Observation of Coherent Sheared Turbulence Flows in the DIII-D Tokamak

Time-resolved measurements of the turbulent density flow field in a tokamak plasma reveal low-frequency ($\sim 15\mathrm{K}\mathrm{H}\mathrm{z}$), coherent oscillations in the poloidal flow, ${\tilde{v}}_{\theta }$. These flow oscillations have a long poloidal wavelength ($m<3$) and narrow radial extent ($k_r{\rho }_i\sim 0.2$). The estimated flow-shearing rate is of the same order of magnitude as the turbulence decorrelation rate and may thus regulate the turbulence amplitude. These features are consistent with theoretically predicted axisymmetric, self-regulating, sheared flows recognized as geodesic acoustic modes.