Description
Magnetic islands induced by the tearing instability in tokamaks can significantly change the magnetic topology, degrade plasma confinement, and affect particle transport [1]. However, the microscopic particle dynamics and energy transfer mechanisms in the presence of magnetic islands remain to be fully elucidated. In this work, we investigate the effect of magnetic islands on particle orbits and acceleration using the global gyrokinetic Particle-in-Cell (PIC) code ORB5 with a test-particle module [2]. Both electrons and ions are tracked as passive test particles with a uniform pitch angle distribution, characterizing the kinetic features of both trapped and passing particles. First, the mode evolution is examined with respect to different mass ratios ($m_i/m_e$, ranging from 100 to the realistic value of 1836), and the results are found to be in agreement with theoretical scaling laws. The dynamically evolving magnetic islands serve as a time-dependent background for the particle tracking. The magnetic island structures are visualized using Poincare plots. The relative phase between particles and islands is tracked and possible resonances are identified. The structures of densities, temperatures and electric fields inside and around the magnetic islands are also examined. Furthermore, particle acceleration near the reconnection X-points is investigated for both linearly unstable and turbulence-driven [3] islands. The contributions of different acceleration mechanisms, particularly the work done by the parallel electric field, are discussed.
References
[1] Freidberg, Jeffrey P. Plasma physics and fusion energy. Cambridge university press, 2008.
[2] Lanti, Emmanuel, et al, Computer Physics Communications 251 (2020): 107072.
[3] Widmer, Fabien, et al, Physics of Plasmas 31 (2024): 112505.