Evolution of the Turbulence Radial Wavenumber Spectrum near the L-H Transition in NSTX Ohmic Discharges

Full text: The measurement of radially extended meso-scale structures such as zonal flows and streamers, as well as the underlying microinstabilities driving them, is critical for understanding turbulence-driven transport in plasma devices. In particular, the shape and evolution of the radial wavenumber spectrum indicate details of the nonlinear spectral energy transfer, the spreading of turbulence, as well as the formation of transport barriers. In the National Spherical Torus Experiment (NSTX), the FMCW backscattering diagnostic is used to probe the turbulence radial wavenumber spectrum (kr = 0 - 22 cm-1 ) across the outboard minor radius near the L- to H-mode transition in Ohmic discharges. During the L-mode phase, a broad spectral component (kr ∼ 2 - 10 cm-1 ) extends over a significant portion of the edge-core from R = 120 to 155 cm (ρ = 0.4 - 0.95). At the L-H transition, turbulence is quenched across the measurable kr range at the ETB location, where the radial correlation length drops from ∼ 1.5 - 0.5 cm. The kr spectrum away from the ETB location is modified on a time scale of tens of microseconds, indicating that nonlocal turbulence dynamics are playing a strong role. Close to the L-H transition, oscillations in the density gradient and edge turbulence quenching become highly correlated. These oscillations are also present in Ohmic discharges without an L-H transition, but are far less frequent. Similar behavior is also seen near the L-H transition in NB-heated discharges. (author)