Energetic particle optimization of quasi-axisymmetric stellarator equilibria

LeViness, Alexandra; Schmitt, J. C.; Lazerson, S.; Bader, A.; Faber, B. J.; Hammond, K. C.; Gates, D.

doi:10.1088/1741-4326/aca4e3

Cited by 6 publications

(16 citation statements)

References 32 publications

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“…If considering configurations within the same class, Γ α and Γ c appear to have better correlations with GC confinement. Good correlation has been observed for a set of QA equilibria close to li383 [72] as well as for W7-X configurations [1].…”

Section: Drift-convective Transportmentioning

confidence: 77%

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Energetic particle loss mechanisms in reactor-scale equilibria close to quasisymmetry

et al. 2022

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Collisionless physics primarily determines the transport of fusion-born alpha particles in 3D equilibria. Several transport mechanisms have been implicated in stellarator configurations, including stochastic diffusion due to class transitions, ripple trapping, and banana drift-convective orbits. Given the guiding center dynamics in a set of six quasihelical and quasiaxisymmetric equilibria, we perform a classification of trapping states and transport mechanisms. In addition to banana drift convection and ripple transport, we observe substantial non-conservation of the parallel adiabatic invariant which can cause losses through diffusive banana tip motion. Furthermore, many lost trajectories undergo transitions between trapping classes on longer time scales, either with periodic or irregular behavior. We discuss possible optimization strategies for each of the relevant transport mechanisms. We perform a comparison between fast ion losses and metrics for the prevalence of mechanisms such as banana-drift convection, transitioning orbits, and wide orbit widths. Quasihelical configurations are found to have natural protection against ripple-trapping and diffusive banana tip motion leading to a reduction in prompt losses.

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Section: Drift-convective Transportmentioning

confidence: 77%

“…The Γ c metric averages |γ c | 2 over velocity space [21,33] in order to quantify the misalignment of flux surfaces and J surfaces. Optimization including Γ c has successfully improved confinement in QH [54] and QA [72] configurations. However, the existence of DC orbits does not always indicate prompt losses.…”

Section: Drift-convective Transportmentioning

confidence: 99%

Energetic particle loss mechanisms in reactor-scale equilibria close to quasisymmetry

et al. 2022

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“…2021; LeViness et al. 2023), classifiers presented here are based on real orbits retaining their actual radial widths which can be compatible with plasma radius (see Albert et al. 2022; Landreman & Paul 2022) and, more importantly, bounce phases.…”

Section: Discussionmentioning

confidence: 94%

“…In contrast to alpha particle confinement metrics based on the bounce-averaged motion (Gori et al 1996;Nemov et al 2005;Velasco et al 2021;LeViness et al 2023), classifiers presented here are based on real orbits retaining their actual radial widths which can be compatible with plasma radius (see Albert et al 2022;) and, more importantly, bounce phases. Therefore, they naturally take into account drift-orbit resonances which may lead to chaotization of the orbits and respective additional losses.…”

Section: Discussionmentioning

confidence: 94%

Alpha particle confinement metrics based on orbit classification in stellarators

et al. 2023

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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.

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“…2021; LeViness et al. 2022; Paul et al. 2022), but relatively little has been studied with respect to AE-driven transport.…”

Section: Introductionmentioning

confidence: 99%

Fast-ion transport in quasisymmetric equilibria in the presence of a resonant Alfvénic perturbation

Paul,

Mynick,

Bhattacharjee

2023

J. Plasma Phys.

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Significant progress has been made in designing magnetic fields that provide excellent confinement of the guiding-centre trajectories of alpha particles using quasisymmetry (QS). Given the reduction in this transport channel, we assess the impact of resonant Alfvén eigenmodes (AEs) on the guiding-centre motion. The AE amplitudes are chosen to be consistent with experimental measurements and large-scale simulations. We evaluate the drift resonance condition, phase-space island width and island overlap criterion for quasisymmetric configurations. Kinetic Poincaré plots elucidate features of the transport, including stiff transport above a critical perturbation amplitude. Our analysis highlights key departures from the AE-driven transport in tokamaks, such as the avoidance of phase-space island overlap in quasihelical configurations and the enhanced transport due to wide phase-space islands in low magnetic shear configurations. In configurations that are closer to QS, with QS deviations $\delta B/B_0 \lesssim 10^{-3}$ , the transport is primarily driven by the AE, while configurations that are further from QS, $\delta B/B_0 \sim 10^{-2}$ , experience significant transport due to the QS-breaking fields in addition to the AE.

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Energetic particle optimization of quasi-axisymmetric stellarator equilibria

Cited by 6 publications

References 32 publications

Energetic particle loss mechanisms in reactor-scale equilibria close to quasisymmetry

Energetic particle loss mechanisms in reactor-scale equilibria close to quasisymmetry

Alpha particle confinement metrics based on orbit classification in stellarators

Fast-ion transport in quasisymmetric equilibria in the presence of a resonant Alfvénic perturbation

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