4.2 ENGINEERING DESIGNS

ISIT completed a full CAD model of the MACE Demonstration Unit using the Autodesk Inventor 2017 software package. Here ISIT describes an overview of the completed design and highlight some aspects of key sub-assemblies within the design. Further discussion of the requirements and design choices that led to the final design is found in Section 5.

4.2.1 Overview of the MACE Demonstration Unit Design

The main function of the MACE Demonstration Unit (Figure 15) is to transfer energy from the HTS primary loop into the normal conductor secondary loop and then out to the external load. To accomplish this, several specialized sub-assemblies needed to be designed and modeled in CAD. These include:

• Primary/Secondary Loop Assembly

• HTS Switch and Dewar Assembly

• Cryocooler and Thermal Coupling Assembly

• Primary and Secondary Demountable Leads Assemblies

• Main Unit Vacuum Vessel Assembly

Figure 15

Figure 15. Main subsystems of the MACE Demonstration Unit.

4.2.2 Primary/Secondary Coil Assembly

The primary coil (Figure 16) is a 188 turn, double pancake-type high temperature superconducting coil. The primary coil is wound in a rectangular hyperellipse geometry measuring 91 cm by 25 cm. After winding, the cross-section of the completed coil measures 25-mm square. The individual turns in the primary coil are electrically insulated from each other by 5 mils of Nomex insulation. The secondary coil is a bipartite solid copper form, which is bolted together around the primary coil to create an annular coil that completely encloses the primary (Figure 17). The wall thickness of this shell (12.5 mm) is sufficient to allow both efficient current transport during firing, and also to withstand the hoop stresses placed on the system during operations.

Figures 16+17

Figure 16. Geometry of the HTS primary coil. Secondary coil is shown as translucent layer encasing the primary coil

Figure 17. Bipartite shell of the secondary coil encasing the primary coil

 

4.2.3 HTS Switch and LN2 Dewar Assembly

The HTS switch assembly (Figure 18) is designed to operate under standby conditions at subcooled LN2 temperatures (70 K). Therefore, the switch coil and its coil form is directly cooled by immersion in LN2. At the time of firing, however, the HTS switch will absorb approximately 1.9 KJ of energy, raising it above its critical temperature, and must therefore be immediately cooled back down to operating temperature. Our design uses a gravity-feed LN2 Dewar system that allows LN2 to flow through the perforated coil form quickly, carrying away heat, and thereby directly cooling the HTS switch after firing.

4.2.4 Cryocooler and Thermal Coupling

 

Figure 18

Figure 18. HTS switch assembly with major subassemblies labeled.

A major challenge in designing the MACE Demonstration Unit was to effectively cool both the primary and secondary coil circuits with a single cryocooler. Additionally, all three (3) assemblies (the primary circuit, the secondary circuit, and the cryocooler), as shown in Figure 19, needed to be thermally coupled but remain electrically isolated from each other due to high voltages that can occur during triggering. The final design overcame these challenges by including a Nomex electrical insulation layer between the mated connections of the copper circuit assemblies to the copper cold head of the cryocooler. The Nomex is thin enough (5 mils) to readily allow rapid cooling of the circuits by the cold head, but also electrically insulate the assemblies from each other.

Figure 19

Figure 19. Thermal coupling of cryocooler with both the primary and secondary circuits

4.2.5 Demountable Leads

Another challenge in designing the MACE Demonstration Unit was the prevention of heat loss to the exterior via the electrical leads. During standby operation, prior to triggering, the use of large diameter, permanently connected external leads would lead to unacceptable heat losses due to the high thermal conductivity of copper. A solution was found such that both the primary and secondary external leads could be coupled or decoupled (“demounted”) from the internal leads, within a matter of seconds. The design (Figures 20 and 21) incorporates the use of an expandable metal vacuum bellows that can be pushed into place to connect the external leads to the internal leads at a bullet-type mounting point. Disconnection is accomplished by pulling the bellows assembly away from the vacuum vessel.

Figures 20+21

Figure 20. Demountable lead system for the primary circuit.

Figure 21. Demountable lead system for the secondary circuit.

4.2.6 Main Unit Vacuum Vessel

The main vessel enclosing the primary/secondary coil assembly as well as the cold head and thermal/electrical coupling system must fulfill two criteria. First, it must be made of a non-metallic material in order to minimize inductive coupling-related losses in the primary/secondary coil during operation. Second, it must be sufficiently rigid to withstand the stresses created when the interior is held in a vacuum. The chosen solution (Figure 22) is to mill the main unit pressure vessel from a solid block of High Density Polyethylene (HDPE).

Figure 22

Figure 22. Main Pressure Vessel of the MACE Demonstration Unit

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