Abstract Details
Abstracts
Author: Thomas E. Foster
Requested Type: Consider for Invited
Submitted: 2025-02-16 16:24:54
Co-authors: F.I.Parra, R.B.White, J.L.Velasco
Contact Info:
Princeton University
10 Lawrence Dr, Apt 504
Princeton, 08540
USA
Abstract Text:
In modern stellarator designs, confinement of the bounce-averaged drift motion of fusion-born alpha particles is achieved by optimising the magnetic field for omnigeneity. Recently, concerns have been raised that additional optimisation may be necessary for adequate confinement of certain classes of alpha particles with unusual orbits. One such class is particles near rational flux surfaces. Simulations have shown that passing alpha particles near rational surfaces can have orbits containing islands, even when the magnetic field possesses nested toroidal flux surfaces [1,2]. Meanwhile, trapped alphas undergoing resonant motion near a rational surface can exhibit poor conservation of the second adiabatic invariant [3]. In this talk, we give a theoretical account of these unusual orbits by deriving simple, analytical expressions for the orbits of energetic particles near rational flux surfaces in a general stellarator. These orbits are determined by conservation of a ‘transit adiabatic invariant’ associated with the closed rational-surface field lines. To ensure accurate results even for energetic particles, we compute higher-order corrections to the transit adiabatic invariant; the resulting theory agrees extremely well with simulations conducted using the ASCOT code. We find that semi-trapped particles – trapped particles that undergo multiple toroidal transits before bouncing – do not conserve the second adiabatic invariant but do conserve the transit adiabatic invariant, which allows their orbits to be described analytically. Finally, we discuss how wide orbits, such as the islands around rational surfaces or trapped-particle orbits in an insufficiently optimised device, lead to enhanced collisional transport of alpha particles.
[1] R.White, A.Bierwage and S.Ethier, Phys. Plasmas 29, 052511 (2022)
[2] R.White, Phys. Plasmas 29, 092504 (2022)
[3] E.J.Paul, A.Bhattacharjee, M.Landreman, D.Alex, J.L.Velasco and R.Nies, Nucl. Fusion 62, 126054 (2022)
Characterization: 1.0
Comments: