Relativistic guiding-center dynamics in the presence of strong flows

A Lagrangian approach to relativistic particle guiding-center dynamics in strongly magnetized plasmas, based on a covariant formulation, is presented, proving that a four-dimensional transformation is required in order to define the correct relativistic gyrophase-averaged Lagrangian. Comparisons with previous approaches show that these become potentially inconsistent, particularly for magnetoplasmas exhibiting relativistic mass-flow velocities and/or deeply relativistic particles, causing the paradox of hyper-relativistic guiding-center velocities. To avoid this inconvenience, the relativistic velocity addition law is adopted to describe the particle motion with respect to the reference frame locally at rest with the fluid. Based on the adoption of the relativistic Hamilton principle, new expressions are determined for the relativistic guiding-center velocity transformation, the relativistic magnetic moment and the relativistic gyrokinetic equations of motion.