Speaker: Ken Hammond, Princeton Plasma Physics Laboratory (PPPL)

Title: Improved stellarator permanent magnet designs through combined discrete and continuous optimizations

Abstract: A common optimization problem in the areas of magnetized plasmas and fusion energy is the design of magnets to produce a given three-dimensional magnetic field distribution to high precision. When designing arrays of permanent magnets for stellarator plasma confinement, such problems have tens of thousands of degrees of freedom whose solutions, for practical reasons, should be constrained to discrete spaces. We perform a direct comparison between two algorithms that have been developed previously for this purpose, and demonstrate that composite procedures that apply both algorithms in sequence can produce substantially improved results. One approach uses a continuous, quasi-Newton procedure to optimize the dipole moments of a set of magnets and then projects the solution onto a discrete space. The second uses an inherently discrete greedy optimization procedure. The approaches are both applied to design arrays of cubic rare-Earth permanent magnets to confine a quasi-axisymmetric plasma with a magnetic field on axis of 0.5 T. The first approach tends to find solutions with higher field accuracy, whereas the second can find solutions with substantially fewer magnets. When the approaches are combined, they can obtain solutions with magnet quantities comparable to the second approach while exceeding the field accuracy of what either approach can achieve on its own. 

Biography: Ken Hammond is a Research Physicist at the Princeton Plasma Physics Laboratory (PPPL). He received a PhD in Applied Physics at Columbia University in 2017 after completing thesis work studying error fields, microwave-heated plasmas, and equilibria on the CNT stellarator experiment. He then worked as a postdoc at the Max Planck Institute for Plasma Physics in Greifswald, Germany, researching edge plasma physics on the Wendelstein 7-X (W7-X) stellarator and specializing in Langmuir probe measurements and diagnostic design. He joined PPPL in 2019, where his projects have included permanent magnet stellarator design and real-time plasma diagnosis. In addition, since 2021, he has served as a Task Force Leader for experiment program planning on W7-X.

This talk will be offered in a hybrid format. If you wish to participate remotely, please send an email to [email protected].