Abstract Details
Abstracts
Author: Thomas Foster
Requested Type: Poster
Submitted: 2026-03-20 10:21:39
Co-authors: F.I.Parra
Contact Info:
Princeton University
Princeton University
Princeton, 08544
USA
Abstract Text:
Recent simulations [1] have shown that trapped alpha-particle orbits in certain quasi-axisymmetric (QA) stellarators contain wide resonance islands, which are likely to degrade alpha-particle confinement. To minimize alpha losses when designing a QA stellarator, it may be prudent to eliminate these "drift-orbit resonances" (also called "bounce-harmonic" resonances) or to minimize their island width via optimization. While such optimization could leverage large simulations or machine learning, an accurate analytical formula for the island shape would offer a faster and more physically informative approach. In this work, we calculate an analytical formula for these resonant orbits by deriving an invariant that they conserve. From a theoretical perspective, drift-orbit resonance islands are intriguing because they indicate non-conservation of the second adiabatic invariant J, even when higher-order corrections to J are included. We suggest a physical mechanism for these drift-orbit resonances that allows them to break J conservation: in QA stellarators with small rotational transform (iota) and small inverse aspect ratio (epsilon), trapped alphas can have such long bounce periods that their bounce points drift through an order-unity toroidal angle in a single bounce. By calculating trapped orbits in a nearly-QA stellarator with iota << 1, we show that, once iota becomes small enough to satisfy sqrt(epsilon) iota^2 ~ rho_star (rho_star is the normalized alpha gyroradius), large drift-orbit resonance islands become possible. This calculation generalizes the [2] mechanism from tokamaks with high-n ripple (n is toroidal mode number) to arbitrary nearly-QA stellarators with n ~ 1 QA-breaking modes.
[1] A. Chambliss, et al., J. Plasma Phys. 91, E74 (2025)
[2] R. J. Goldston, et al., Phys. Rev. Lett. 47, 647 (1981)
This work is supported by US-DOE Contract DE-AC02-09CH11466
Characterization: 1.0
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