Direct measurements of residual stress (force) have been executed at the edge of the TEXTOR tokamak using multitip Langmuir and Mach probes, together with counter-current NBI torque to balance the existing toroidal rotation. Substantial residual stress and force have been observed at the plasma boundary, confirming the existence of a finite residual stress as possible mechanisms to drive the intrinsic toroidal rotation. In low-density discharges, the residual stress displays a quasi-linear dependence on the local pressure gradient, consistent with theoretical predictions. At high-density shots the residual stress and torque are strongly suppressed. The results show close correlation between the residual stress and the Er × B flow shear rate, suggesting a minimum threshold of the E × B flow shear required for the k∥ symmetry breaking. These findings provide the first experimental evidence of the role of Er × B sheared flows in the development of residual stresses and intrinsic rotation.
Global flux-driven gyrofluid simulations of internal transport barrier formation by external torque in weak magnetic shear configuration