Description
Transport barriers in magnetically confined plasmas significantly improve confinement by reducing energy and particle transport through resonant interactions between drift orbits and non-axisymmetric perturbative modes. The kinetic-$q$ factor ($q_{kin}$), defined as the ratio of bounce/transit-averaged toroidal precession to poloidal frequency of the guiding center motion [1], determines both resonance conditions and transport barrier locations which occur at its local extrema, where the condition for zero kinetic shear is satisfied [2]. Radial electric fields drastically modify $q_{kin}$, enabling or suppressing specific resonances and creating transport barriers through elimination of kinetic shear [3], with direct implications for H-mode operation and edge transport barrier dynamics [4].
In this work, we develop a comprehensive 3D mapping of $q_{kin}$ extrema across the constants of motion (COM) space ($\mu, E, P_\zeta$) in axisymmetric toroidal equilibria, identifying shearless positions corresponding to persistent transport barriers. This approach provides, for the first time, an a priori predictive map of transport barrier locations that is independent of the applied perturbative modes. The mapping reveals a comprehensive transport barrier landscape across different energies and pitch angles, whose topology is strongly modulated by the radial electric field profile, explaining the selective confinement of particles with specific kinetic characteristics, by suppressing radial stochastic transport, even under overlapping resonance conditions. This framework paves the way for predictive optimization of fusion plasma confinement through tailored design of equilibrium and electric field configurations in present and future devices.
References
[1] Y. Antonenas, G. Anastassiou, Y. Kominis, Phys. of Plasmas 31, 102302 (2024)
[2] I. L. Caldas et al., Plasma Phys. Control. Fusion 54, 124035 (2012)
[3] G. Anastassiou, P. Zestanakis, Y. Antonenas, E. Viezzer and Y. Kominis, J. Plasma Phys. 90, 905900110 (2024)
[4] G. J. Kramer et al., Nucl. Fusion 64, 106035 (2024)