It is well known that electrostatic fields in tokamaks can vary on relatively short scale lengths, approaching the ion banana width. The resulting "squeezed" ion orbits are associated with significantly reduced neoclassical transport. It is shown here that an analogous process occurs, for steeper field variation, at the level of particle gyration: potentials varying on a scale comparable to the ion gyro-radius distort gyro-orbits and thus modify classical transport. The gyro-distortion can take one of three forms, depending upon the sign and size of the electric field shear; reduction in orbit width occurs only in a potential well. In this case, and assuming that the ion density and temperature vary slowly on the scale of the shrunken orbit, the classical ion heat flux is computed. It is shown that this flux is reduced by a factor of S-2, where S ≡ 1 + cΦ"(r)/BΩ. The sharp potential variation required for large S might result from steep electron temperature gradients near the separatrix of a spherical tokamak (with comparable poloidal and toroidal field components) or reversed field pinch.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics