Effect of the externally driven velocity shear on the turbulence radial wavenumber spectral shift and its amplitude

It has been observed that the plasma velocity shear can be strongly affected by external source input, such as neutral beam injection, lower hybrid current drive, supersonic molecule beam injection, and impurity injection. Such a change in velocity shear can lead to turbulence suppression or enhancement by inducing a spectral shift in the turbulence radial wavenumber spectrum. A theoretical model, based on the nonlinear evolution of the turbulence amplitude with an externally driven velocity shear, combined with the thermal transport equation, has been used to describe the turbulence evolution. A critical growth rate γ₀ for the turbulence regulation has been identified in this theoretical model, and the parametric dependency of gamma(0) is investigated. A threshold U_c of the reduction value of the velocity shear rate U, intimately related to γ₀, has been found for the triggering radial wavenumber shift. This indicates that the turbulence could be regulated by external source input, when U > U_c. (authors)