Classification of particle orbits in high-beta spherical tokamaks

Orbits of charged particles in magnetic configurations with high beta (rati o of the plasma pressure to the pressure of the magnetic field) and strong elongation typical for modern spherical tokamaks are analyzed. Using a mode l representation of the magnetic field, the dependence of the topology and the shape of particle orbits on constants of motion (the magnetic moment an d the canonical angular momentum of a particle) is systematically studied. The resulting classification of particle orbits takes into account the poss ible existence of the so-called "magnetic valley" (local minimum of the mag netic 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 orbi ts, including banana and passing ones, have unusual shape with the maximum and/or minimum radial coordinate reached outside the equatorial plane of th e torus. The boundaries between the regions of orbits of usual and unusual shape on the plane of constants of motions are delineated. (C) 2001 America n Institute of Physics.