L-H Transition and Turbulence Behavior in Ohmic H-modes on NSTX

The L-H transition and H-mode behavior and turbulence have been studied in NSTX ohmically heated (OH) plasmas, which offer the advantage of an absence of the NBI sources of particles and momentum. The OH H-modes have density profiles that are peaked in the core, rather than the edge as in NBI-heated H-modes, making the core accessible for turbulence studies using correlation reflectometry. A decrease by more than a factor 2 in the core correlation length across the L-H transition was observed using correlation reflectometry. Concurrently gas-puff imaging (GPI) shows the edge becoming very quiescent as edge turbulent “blob” activity subsides. No consistent precursor MHD activity to the L-H transition has been observed for OH H-modes in NSTX. However, an edge rotation diagnostic showed the edge electric field becoming more negative up to 20 ms before the transition in several OH H-mode plasmas. Observations of Te, Ti, and Vf before and after the transition and from other diagnostics will be presented in discussions of the physics of NSTX OH H-modes.