Classification of particle orbits in high-β spherical tokamaks

Orbits of charged particles in magnetic configurations with high beta (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 orbits of usual and unusual shape on the plane of constants of motions are delineated. (C) 2001 American Institute of Physics.