C.T. Wilson

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. >

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.

Investigating thermal hydraulic quenchback in a cable-in-conduit superconductor

Quench propagation of a cable-in-conduit force-cooled superconductor plays a very important role in the protection of a magnet built with such a conductor as in a superconducting magnetic energy storage (SMES) system. Some thermal analysis showed that the compressional and frictional heating exerted by the expanding hot helium could heat the helium away from the normal zone above the superconductor current sharing temperature. Thus, an acceleration of the quench propagation could be realized. This phenomenon is called thermal hydraulic quenchback (THQ). A setup was built specifically to investigate this phenomenon. The test sample consists of a 50 m long NbTi superconducting cable enclosed in a stainless steel conduit. Heaters 0.2 to 8 m long are provided to quench the conductor. The authors report experimental finding of THQ and its dependence on the initial normal zone length, the conductor current, the magnetic field, and the coolant temperature.<<ETX>>

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.

Test results of superconducting AC magnets for magnetic refrigeration experiment

Magnetic refrigeration can be achieved by cycling the magnetic field while leaving the magnetic material and the magnet stationary to avoid the large electromagnetic force problem. Two superconducting magnets were used to test this approach. First we reconfigured a force-cooled cable-in-conduit magnet to operate with liquid rather than supercritical helium. Limited by the available power supply voltage, the fastest charging rate achieved was 10 s to 5 T. A second low loss magnet was acquired for operation to 7 T with a 6-s duty cycle. This is a bath-cooled magnet with potted sub-coils. The conductor is a 20-strand Rutherford-type cable with Ebanol insulation on each strand. This magnet quenched prematurely at 6.4 T. It was charge rate sensitive and the fastest charging rate achieved was 10 s to 5.8 T. >

Operating Experience of the IFSMTF Vapor-Cooled Lead System

The International Fusion Superconducting Magnet Test Facility (IFSMTF) uses six pairs of vapor-cooled leads (VCLs) to introduce electric power to six test coils. Each VCL is housed in a dewar outside the 11-m vacuum vessel and is connected to the coil via a superconducting bus duct; the various VCLs are rated at 12 to 20 kA. Heat loss through the leads constitutes the single largest source of heat load to the cryogenic system. Concerns about voltage breakdown if a coil quenches have led to precautionary measures such as installation of a N2-purged box near the top of the lead and shingles to collect water that condenses on the power buses. A few joints between power buses and VCLs were found to be inadequate during preliminary single-coil tests. This series of tests also pointed to the need for automatic control of helium flow through. the leads. This was achieved by using the resistance measurements of the leads to control flow valves automatically. By the time full-array tests were started, a working scheme had developed that required little attention to the leads and that had little impact on the refrigerator between zero and full current to the coils. The operating loss of the VCLs at full current is averaging at about 7.4 g/s of warm flow and 360 W of cold-gas return load. These results are compared with predictions that were based on earlier tests.

Test results of the US-LCT pool-boiling coils in the International Fusion Superconducting Magnet Test Facility (IFSMTF)

The international Large Coil Task (LCT) designed, built, and successfully tested six different toroidal-field coils. Included in the torus are two pool-boiling coils designed and fabricated by US firms, General Dynamics/Convair Division (GD) and General Electric/Oak Ridge National Laboratory (GE). Both coils were instrumented for studies of electromagnetic, mechanical and thermodynamic properties and performed well and met design specifications. The authors summarize the complete test results, including the extended-condition test in which both coils demonstrated capability for operation beyond design points. The highlights of all major experiments since April 1987 that involved the GD or GE coils are presented in tabular form. In the extended-condition tests, both coils operated stably at 100% design current and above 9 T, even with bath temperature higher than 4.3 K. The mechanical behavior of both coils was generally in good agreement with calculations. Both coils were also safely discharged several times in the extended-condition tests. All results indicate that the technology developed for these two pool-boiling LCT coils can be directly applied for future large-scale applications. >

Single Magnet Test Results of the First EBT-P Development Magnet

The first development coil (D1) for the ELMO Bumpy Torus Proof-of-Principle (EBT-P) machine was successfully tested1 in a large “bucket” dewar — the “open dewar test”. Since then, it has been welded closed and installed in an individual dewar by a McDonnell Douglas/General Dynamics magnet development team. During the open dewar test, the bath contained 500 to 600 L of liquid helium surrounding the coil. In contrast, there is only a volume of about 15 L left for helium inside the individual dewar. This, plus the heat load introduced by the support structure, changed the cryogenic environment of the coil.

Large Coil Task instrumentation and diagnostics-a review

The goal of the Large Coil Task (LCT) was to develop large toroidal superconducting magnets for fusion reactors. Each of the six coils built for this task was heavily instrumented with some 200 to 400 sensors and diagnostic voltage taps to test its performance and characteristics. In addition, more than a thousand sensors were installed in the facility and test-stand components to ensure safe and controlled operation. A review is presented of the various types of thermometers, pressure transducers, flowmeters, strain gages, displacement transducers, acoustic emission sensors, field probes, and other diagnostic instrumentation used in the LCT. The usefulness of the sensors, the difficulties with some of them, and the reliability of different groups of sensors in this task are described. >