J.N. Luton

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. >

Critical current measurements on Ag/Bi-Pb-Sr-Ca-Cu-O composite coils as a function of temperature and external magnetic field

Transport critical currents have been measured on two coils of high-temperature superconducting (HTSC) tape as a function of temperature and external magnetic field. The sample coil windings have inside and outside diameters of roughly 25 mm and 40 mm, respectively, and a length of 50 mm. They contain about 300 turns of filamentary Bi-Pb-Sr-Ca-Cu-O 2223 HTSC material sheathed in Ag to form a 0.18-mm by 2.54-mm tape, with a total length of about 30 m. Critical current results are reported for temperatures between 4.2 K and 90 K, in magnetic fields ranging up to 5.5 T.<<ETX>>

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.

Test Results of Two Small, High-Temperature Superconducting Coils

Electrical Measurements were performed on two high-temperature superconducting coils made by American Superconductor Corporation. One coil measured 24-mm i.d., 59-mm o.d, 50-mm long, and used 85-m long Y-124 tape conductor. The other coil measured 29-mm i.d., 44-mm o.d., 43-mm long, and used 35-m long Bi-2223 tape conductor. We obtained V-I curves from room to helium temperature by using a variable temperature cryostat cooled by helium gas in external fields up to 4 T. Without an external field, the better performing Bi-2223 coil had a critical current, I c , of 14.1 A (2,820 A/cm2 over the conductor) at 4.2 K and 1.8 A (360 A/cm2) at 77 K. At 4 T, the I c was 4.9 A (980 A/cm2) at 4.2 K and 2.0 A (400 A/cm2) at 50 K. Reduced critical current, I c (B)/I c (0) vs. field plots indicated that a single smooth curve could fit all the data up to 50 K. The reduction in critical currents with external fields for the Y-124 coil was more than 80% at i T. For the Bi-2223 coil, it was about 42% at 1 T and about 61% at 4 T.

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.

Analysis and performance of an axial-gap superconductor motor

The performance of a variable-speed, AC, superconducting motor has been evaluated. A novel axial-gap geometry was chosen for four-pole, 1800-r/min operation, using a superconducting stator and normal armature. The pool-boiling cryostat contains four solenoidal field windings of filamentary Nb-Ti conductor and has a maximum field of 7 T and an air-gap field of 2-3 T. The armature windings are formed from copper Litz wire and are arranged in 48 radial slots. The wires inside diameter and outside diameter are 17.18 and 69.24 cm, respectively. The armature is driven by three-phase power supplied via slip rings and an adjustable-speed drive. The maximum design power is 100 hp, which can be doubled by using two armatures. Motor-performance data as a function of speed and air-gap flux density are presented for initial low-power tests with a drive capable of delivering 60 A per phase to the armature.

Results of the international large coil task: a milestone for superconducting magnets in fusion power

Abstract The aim of the Large Coil Task (LCT) was to demonstrate the reliable operation of large superconducting toroidal field (TF) coils and to prove the design principles and fabrication techniques to be applied for the magnets in a tokamak experimental power reactor. This has been achieved by an outstanding international development effort during more than ten years of cooperation within an IEA agreement. Parties were the US DOE, EURATOM, JAERI and the Swiss government. Six different D-shaped test coils were separately designed, developed and constructed by the LCT participants, then extensively tested together in a compact toroidal array. The ORNL acted for DOE as the LCT operating agent, building and operating the required test facility. The US also provided three test coils; the other three participants one coil each. Detailed information on coil design and manufacture and all test data were shared among the LCT participants. After facility shakedown operations and preliminary coil tests, the full six-coil array tests were carried out in a continuous period from the beginning of 1986 until September 1987. Beside the originally planned tests to reach an 8 T design point performance, the tests went well beyond this goal, reaching 9 T peak field in each coil. The experiments also delineated the limits of operability and demonstrated the coil safety under abnormal conditions. For fusion application the transient a.c. field behaviour in the coils was also of great interest. Three of the coils have been tested in this respect and showed excellent performance, with loss values in agreement with the theoretical predictions. At the time of International Experimental Reactor (ITER) activities, it might be worthwhile to mention that LCT demonstrated an effective multinational collaboration in an advanced technology project, involving large scale hardware produced in several countries then assembled and operated as a tightly integrated system.

Preliminary results of the U.S. pool-boiling coils from the IFSMTF full-array tests

The Large Coil Task to develop superconducting magnets for fusion reactors, is now in the midst of full-array tests in the International Fusion Superconducting Magnet Test Facility at Oak Ridge National Laboratory. Included in the test array are two pool-boiling coils designed and fabricated by U.S. manufacturers, General Dynamics/Convair Division and General Electric/Union Carbide Corporation. So far, both coils have been energized to full design currents in the single-coil tests, and the General Dynamics coil has reached the design point in the first Standard-I full-array test. Both coils performed well in the charging experiments. Extensive heating tests and the heavy instrumentation of these coils have, however, revealed some generic limitations of large pool-boiling superconducting coils. Details of these results and their analyses are reported.

Acoustic Emission Measurements for Locating High-Voltage Breakdowns in Large Superconducting Magnet Systems

A disappointingly low withstand voltage capability was found during high-potential testing of an electrical system consisting of a large superconducting coil and the equipment connected as it was installed in the International Fusion Superconducting Magnet Test Facility (IFSMTF)— two superconducting buses, two vapor-cooled leads, and 120 sensor cables with ambient temperature and cryogenic vacuum feedthrough connectors. DC and transmission line techniques were unsuitable for finding the location of the dielectric breakdowns. An acoustic emission (AE) measurement system was developed with which to determine the location of breakdowns in large coils after installation in IFSMTF. Using triangulation with AE sensors, the system measures the difference in time-of-arrival of transient waveforms caused by the dc voltage discharge. The system was calibrated on a stainless steel surface representing the coil case, and its accuracy was found to be better than 5 cm. This paper describes both the acoustic emission measurements and the high-voltage testing system used concurrently. Also presented are the experimental results of a series of high-voltage tests that led to the determination of the exact location of one breakdown in the GD/C coil system of the Large Coil Task (LCT) and the results of the successful repair.