Abstract Details
Abstracts
Author: Thomas E. Foster
Requested Type: Poster
Submitted: 2024-04-11 19:44:53
Co-authors: F.I.Parra, R.B.White
Contact Info:
Princeton University
10 Lawrence Dr, Apt 504
Princeton, NJ 08540
USA
Abstract Text:
Recent simulations [1, 2] have shown that, even when the equilibrium magnetic field of a stellarator possesses intact flux surfaces, the orbits of energetic particles can exhibit islands around rational surfaces. These 'drift islands' may be a source of enhanced, resonant transport near rational surfaces [3], flattening the alpha density profile. In this work, we calculate the resonant orbits of energetic particles near a low-order rational flux surface in a general stellarator. We show that, within a narrow region around the rational surface, the orbits of passing and semi-trapped particles (trapped particles that complete multiple toroidal transits before bouncing) are determined by conservation of a new adiabatic invariant associated with the closed rational-surface field lines. We derive higher-order corrections to this adiabatic invariant to ensure accurate results even for energetic particles. The island width scales as sqrt(rhostar*delta/s) ('rhostar' is the energetic-particle gyroradius divided by a typical length scale of the field, 'delta' is the deviation from omnigeneity, and 's' is the magnetic shear), so drift islands could be large in stellarators with low shear that are insufficiently omnigeneous. We find that semi-trapped particles experience no net radial drift through a rational surface in a stellarator-symmetric device, even if it is poorly omnigeneous; therefore, stellarator symmetry may be beneficial for particle confinement.
[1] R.B.White et al, Phys. Plasmas 29, 5 (2022).
[2] R.B.White, Phys. Plasmas 29, 9 (2022).
[3] E.J.Paul et al, Nucl. Fusion 62, 12 (2022).
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