Ion-temperature-gradient-driven turbulence and transport in a sheared magnetic field

An analytical and numerical investigation has been completed of the nonlinear growth and saturation of long-wavelength ion-temperature-gradient-driven turbulence in a sheared magnetic field for the case of an isolated rational surface. The radial correlation length of the turbulence is found to be of order ρiLs/ Lt, with ρi the ion Larmor radius, Ls the magnetic shear length, and LT the temperature scale length. The scaling of the resulting anomalous cross-field diffusivity is χ⊥∝g(ηi)(ρ2i/LTLy) (Ls/LT)2ρivti, where g(ηi) is obtained from the numerical results. In the poloidal direction, the spectrum collapses to the longest wavelength, Ly, available. Explicit results for the function g(ηi) are presented for values of ηi ranging from just above marginal stability ηic to ηi=∞. Although the quasilinear temperature profile is held fixed, the transport rates are very small as a result of local flattening of the temperature profile near the rational surface. The implications of these results for understanding anomalous transport in tokamaks are discussed.