EFFECTS OF ORBIT DISTORTION ON CLASSICAL TRANSPORT

It is well known that electrostatic fields in tokamaks can vary on rel atively short scale lengths, approaching the ion banana width. The res ulting ''squeezed'' ion orbits are associated with significantly reduc ed 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 dist ort gyro-orbits and thus modify classical transport. The gyro-distorti on can take one of three forms, depending upon the sign and size of th e electric field shear; reduction in orbit width occurs only in a pote ntial well. In this case, and assuming that the ion density and temper ature vary slowly on the scale of the shrunken orbit, the classical io n heat flux is computed. It is shown that this flux is reduced by a fa ctor of S-2, where S=1 + c Phi ''(r)/B Omega. The sharp potential vari ation required for large S might result from steep electron temperatur e gradients near the separatrix of a spherical tokamak (with comparabl e poloidal and toroidal field components) or reversed field pinch. (C) 1998 American Institute of Physics. [S1070-664X(98)02810-9].