Magnetically confined charged particles: From steep density profiles to the breaking of the adiabatic invariant

TL;DR

This paper investigates the stability of magnetically confined plasma equilibria, revealing that under certain conditions, chaotic particle dynamics can break adiabatic invariants, potentially impacting plasma confinement models.

Contribution

It introduces a perturbative approach to analyze plasma equilibria and demonstrates the emergence of chaos affecting magnetic moment conservation in burning plasmas.

Findings

Equilibria align with inviscid MHD predictions

Chaotic dynamics emerge at high energies

Potential impact on gyrokinetic modeling

Abstract

This study examines the stability of Vlasov equilibrium solutions for magnetically confined plasmas, derived through the principle of maximum entropy. By treating the toroidal limit as a perturbation from an analytical cylindrical solution, we demonstrate that these equilibria align well with the inviscid magnetohydrodynamic (MHD) description. Using the aspect ratio as a perturbation parameter, we compute particle trajectories sampled from the kinetic equilibrium distribution, confirming the overall stability of the solutions. However, under burning plasma conditions, chaotic dynamics emerge for particles with supra-thermal and even thermal energies. This destroys the adiabatic invariance of the magnetic moment. The exact consequences are unclear, but they could undermine the foundational assumptions of gyrokinetic modelling in burning plasmas. Nevertheless, these results suggest the possibility of unaccounted transport losses in future burning plasma operations. The interplay between turbulence and energetic particles in the presence of Hamiltonian chaos certainly warrants further investigation.