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Alpha particle confinement metrics based on orbit classification in stellarators

Published online by Cambridge University Press:  05 May 2023

Christopher G. Albert Open the ORCID record for Christopher G. Albert [Opens in a new window] ,
Rico Buchholz ,
Sergei V. Kasilov ,
Winfried Kernbichler  and
Katharina Rath Open the ORCID record for Katharina Rath [Opens in a new window]
Christopher G. Albert*
Affiliation:
Fusion@ÖAW, Institute of Theoretical & Computational Physics, Graz University of Technology, 8010 Graz, Austria
Rico Buchholz
Affiliation:
Fusion@ÖAW, Institute of Theoretical & Computational Physics, Graz University of Technology, 8010 Graz, Austria
Sergei V. Kasilov
Affiliation:
Fusion@ÖAW, Institute of Theoretical & Computational Physics, Graz University of Technology, 8010 Graz, Austria Institute of Plasma Physics, National Science Center ‘Kharkov Institute of Physics and Technology’, 61108 Kharkiv, Ukraine Department of Applied Physics and Plasma Physics, V.N. Karazin Kharkiv National University, 61022 Kharkiv, Ukraine
Winfried Kernbichler
Affiliation:
Fusion@ÖAW, Institute of Theoretical & Computational Physics, Graz University of Technology, 8010 Graz, Austria
Katharina Rath
Affiliation:
Max Planck Institute for Plasma Physics, 85748 Garching, Germany Department of Statistics, Ludwig-Maximilians-Universität München, 80539 Munich, Germany
*
†Email address for correspondence: albert@tugraz.at
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Abstract

We present orbit classification schemes for use as fast metrics for fusion alpha particle losses implemented in the symplectic guiding-centre code SIMPLE. Two variants respectively based on conservation of the parallel adiabatic invariant, and topology of footprints in Poincaré sections are introduced. Like an existing approach based on the Minkowski fractal dimension, those methods estimate whether a guiding-centre orbit is regular and therefore expected to be confined for infinite time in the collisionless case, or chaotic, which might lead to its loss. Compared with the existing approach, the required orbit tracing time for the novel classifiers is shorter by at least an order of magnitude. This enables massive sampling of orbits across the whole phase space to identify regular and chaotic regions for the purpose stellarator optimization. Based on conservation of the perpendicular invariant, we demonstrate how extended regular regions may act as radial barriers for orbits from the chaotic regions on the radially inboard side. We propose to use a quantified version of this property as a new metric for collisionless fusion alpha losses. As pitch-angle scattering becomes only relevant after alphas have already deposited a significant fraction of their energy, such a metric remains useful also for the case with collisions. This is illustrated by comparison with collisional loss computations. Results are presented for applications to two quasi-isodynamic configurations, a quasi-helical configuration and two quasi-axisymmetric configurations. In addition, the Hamiltonian action-angle formalism is used in quasi-axisymmetric configurations to investigate the overlap of drift-orbit resonances leading to chaos. The respective analysis is performed with the NEO-RT code originally developed for investigation of neoclassical toroidal viscous torque in tokamak plasmas.

Keywords

fusion plasmaplasma simulation
Type
Research Article
Information
Journal of Plasma Physics , Volume 89 , Issue 3 , June 2023 , 955890301
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Copyright
Copyright © The Author(s), 2023. Published by Cambridge University Press

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