April 4-6

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Author: Elizabeth Paul
Requested Type: Poster
Submitted: 2022-03-04 08:58:34

Co-authors: A. Bhattacharjee, M. Landreman, R. Nies, S. R. Hudson, D. Alex

Contact Info:
Princeton University - Department of Astrophysical
4 Ivy Lane
Princeton, New Jersey   08544
United States

Abstract Text:
Magnetic confinement reactors must confine energetic particle populations as excessive losses of fusion-born alpha particles can inflict damage to material structures and reduce heating of the bulk plasma. When 3D magnetic fields are introduced, such as in a stellarator or rippled tokamak, collisionless particle orbits are no longer confined.

Using new optimization strategies [1,2], we demonstrate that excellent collisionless confinement can be obtained by achieving precise quasisymmetry on one surface or throughout a volume. We uncover the strong dependence of fast ion confinement on the orbit width by analyzing a set of nearly quasisymmetric equilibria with varying rotational transform profiles and aspect ratios. To gain further insight into the nature of energetic particle confinement in 3D magnetic fields close to quasisymmetry, we perform a classification of loss mechanisms in a set of magnetic configurations. We identify the relative impact of banana diffusion, super-banana orbits, ripple trapping, transitions between particle classes, and complex periodic orbits. Since small deviations from symmetry may be required in order to accommodate other physics objectives, we propose strategies to mitigate each loss type.

[1] M. Landreman and E. J. Paul, Phys. Rev. Lett. 128, 035001 (2022).
[2] R. Nies et al, J. Plasma Phys. 88, 905880106 (2022).