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

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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.