M.S. Lubell

State-of-the-art of superconducting magnets

Abstract A survey of the most recent developments in superconducting magnet materials is presented, and complete data on the upper critical field and transition temperature for the NbTi alloy system are given. The overall critical current density of compound conductors is shown for both low and high field commercial superconductors. A tabulation is given of high field and large bore solenoids, comparing design and test data. Comparative data are also given for some nonsolenoidal coils, and details are listed for the systems under construction or design. A criterion is derived for the stable current density attainable in extremely large magnet systems such as those envisioned for fusion reactors: jα (stored energy) − 1 6 . The review concludes with summaries concerning the structural materials useful in large magnets and the effects of radiation on superconducting magnets.

Quenches in a high-temperature superconducting tape and pancake coil

Abstract Stability and normal-zone propagation experiments have been performed on singlelength and double-pancake coils made of Bi-2223/Ag high-temperature superconducting tape. Experiments were conducted with liquid nitrogen and gaseous helium cooling in temperatures from 5 to 77 K. No distinctive normal-zone propagation was observed in the single-length conductor tests. Non-uniform critical currents were, however, observed over the length of the conductor. To dramatize the effect on the stability of a magnet wound with this kind of non-uniform conductor, a pancake coil was wound with conductors composed of a two-tape bundle on the inner turns and a single-tape on the outer turns. Quenches of the coil were observed when currents higher than the critical current of the single-tape section were applied and held for more than a few minutes. The coil quenched without a distinctive ‘normal’ front propagation or a large temperature gradient.

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>>

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>>

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.

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.

AC losses of HTS tapes and bundles with de-coupling barriers

Plastronic Inc. is working on a project for the Ballistic Missile Defence Organization to improve the strength and AC loss characteristics of Bi-based high temperature superconductors (HTS) for AC applications like motors, generators, and transmission cables. High strength, high amperage conductor is made by encapsulating bundles of HTS tapes with a nonsilver sheath. With this technique, Cu-alloy spacers can be placed between Bi-2223/Ag HTS tapes to de-couple the tapes under AC currents. AC losses were measured on conductor bundles with and without Cu-alloy spacers at AC currents up to 70 A and frequencies from 50 to 400 Hz. At frequencies up to about 100 Hz, it was found that the bundle with resistive barriers had AC loss equal to the sum of individual tapes as expected from the Norris ellipse model. At higher frequencies, the Cu-alloy barrier was not as effective in reducing the AC loss. For single tapes, Zr-oxide was used to coat the HTS filaments before stacking to form a multifilament tape. AC loss measurements of 6-filament tapes with and without the oxide coating showed little effect of the coating when the loss was normalized to the square of the critical current as the theory suggests.

Test Results for Different High Temperature Superconducting Transmission Cable Prototypes

Development has begun on high temperature superconductor (HTS) cable systems for power transmission. Many design factors affect the performance of a HTS transmission cable. Typical constructions will have multiple layers of superconductor. It is known that the current distribution among the layers has an effect on the performance of the cable. Measurements on two different prototype cable constructions have been performed. The prototypes are approximately 1 m long and were fabricated by helically winding Ag sheathed Bi-2223 high temperature superconductor tape on a stainless steel tube former. Both prototypes had four HTS layers consisting of 2 pairs of oppositely wound tapes. Comparisons of the measured performance of the two prototypes and the measured current distributions are compared and discussed.

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.

Large-bore, superconducting magnets for high-energy density propellant storage

A study has been conducted on the design of large-bore, superconducting solenoid magnets in an effort to determine how weight and cost scales with field and size. The fields considered ranged from 0.5 to 20 T and bore sizes from 3 to 10 m. The designs are based on light-weight, high-performance superconducting magnet designs using cable-in-conduit niobium-titanium and niobium-tin conductors. All methods to reduce the weight of the magnets are used, and both aluminum and stainless steel are considered for the structure, With the length of the magnets fixed at 10 m, the amount and cost of the conductor is determined for each field and bore size. The largest magnets are determined for each field value that have weights under 10/sup 4/, 10/sup 5/, and 10/sup 6/ kg. The cost scales as the square root of stored energy and in millions of dollars is twice the product of field (T) and bore (m).