Overview of L-H power threshold studies in NSTX

A summary of results from recent L-H power threshold (P_LH) experiments in the National Spherical Torus Experiment is presented. First P_LH (normalized linearly by plasma density) was found to be a minimum in double-null configuration, tending to increase as the plasma was shifted more strongly towards lower- or upper-single null configuration with either neutral beam or rf heating. The measured P_LH/n_e was comparable with neutral beam or rf heating, suggesting that rotation was not playing a dominant role in setting the value of P_LH. The role of triangularity (δ_bot) in setting P_LH/n_e is less clear: while 50% less auxiliary heating power was required to access H-mode at low δ_bot than at high δ_bot, the high δ_bot discharges had lower ohmic heating and higher dW/dt, leading to comparable loss of power over a range of δ_bot. In addition, the dependences of P_LH on the density, species (helium versus deuterium), plasma current, applied non-axisymmetric error fields and lithium wall conditioning are summarized.