ENERGETIC PARTICLE ORBITS IN THE NATIONAL-SPHERICAL-TOKAMAK-EXPERIMENT

The trajectories of neutral beam injected energetic ions in spherical tokamaks are examined. The large poloidal magnetic field in the outboa rd region of spherical tokamaks causes neutral beam injected ions to b e born on trapped orbits even with cotangential injection. Numerical s olutions to the equations for particle motion and for guiding center d rifts are compared in several magnetic equilibria for a range of parti cle initial conditions. Even when r(L)/a similar to 1/4 the guiding ce nter orbits closely resemble the path of the instantaneous center of g yration of the particle motion; exceptions occur primarily for orbits near the trapped/passing boundary. Finite Larmor radius effects are in cluded in guiding center simulations of prompt orbit loss in the Natio nal Spherical Tokamak Experiment (NSTX) [J. Spitzer et al., Fusion Tec hnol. 30, 1337 (1996)]. Orbit loss in the NSTX is caused primarily by collisions with the close fitting conducting shell, and severe losses would be expected with counter directed injection. While most orbits a re similar to those found in conventional tokamaks, additional orbit t ypes are possible in spherical tokamaks. (C) 1997 American Institute o f Physics.