Speaker
Description
Typically, stellarator optimization relies on the notion of omnigenity [1, 2], which guarantees small radial neoclassical transport at the low collisionalities that are characteristic of fusion-grade plasmas. In omnigenous magnetic fields, collisionless particles do not move radially on average and, in the course of their motion along flux surfaces, they never undergo transitions between different types of orbits. Quasi-isodynamic fields are a subset of omnigenous fields that give zero bootstrap current at low collisionality [3], a property that makes them compatible with an island divertor, an advanced exhaust solution for stellarators. Quasi-isodynamicity is the approach Wendelstein 7-X is based on [4], and probably the most mature concept for stellarator reactors.
The recent introduction of the notion of piecewise omnigenity [5] represents a major theoretical breakthrough. In piecewise omnigenous fields, the average radial displacement of collisionless particles is zero, but in their motion over magnetic surfaces, particles experience transitions between different types of trapped orbits. The discovery of piecewise omnigenous fields radically expands the space of known configurations that are optimized with respect to radial neoclassical transport. In particular, piecewise omnigenous fields are free from certain topological constraints obeyed by magnetic field strength contours in omnigenous configurations that often lead to complex coils.
In this conference contribution, we will show that the potential of piecewise omnigenity goes far beyond the optimization of radial neoclassical transport by proving that there exist piecewise omnigenous fields that give zero bootstrap current at low collisionality [6]. We will analytically derive the mathematical condition for zero bootstrap current and will confirm the accuracy of this condition by means of neoclassical simulations. Our results imply that two fundamental properties of plasmas confined in quasi-isodynamic fields (small radial neoclassical transport and vanishing bootstrap current) can also be attained by piecewise omnigenous fields, providing a pathway to new, possibly simpler stellarator reactor designs.
[1] J. R. Cary and S. G. Shasharina, Phys. Rev. Lett. 78, 674 (1997).
[2] F. I. Parra, I. Calvo, P. Helander et al., Nucl. Fusion 55, 033005 (2015).
[3] P. Helander and J. Nührenberg, Plasma Phys. Control. Fusion 51, 055004 (2009).
[4] H. Wobig, Plasma Phys. Control. Fusion 35, 903 (1993).
[5] J. L. Velasco, I. Calvo, F. J. Escoto et al., Phys. Rev. Lett. 133, 185101 (2024).
[6] I. Calvo, J. L. Velasco, P. Helander et al., arXiv:2505.02546. Submitted to Phys. Rev. Lett.
