  The effect of momentum injection on the temperature gradient in tokamak
plasmas is studied. A plausible scenario for transitions to reduced transport
regimes is proposed. The transition happens when there is sufficient momentum
input so that the velocity shear can suppress or reduce the turbulence.
However, it is possible to drive too much velocity shear and rekindle the
turbulent transport. The optimal level of momentum injection is determined. The
reduction in transport is maximized in the regions of low or zero magnetic
shear.
