Split Florida Helix
3-D CAD model (top) and actual working model (bottom) of the Split Florida-Helix employed near the mid-plane of the resistive split magnet.
A preliminary design of a resistive magnet with a mid-plane split for use in far-infrared photon scattering experiments was completed in 2007 at the conceptual level. This user magnet, to be operated at our own facility, should provide a flux-density in the range of 25-30 tesla depending on the final gap dimensions using less than 28 megawatts of DC power. This magnet is expected to be operational by 2010.
The user magnet requires a traditional bore tube as well as four large scattering ports of elliptical shape to provide adequate experimental space at the mid-plane (each with an opening angle: horizontal=45°/vertical=11.4°/). While the mid-plane of a traditional solenoid typically consists of high current-density conductor, the mid-plane of this split magnet will be more than 50 percent vacuum space. A significant fraction of the mid-plane also will be steel to isolate the vacuum from the cooling water. The remaining space is further compromised to provide sufficient room for cooling-water passages. Eventually, there needs to be adequate coil structure to not only carry the current, but also to react the clamping forces and the torque pulling and twisting the two halves of the magnet together. These unique design challenges are especially severe for the windings in the mid-plane region of the innermost coils. Consequently, the Magnet Lab developed a new technology called Split Florida Helix, a modification of the previously described Florida Helix technology [2].
3-D Finite Element Model for analysis with Ansys(TM) evaluating the thermal-electric and structural performance.
To demonstrate the technology suitable for a 30 T split magnet, we designed and built model coils to match or exceed the critical design parameters of the conceptual design of the user magnet. The working model summarized in [2] consists of two coils, and only the innermost coil includes a mid-plane gap. The Split Florida Helix is employed near the mid-plane and Florida Bitter technology elsewhere. As in most of our other high-performance solenoids, we incorporated axial current grading including a different cooling hole pattern for the end turns.
Two model coil tests (with different coil lengths to consider varying coil clamping forces) were conducted at the appropriate level of current-density, power-density, stress, and field by inserting them in the existing 20-T, 200-mm bore resistive magnet (Large Bore Magnet). While designed for 15.6kA, we eventually increased the current in the working model up to 18.0 kA, tested a range of different cooling water flow rates (± 7 percent), and cycled the magnet at least 20 times. The measurements confirmed a center field of 32.1 T while the resistance of the split resistive insert coil never increased by more than 2.1 percent.
Conclusions
A working model for a large split resistive magnet was designed, built, and successfully tested at a high magnetic field above 32 T at the Magnet Lab. The test results fell well within the expectations and confirmed the design assumptions. Complex and sophisticated FEA proved to be fundamental for the implementation of the new technology called Split Florida Helix.
Acknowledgements
This work was supported by the State of Florida and the National Science Foundation through NSF Cooperative Grant No. DMR 9016241.
References
[1] Bird, M.D., "Florida-Helix Magnets", IEEE Trans. On Superconductivity, vol. 14, no. 2, 2004, pp. 1271-1275.
[2] Toth, J., et al., "FEA-aided Design for a Working Model of a Split Florida-Helix", presented at MT-20, Aug. 27 – 31,
2007, Philadelphia, Pennsylvania, paper 4S05, to be published in IEEE Trans. Appl. Supercond.
Links
For more information, please contact project director Jack Toth.
