The nonlinear evolution and spectral properties of ion temperature gradient-driven modes in a three-dimensional sheared magnetic field with multiple rational surfaces have been investigated using gyrokinetic plasma simulation techniques. A detailed study of the electrostatic potential fluctuation spectra shows that it is dominated by the modes with moderate poloidal wavelength (0.1≤${\mathit{k}}_{\mathit{y}}$${\mathrm{\rho }}_{\mathit{s}}$≤0.5). The measured thermal flux in the steady state, which is lower than the quasilinear value at saturation, is found to decrease weakly with an increase in the magnetic shear strength.

• Received 21 November 1988

DOI:https://doi.org/10.1103/PhysRevLett.64.2015