Speaker
Description
The presence of non-axisymmetric perturbations of an axisymmetric toroidal magnetic field results in the chaoticity of the magnetic field lines and strongly affects the charged particle motion and therefore the particle, energy and momentum transport in toroidal plasmas [1-2]. Particle chaoticity is determined by resonance conditions relating the unperturbed Orbital Frequencies of the particles with the toroidal and poloidal numbers of the perturbative modes [3]. The Guiding Center (GC) motion [4] of low-energy particles approximately follows the magnetic field lines so that magnetic and kinetic chaos have similar characteristics. However, higher-energy particles may undergo large drifts across the magnetic field lines and the chaoticity characteristics of their GC motion can be quite different from those of the underlying magnetic field. In fact, kinetic chaos may take place in different spatial locations from those of magnetic chaos, and the degree as well as the extent of the two chaoticities can be different depending on the particle orbit width. The systematic comparison of magnetic and kinetic chaos necessitates: (a) the efficient detection and quantification of chaos, and (b) the compact representation of particle orbits in a kinetic parameter space.
In this work, we introduce the Smaller Alignment Index (SALI) [5-6], utilized for the first time in the context of plasma physics, as an efficient measure for detecting and quantifying magnetic and kinetic chaos, and discuss its advantages in comparison with other standard chaos measures. Magnetic and kinetic chaos are visually compared in terms of appropriate Poincare surfaces of section for characteristic cases of low-energy, thermal, and higher-energy particles. Moreover, we construct detailed diagrams by assigning a SALI value to every point in the kinetic parameter space uniquely representing a particle orbit in terms of three Constants of the Motion, namely the energy, the canonical toroidal momentum and the magnetic moment. These diagrams contain aggregated information for the role of various non-axisymmetric perturbations on the chaoticity of specific particles characterized by being trapped or passing, and confined or lost, providing a valuable tool for understanding the interplay between perturbations and for transport prediction and control.
This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.
References:
[1] A. Rechester and M. Rosenbluth, Phys. Rev. Lett 40, 38 (1978)
[2] K. Shinohara, et al., Nucl. Fusion 58, 082026 (2018)
[3] Y. Antonenas, G. Anastassiou and Y. Kominis, J. Plasma Phys. 87, 855870101 (2021)
[4] R. B. White and M. S. Chance, Phys. Fluids 27, 2455–2467 (1984)
[5] Ch. Skokos, J. Phys. A 34, 10029-10043 (2001)
[6] Ch. Skokos, Lect. Notes Phys. 790, 63-135 (2010)
