Effect of a large-gradient radial electric field on plasma current in a spherical torus

摘要

A description of the guiding field line (GFL) model is presented for spherical torus geometry. The model is extended to cover small banana orbits that are trapped either above or below the midplane. Comparison of the model with the fluid and neoclassical theories demonstrates correctness of the model, though it shows that the model is not exactly equivalent to them. The GFL model provides a more general description for the orbit-squeezing effect resulting from the large gradient of the radial electric field. It shows that not only does the deformation of banana orbits, but also the change of the passing particles' orbits, affect the current in plasma. Besides, it appears that the change of the orbits' shape, as projected on the magnetic surface, contributes to modification of the current along with the radial squeezing/expanding. Distortion of the current by the gradient of the electric field is found to be predominantly in the direction parallel to the magnetic field, while almost no effect is seen for the perpendicular component.