The interplay of an external torque and E×B structure formation in tokamak plasmas

The interplay between an external torque and spontaneously occurring mesoscale structures, known as staircases, is investigated. Gyrokinetic simulations show that the E × B shear connected with the external torque does not simply add to the shear of the mesoscale structures. A positive (negative) externally forced E × B shear leads to a broadening of the positive (negative) region of the staircase but does not significantly change the plateau value or the narrow zero shear layer. In consequence, while the space and time averaged shearing rate is enhanced by the external torque, there is little or no effect on the turbulent transport. This raises doubts about the importance of driven or intrinsic rotation as a means to improve plasma confinement close to the stability threshold.The interplay between an external torque and spontaneously occurring mesoscale structures, known as staircases, is investigated. Gyrokinetic simulations show that the E × B shear connected with the external torque does not simply add to the shear of the mesoscale structures. A positive (negative) externally forced E × B shear leads to a broadening of the positive (negative) region of the staircase but does not significantly change the plateau value or the narrow zero shear layer. In consequence, while the space and time averaged shearing rate is enhanced by the external torque, there is little or no effect on the turbulent transport. This raises doubts about the importance of driven or intrinsic rotation as a means to improve plasma confinement close to the stability threshold.