field, flow pressure, strain, displacement, and acoustic emission sensors sufficient for penetrating analysis of performance field. Shakedown operation of the test facility and preliminary tests of the first three coils were accomplished in 1984. Tests of the full six-coil toroidal array began early in 1986 and have progressed to the stage of design-current, design-field stability tests. Results to date have elucidated complex structural and electrical interactions in a multicoil array and provide gratifying assurance of coil performance.

Stability Measurements of A Large Nb3Sn Force-Cooled Conductor

The Westinghouse coil for the Large Coil Program (LCP) [1] at Oak Ridge National Laboratory will use a cable-in-conduit conductor made of copper-stabilized, multifilamentary (MF) Nb3Sn strands enclosed in a stainless steel jacket, which provides a channel around the conductor to allow forced cooling by supercritical helium. The operating current will be 16 kA with an 8-T maximum field. In this study, the performance of a subsize conductor similar in construction, but with only one-third as many active strands in the cable, was investigated.

Superconductors for tokamak poloidal field coils

The poloidal field (PF) system in a tokamak fusion device provides a time-varying flux, which is used initially to break down the plasma and build up the plasma current and, subsequently, to compensate for plasma losses in sustaining the burn. Although the coil parameters vary somewhat from device to device, maximum fields up to 7 T and field sweep rates up to 7 T/sec are common. This combination of maximum field and field sweep rate is somewhat beyond the state of the art and, when combined with the large size of the magnets (stored energy ∼ 1 G J), presents a rather demanding technological challenge. In this paper, stability and loss measurements on superconducting cables similar to those that could be used in PF coils are presented, along with a model, which aids in the interpretation of the loss.