5.2.1 Primary Coil Design

Choice of Superconductor:

The most important requirement of the MACE primary coil is to generate as strong a stable magnetic field as possible without losing any energy to resistive losses during the energy storage or “charged” phase of MACE operation. Therefore, MACE requires a High Temperature Superconductor (HTS) that exhibits excellent critical current capacity (Ic) while exposed to high magnetic fields. Second generation HTS tapes using YBCO superconductors commercially-available from Superpower Inc. (Schenectady, New York) have an Ic of approximately 1500 A per 12 mm width when operating at 30 K and 2 Tesla magnetic field, or almost double the likely design requirement for the MACE Demonstration Unit (Option 1).

Superpower’s HTS product appears to offer the best combination of Ic, in-field performance, mechanical strength, overall tape thinness, long piece lengths, and price.

Operating Temperatures: As with all HTS tapes, critical current increases with decreasing temperature (Figure 25). Thus, it is possible to generate the same strength magnetic field using less HTS tape if MACE is operated at lower temperatures. The main trade when determining MACE operating temperature is the cost and complexity of the cryogenic system versus the cost savings of using less HTS at lower temperatures. A liquid helium-cooled system (4 K) would offer far greater magnetic energy storage per unit volume than a liquid nitrogen cooled system (77 K), but at greater cost and reduced system reliability. It appears that a MACE unit operating at 30K would increase Ic in the HTS tape by 3-fold over liquid nitrogen https://doi.org/10.6084/m9.figshare.3759321.v1 temperatures. A 30K system would also allow cryogen-free, conduction-based cooling of the primary coil using standard commercially-available cryocoolers. (Figure 25)

Figure 25

Figure 25. Temperature dependence of Ic in HTS

Choice of Geometry: An important goal of the next phase (“Option 1”) of the MACE program is to demonstrate efficient transfer of stored energy from the MACE coil to an external load within 10 msec or less. There are several different coil geometries that could accomplish this goal, including circular coils, square coils, and rectangular coils. From a fabrication viewpoint, a circular coil would provide the simplest form to build while also minimizing the amount of required HTS. However, in designing MACE for its final field-deployable form, a rectangular coil is ultimately preferable since MACE will ultimately use a toroid geometry to minimize stray fields that could impact personnel and sensitive equipment. Compared with circular and square cross-sections, toroids with rectangular cross-sections offer the best combination of volumetric energy density, form factor design freedom, and economy of required HTS (Figure 26).

Figure 26

Figure 26. Loop geometry choices for MACE. While a circular or square geometry might be most efficient for a single primary coil, when multiple units are integrated into a toroidal form, rectangular loops offer more stored energy for an equivalent length of HTS.


5.2.2 Secondary Coil Design


Operating Principle of the Annular Shell Design: The MACE secondary coil is a single turn coil that multiplies the current flowing to an external load by a factor proportional to the turns in the primary coil.  To maximize energy transfer efficiency, the secondary coil in the MACE concept is designed as an annular shell (Figure 27) that completely encloses the HTS primary coil to capture energy stored in magnetic field surrounding the primary coil when the MACE device is triggered. An additional advantage of the annular shell design is that for the purposes of the MACE Demonstration Unit, a metal secondary coil is robust enough to provide full structural support for the primary coil, restraining it from hoop stresses while operational.

Figure 27

Figure 27. Preliminary design of the MACE Demonstrator Unit primary and secondary coil : A: CAD rendering of the HTS primary coil embedded within the transparent secondary shell.  B: Dimensions of the MACE secondary shell.  C: Cross section view of the coils.

Choice of Material: The secondary coil must provide a combination of low resistivity (to maximize energy transfer), low magnetoresistance (to minimize changing electrical conductivity in changing fields), high thermal conductivity (to enable rapid cooling), with good structural characteristics (to restrain the primary coil). The two obvious candidates for material are copper and aluminum. Copper is traditionally used when constructing coil forms for superconducting magnets for small to medium size magnets since copper presents superior characteristics, especially at cryogenic temperatures, and is also less susceptible to oxide layer formation which can inhibit electrical and thermal conductivity. Aluminum may be used in the case of very large magnets to reduce overall weight. For the MACE Demonstration Unit, copper is the clear material of choice for the secondary coil. 

Solid vs Litz Wire Shell: Initially, it was believed that the energy contained in the collapsing magnetic field would best be captured by a annular secondary coil made from Litz wire. In this application, it was believed that the use of Litz wire would reduce the skin effect exhibited in the secondary coil and thus reduce ohmic losses as the current was transferred to an external load. However, preliminary calculations indicate that a solid shell is sufficient for the MACE Demonstration Unit. The solid shell option also provides the needed structural support, and is easier to fabricate.

Operating Temperature: Unlike the HTS primary coil, which must operate at cryogenic temperatures to maintain superconductivity, the secondary coil could operate at ambient temperatures and still transfer current to the external load. However, operating the secondary coil at either 30 K (the temperature of the primary coil) or 70 K (the temperature of subcooled liquid nitrogen) presents numerous advantages, including lower overall electrical resistance and lower temperature differentials between primary and secondary (enabling more efficient cooling of the primary and simpler thermal insulation structures). In Phase II, the MACE Demonstration Unit, the cooling of the primary coil will be achieved via conduction cooling of the secondary coil physically coupled to a cryocooler cold head. Thus, for the demo unit, the operating temperature of the secondary coil will be 30 K. However, for the subsequent Phase III (the MACE Field Unit prototype), the power dissipation in the secondary is anticipated to be 1000-X higher, leading to much larger resistive heating in the secondary. In this case, the secondary will likely be directly cooled with liquid nitrogen subcooled to 70 K. 

5.2.3 Preferred Design and Operating Characteristics for MACE Demonstration Unit

For the MACE Demonstration Unit (Figure 27), ISIT anticipates building a rectangular primary coil and an encasing secondary coil with the specifications listed in Table 1.

Table 1



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