Figure 1. Two-layer multi-pole DDH magnet.
Recently, Palm Bay, FL-based AML developed a new magnet topology—Direct Double-Helix (DDH) magnets—that allow for a significant increase in power density, performance in field generation, and field quality. The unique characteristics of the DDH magnets, part of the Double-Helix (DH) magnet family invented by AML’s founder, Dr. Rainer Meinke, could lead to more affordable systems and potentially portable devices.
The magnets have a large number of applications, including high-speed generators and charged particle beam optics. AML’s customers include the Department of Energy, NASA, the European Organization for Nuclear Research (CERN), the Center for Advanced Power Systems (CAPS), the National High Magnetic Field Laboratory (NHMFL) and GE Medical Systems.
Unlike conventional magnets based on saddle or racetrack coil configurations, AML’s processes and designs enable magnets of any multi-pole fields with field homogeneity. Specifically, the DH and DDH magnets are composed of modulated tilted helices (see Figure 1) that produce magnetic fields with pure multi-pole content. However, while DH magnets are based on accurate positioning of wire in machined grooves, DDH technology enables the design and manufacturing of magnet coils in one step, without conventional conductor and winding processes.
Dr. Philippe J. Masson, left, and the creator of the Direct Double Helix magnet configuration, Dr. Rainer Meinke, showcase their breakthrough magnets.
“DDH magnets are created in-situ, and conducting paths are machined directly out of a resistive object such as a conductive cylinder,” states Dr. Philippe Masson, an AML senior research scientist. “The machined grooves serve as electrical insulation, and follow a mathematical equation—leading to non-uniform cross-section of the conductive paths.”
This results in a reduction of the total electrical resistance of a DDH magnet, meaning more current can be flown into the conductor—hence, smaller magnets are produced.
“Another great benefit is that each layer of a DDH magnet presents a large surface, allowing for improved cooling,” Masson says. “With a simple water-cooling system, small DDH coils can carry an excess of 200 A/mm2 of peak current density in steady state.”
Optimized Magnet Design
According to Masson, when DDH was invented, the advantages were clear in concept—outstanding performances were obtained experimentally and applied to commercial products. But to really understand the physics behind the measurements, a detailed numerical analysis was required.
COMSOL Multiphysics was used to visualize the current distribution in the magnet (see Figure 2) and to simulate the heat transfer in steady state operation through the coupling of the Conductive Media DC and General Heat Transfer physics interfaces.
Figure 2. Current density distribution in a few turns of a 4-pole DDH magnet.
Figure 3. Heat flux distribution in one turn of a 4-pole DDH magnet.
“The main question is, ‘what is the limit of the technology?’” Masson says.
Magnet design is a multiphysics process dealing with electromagnetism, thermal analysis, structural analysis and fluid dynamics. The software allowed for all those aspects of the design to be simulated—and moreover, to be coupled together, “making it the ideal tool for a magnet company,” according to Masson.
Because the resistance of the DDH coils strongly depends on the current density distribution, the first questions Masson and his team needed to answer were how the current actually distributed in the conductor, and then how to accurately predict the resistance. Secondly, the team needed to figure out how the non-uniform distribution of current density in the conductor affected the field homogeneity. Finally, they worked on how the heat propagates in the coil and to the coolant.
“All these questions were answered, and the results are used to help push the limits of the technology and improve the coil design,” Masson says. “Using COMSOL Multiphysics allows for the design to be optimized numerically before anything is actually fabricated. All the trial and error can be performed through simulation, thus saving time and money.”