Multi-scale Nonlinear Turbulence Dynamics Studies

Critical gradient transport dynamics, breaking of gyro-Bohm transport scaling, transport bifurcation, and transport barrier formation all have a profound effect on confinement in magnetic fusion devices, and are thought to be both generated and regulated by the nonlinear interaction between gradient-driven small scaled ( k!"s <1) drift turbulence and larger meso-scaled structures such as zonal flows and streamers with k!"s << 1 [1, 2] that are generated from the turbulence. Zonal flow formation from turbulence, regulation of the turbulence by the zonal flow, effects of flow damping rate on turbulence saturation, and introduction of device-scaled transport events due to the breakdown of zonal flows and formation of propagating structures are the key nonlinear physics processes that lead to these macroscopic confinement properties. Direct study of the underlying physics requires simultaneous spatiotemporal turbulence measurements that cannot be made in existing large confinement devices. Suitably designed and diagnosed smaller laboratory experiments such as the TJ-K toroidal device and the CSDX, VINETA, and LMD linear devices have recently provided direct experimental study of these issues, and thus provide a detailed stress test of theory and simulation.