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

摘要

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 E(sub r), as is often observed in the edge region. For simplicity, the ordering with (rho)(sub pi)(vert bar)d ln E(sub r) /dr(vert bar) (much >) 1 but c(vert bar)E(sub r)(vert bar)/B(sub p)(upsilon)(sub ti) < 1 is adopted. Here, (rho)(sub pi) is the ion poloidal gyroradius, B(sub p) is the poloidal magnetic field strength, (upsilon)(sub ti) 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 (chi)(sub i) is reduced by a factor of roughly S(sup (minus)3/2), where S = 1 (minus) ((rho)(sub pi)d ln E(sub r)/dr)(cE(sub r)/B(sub (rho))(upsilon)(sub ti)). The result reflects more than simple orbit squeezing: the fraction of trapped ions is also modified by S.