J.W. Lue

Pressure drop measurement on forced flow cable conductors

Forced flow cable conductors being developed for use in LCP coils and other large superconducting magnets utilize supercritical helium flowing through narrow, uneven channels with large cooling surfaces. Extensive measurements on pressure drop of a variety of samples were performed. It is found that the friction factor versus Reynolds number plots of all the data are clustered together and behave in a universal way. A factor of two to three higher in friction factor than the smooth tube value in turbulent helium flow regime can be expected for this type of conductor.

Review of stability experiments on cable-in-conduit conductors

Abstract The stability of a cable-in-conduit conductor (CICC) cooled with supercritical helium can be very high if it is operated below a certain limiting current. This limiting current can be determined by a Stekly type heat balance equation. However, an effective heat transfer coefficient that depends not only on the conductor and the coolant, but also on the heating condition, should be used in the equation. By reviewing the stability experiments performed on CICCs, it is shown that, depending on the effectiveness of the heating induced flow, the effective heat transfer coefficient can be as low as 400 W m −2 K −1 or as high as 1400 W m −2 K −1 . Based on this review, conclusions and comments are made with regard to the permissible current density, ramp rate limitation and dual cooling channel in a CICC.

Parametric study of the stability margins of cable-in-conduit superconductors: Experiment

In a previous experiment on the stability of cable-in-conduit superconductors, we sometimes observed multivalued stability margins, which we attributed to strong heating-induced transient flows. We proposed a schematic theory from which we derived a scaling relation for the limiting current below which the stability margin is always singlevalued. Measurements at different magnetic fields are used to test the scaling with critical temperature and resistivity. We also examine the scaling with heated length and heat pulse duration. The results of these experiments are given and compared with theory.

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.

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.

Extending an internally cooled superconducting magnet to higher fields

An internally cooled superconducting (ICS) magnet was built and tested earlier at Oak Ridge National Laboratory (ORNL), and its stability margin as a function of operating fields was measured at an ambient helium temperature of 4.2 K. In the experiment reported here, we retested this magnet with lower bulk helium temperatures. The stability margins of the magnet as a function of temperature were measured. The results demonstrate clearly the flexibility offered by an ICS magnet: The stability margin can be improved substantially or the field output can be extended without sacrificing the stability margin simply by feeding the magnet with lower temperature helium. The analysis made in a previous paper on extended field operation of an ICS magnet is thus confirmed.

Quench propagation in a cable-in-conduit force-cooled superconductor-preliminary results

A preliminary test was performed to measure quench propagation in a cable-in-conduit superconductor. Although the data are not extensive, the behavior of the sample was similar to that reported by T. Ando et al. (1989) for tests performed at the same current densities (though at 7-T field). The propagation increased with time, a phenomenon that can only be explained by thermal hydraulics of the coolant. The maximum propagation velocity was about 5 m/s at a current density of 100 A/mm/sup 2/. The propagation velocity (tens of meters per second) predicted by others was not observed. Based on the measured initial normal zone hot helium expansion velocity, the condition for use of the finish time formula of L. Dresner was not met in either the present experiment or in Andos experiment. It is not clear whether the observed slightly higher power dependence of normal zone velocity on elapsed time is due to changes in helium expansion velocity or is a result of THQ (thermal hydraulic quenchback). Further studies, both analytical and experimental, are needed before the existence of THQ can be verified.

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.