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Investigation of Turbulence-Driven Shear Flows in Toroidal and Cylindrical Plasmas


The turbulence-driven shear flows in both transverse and parallel direction have been investigated in a tokamak (HL-2A) and a linear device (CSDX). The first part of this study is devoted to the evolution of zonal flows and their turbulent drive near Greenwald limit in HL-2A tokamak. As the normalized line-averaged density $ \bar{n}_e/n_G $ is raised, the shearing rate of the poloidal $ {\bf E \times B} $ and turbulent phase velocities drop. Meanwhile, the turbulent drive for the low-frequency zonal flow (the Reynolds power) collapses at higher $ \bar{n}_e/n_G $ values. Correspondingly, turbulent particle transport increases in high collisionality plasmas. Besides damping the edge shear flows through increased collisional dissipation, the increased collisionality can also be connected to an increased non-adiabatic electron response at high densities, which raises the cross-correlation between density and potential perturbations. The second part is focused on the generation of intrinsic parallel flow in a linear magnetized device--CSDX. The axial flow shearing rate $V_{z}^{\prime}$ and the axial Reynolds power $\mathcal{P}_{z}^{Re}$ track the rising density gradient as the B field is raised. The axial Reynolds stress is the dominant source driving the axial flow as the density gradient grows during the magnetic field scan. The joint probability density function of radial and axial velocity fluctuations, $\mathsf{P}\left(\tilde{v}_{r},\tilde{v}_{z}\right)$, is highly tilted and anisotropic at higher magnetic field, indicating enhanced imbalance of the spectral correlator $\langle k_{\theta}k_{z} \rangle $ which can give rise to nonzero residual stress according to the dynamical symmetry breaking model. Furthermore, the axial velocity shear $V_{z}^{\prime}$ and its turbulent drive $\mathcal{P}_{z}^{Re}$ rise with increasing azimuthal velocity shear $V_{\theta}^{\prime}$ when $B<800$ G; after 800 G, $V_{\theta}^{\prime}$ and its turbulent drive collapse. The direct energy exchange between axial and azimuthal flows is low, since the $ \mathcal{P}_{z}^{Re} $ is about 10 times smaller than $ \mathcal{P}_{\theta}^{Re} $.

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