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

This study examines the stability of Vlasov equilibrium solutions for magnetically confined plasmas, derived using the maximum entropy principle. We start from a non-resistive plasma in cylindrical geometry and demonstrate that these kinetic equilibria match their magnetohydrodynamic (MHD) counterpart in the limit of vanishing viscosity. By treating the toroidal limit as a perturbation, and using the aspect ratio as a perturbation parameter, we compute particle trajectories sampled from the kinetic equilibrium distribution. This confirms the overall stability of the solutions. However, we demonstrate that chaotic dynamics emerge for particles with both supra-thermal and thermal energies, under typical burning plasma conditions, which destroys the adiabatic invariance of the magnetic moment. The invalidity of this crucial hypothesis for gyrokinetic modelling of burning plasmas in an ideal setting warrants further investigation, while the full consequences remain to be determined.