Larmor radius effect on the control of chaotic transport in tokamaks

TL;DR

This study explores how the finite Larmor radius influences chaotic transport in tokamaks, revealing that larger radii strengthen transport barriers and reduce chaos, which could improve plasma confinement.

Contribution

Introduces a second-order gyro-averaged model to analyze the Larmor radius effect on chaotic transport and transport barrier robustness in tokamaks.

Findings

Larger Larmor radius enhances transport barrier robustness.

Increasing Larmor radius raises the amplitude needed to break barriers.

Escape rates decrease with larger Larmor radius, reducing chaos.

Abstract

We investigate the influence of the finite Larmor radius on the dynamics of guiding-center test particles subjected to an $\mathbf{E} \times \mathbf{B}$ drift in a large aspect-ratio tokamak. For that, we adopt the drift-wave test particle transport model presented by W. Horton [Physics of Plasmas \textbf{5}, 3910 (1998)] and introduce a second-order gyro-averaged extension, which accounts for the finite Larmor radius effect that arises from a spatially varying electric field. Using this extended model, we numerically examine the influence of the finite Larmor radius on chaotic transport and the formation of transport barriers. For non-monotonic plasma profiles, we show that the twist condition of the dynamical system, i.e.,\ KAM theorem's non-degeneracy condition for the Hamiltonian, is violated along a special curve, which, under non-equilibrium conditions, exhibits significant resilience to destruction, thereby inhibiting chaotic transport. This curve acts as a robust barrier to transport and is usually called shearless transport barrier. While varying the amplitude of the electrostatic perturbations, we analyze bifurcation diagrams of the shearless barriers and escape rates of orbits to explore the impact of the finite Larmor radius on controlling chaotic transport. Our findings show that increasing the Larmor radius enhances the robustness of transport barriers, as larger electrostatic perturbation amplitudes are required to disrupt them. Additionally, as the Larmor radius increases, even in the absence of transport barriers, we observe a reduction in the escape rates, indicating a decrease in chaotic transport.