Classification of particle orbits in high-β spherical tokamaks

Orbits of charged particles in magnetic configurations with high β (ratio of the plasma pressure to the pressure of the magnetic field) and strong elongation typical for modern spherical tokamaks are analyzed. Using a model representation of the magnetic field, the dependence of the topology and the shape of particle orbits on constants of motion (the magnetic moment and the canonical angular momentum of a particle) is systematically studied. The resulting classification of particle orbits takes into account the possible existence of the so-called “magnetic valley” (local minimum of the magnetic field, B) and the nonmonotonic dependence of B on the poloidal angle. Special attention is paid to the orbit types that do not exist in standard tokamaks (“tear drops,” “dumb-bells”). It is found that some particle orbits, including banana and passing ones, have unusual shape with the maximum and/or minimum radial coordinate reached outside the equatorial plane of the torus. The boundaries between the regions of o...