Effects of orbit squeezing on ion transport in the banana regime in tokamaks

Abstract

It is shown that ion transport in the banana regime in tokamaks is reduced in the presence of a strong shear in the radial electric field Er , as is often observed in the edge region. For simplicity, the ordering with ρpi‖d ln Er /dr‖ ≫ 1 but c‖Er‖/Bpvti < 1 is adopted. Here, ρpi is the ion poloidal gyroradius, Bp is the poloidal magnetic field strength, vti is the ion thermal speed, and c is the speed of light. A kinetic transport theory similar to those for bumpy tori and stellarators is developed to show that the ion thermal conductivity χi is reduced by a factor of roughly S−3/2, where S = 1 − (ρpi d ln Er /dr)(cEr Bpvti). The result reflects more than simple orbit squeezing: The fraction of trapped ions is also modified by S.