Optimization for Quasi-Poloidal Symmetry

Optimization for quasisymmetry, specifically quasi-axisymmetric and quasi-helical symmetry, has led to stellarators with exceptional confinement properties [1]. Building on this success, we aim to extend the optimization to quasi-poloidal (QP) configurations—a subclass of quasi-isodynamic (QI) stellarators. QI configurations have already demonstrated good confinement and stability [2]; however, QP configurations remain largely unexplored. Optimizing for QP can be difficult due to the highly shaped geometries that arise, which often cause equilibrium codes to fail to converge. Despite these obstacles, quasi-poloidal stellarators hold promise, as they are expected to generate larger, sheared flows that could suppress microturbulence [3].

This study focuses on optimizing equilibrium for quasi-poloidal symmetry using DESC [4], minimizing the QP-symmetry breaking amplitude. We use the quasi-symmetry two-term objective and add penalties to suppress highly elongated cross-sections. We conduct scans of mirror ratio, aspect ratio, elongation, and rotational transform and look at trends in the converged results. In addition, several configurations are obtained with excellent confinement of fusion born alpha particles at reactor scale.

[1] ​​M. Landreman et al (2021). Journal of Open Source Software, 6(65), 3525

[2] T. S. Pedersen et al (2022) Nucl. Fusion 62 042022

[3] P. Helander and J. Nührenberg (2009) Plasma Phys. Control. Fusion 51 055004

[4] D. Dudt et al (2020) Phys. Plasmas 27, 102513

[5] D.A. Spong et al (2005) Nucl. Fusion 45 918