Rotation shear induced fluctuation decorrelation in a toroidal plasma

The enhanced decorrelation of fluctuations by the combined effects of the E×B flow (V_E) shear, the parallel flow (V_∥) shear, and the magnetic shear is studied in toroidal geometry. A two‐point nonlinear analysis previously utilized in a cylindrical model [Phys. Fluids B 2, 1 (1990)] shows that the reduction of the radial correlation length below its ambient turbulence value (Δr₀) is characterized by the ratio between the shearing rate ω_s and the ambient turbulence scattering rate Δω_T. The derived shearing rate is given by ω²_s= (Δr₀)²[(1/Δφ²){(∂/∂r)(qV_E/r)}² +(1/Δη²){(∂/∂r)(V_∥/qR)}²], where Δφ and Δη are the correlation angles of the ambient turbulence along the toroidal and parallel directions. This result deviates significantly from the cylindrical result for high magnetic shear or for ballooning‐like fluctuations. For suppression of flute‐like fluctuations, only the radial shear of qV_E/r contributes, and the radial shear of V_∥/qR is irrelevant regardless of the plasma rotation direction.